Auto merge of #134299 - RalfJung:remove-start, r=compiler-errors

remove support for the (unstable) #[start] attribute

As explained by `@Noratrieb:`
`#[start]` should be deleted. It's nothing but an accidentally leaked implementation detail that's a not very useful mix between "portable" entrypoint logic and bad abstraction.

I think the way the stable user-facing entrypoint should work (and works today on stable) is pretty simple:
- `std`-using cross-platform programs should use `fn main()`. the compiler, together with `std`, will then ensure that code ends up at `main` (by having a platform-specific entrypoint that gets directed through `lang_start` in `std` to `main` - but that's just an implementation detail)
- `no_std` platform-specific programs should use `#![no_main]` and define their own platform-specific entrypoint symbol with `#[no_mangle]`, like `main`, `_start`, `WinMain` or `my_embedded_platform_wants_to_start_here`. most of them only support a single platform anyways, and need cfg for the different platform's ways of passing arguments or other things *anyways*

`#[start]` is in a super weird position of being neither of those two. It tries to pretend that it's cross-platform, but its signature is  a total lie. Those arguments are just stubbed out to zero on ~~Windows~~ wasm, for example. It also only handles the platform-specific entrypoints for a few platforms that are supported by `std`, like Windows or Unix-likes. `my_embedded_platform_wants_to_start_here` can't use it, and neither could a libc-less Linux program.
So we have an attribute that only works in some cases anyways, that has a signature that's a total lie (and a signature that, as I might want to add, has changed recently, and that I definitely would not be comfortable giving *any* stability guarantees on), and where there's a pretty easy way to get things working without it in the first place.

Note that this feature has **not** been RFCed in the first place.

*This comment was posted [in May](https://github.com/rust-lang/rust/issues/29633#issuecomment-2088596042) and so far nobody spoke up in that issue with a usecase that would require keeping the attribute.*

Closes https://github.com/rust-lang/rust/issues/29633

try-job: x86_64-gnu-nopt
try-job: x86_64-msvc-1
try-job: x86_64-msvc-2
try-job: test-various

compiler/rustc_ast/src/entry.rs

@@ -18,12 +18,6 @@ pub enum EntryPointType {
     /// fn main() {}
     /// ```
     RustcMainAttr,
-    /// This is a function with the `#[start]` attribute.
-    /// ```ignore (clashes with test entrypoint)
-    /// #[start]
-    /// fn main() {}
-    /// ```
-    Start,
     /// This function is **not** an entrypoint but simply named `main` (not at the root).
     /// This is only used for diagnostics.
     /// ```
@@ -40,9 +34,7 @@ pub fn entry_point_type(
     at_root: bool,
     name: Option<Symbol>,
 ) -> EntryPointType {
-    if attr::contains_name(attrs, sym::start) {
+    if attr::contains_name(attrs, sym::rustc_main) {
-        EntryPointType::Start
-    } else if attr::contains_name(attrs, sym::rustc_main) {
         EntryPointType::RustcMainAttr
     } else if let Some(name) = name
         && name == sym::main

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -230,18 +230,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 }
             }
 
-            ast::ItemKind::Fn(..) => {
-                if attr::contains_name(&i.attrs, sym::start) {
-                    gate!(
-                        &self,
-                        start,
-                        i.span,
-                        "`#[start]` functions are experimental and their signature may change \
-                         over time"
-                    );
-                }
-            }
-
             ast::ItemKind::Struct(..) | ast::ItemKind::Enum(..) | ast::ItemKind::Union(..) => {
                 for attr in attr::filter_by_name(&i.attrs, sym::repr) {
                     for item in attr.meta_item_list().unwrap_or_else(ThinVec::new) {

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -204,11 +204,11 @@ impl<'a> MutVisitor for EntryPointCleaner<'a> {
         ast::mut_visit::walk_item(self, item);
         self.depth -= 1;
 
-        // Remove any #[rustc_main] or #[start] from the AST so it doesn't
+        // Remove any #[rustc_main] from the AST so it doesn't
         // clash with the one we're going to add, but mark it as
         // #[allow(dead_code)] to avoid printing warnings.
         match entry_point_type(&item, self.depth == 0) {
-            EntryPointType::MainNamed | EntryPointType::RustcMainAttr | EntryPointType::Start => {
+            EntryPointType::MainNamed | EntryPointType::RustcMainAttr => {
                 let allow_dead_code = attr::mk_attr_nested_word(
                     &self.sess.psess.attr_id_generator,
                     ast::AttrStyle::Outer,
@@ -217,8 +217,7 @@ impl<'a> MutVisitor for EntryPointCleaner<'a> {
                     sym::dead_code,
                     self.def_site,
                 );
-                item.attrs
+                item.attrs.retain(|attr| !attr.has_name(sym::rustc_main));
-                    .retain(|attr| !attr.has_name(sym::rustc_main) && !attr.has_name(sym::start));
                 item.attrs.push(allow_dead_code);
             }
             EntryPointType::None | EntryPointType::OtherMain => {}

compiler/rustc_codegen_cranelift/src/main_shim.rs

@@ -1,7 +1,7 @@
 use cranelift_frontend::{FunctionBuilder, FunctionBuilderContext};
 use rustc_hir::LangItem;
 use rustc_middle::ty::{AssocKind, GenericArg};
-use rustc_session::config::{EntryFnType, sigpipe};
+use rustc_session::config::EntryFnType;
 use rustc_span::{DUMMY_SP, Ident};
 
 use crate::prelude::*;
@@ -14,10 +14,9 @@ pub(crate) fn maybe_create_entry_wrapper(
     is_jit: bool,
     is_primary_cgu: bool,
 ) {
-    let (main_def_id, (is_main_fn, sigpipe)) = match tcx.entry_fn(()) {
+    let (main_def_id, sigpipe) = match tcx.entry_fn(()) {
         Some((def_id, entry_ty)) => (def_id, match entry_ty {
-            EntryFnType::Main { sigpipe } => (true, sigpipe),
+            EntryFnType::Main { sigpipe } => sigpipe,
-            EntryFnType::Start => (false, sigpipe::DEFAULT),
         }),
         None => return,
     };
@@ -31,14 +30,13 @@ pub(crate) fn maybe_create_entry_wrapper(
         return;
     }
 
-    create_entry_fn(tcx, module, main_def_id, is_jit, is_main_fn, sigpipe);
+    create_entry_fn(tcx, module, main_def_id, is_jit, sigpipe);
 
     fn create_entry_fn(
         tcx: TyCtxt<'_>,
         m: &mut dyn Module,
         rust_main_def_id: DefId,
         ignore_lang_start_wrapper: bool,
-        is_main_fn: bool,
         sigpipe: u8,
     ) {
         let main_ret_ty = tcx.fn_sig(rust_main_def_id).no_bound_vars().unwrap().output();
@@ -94,8 +92,8 @@ pub(crate) fn maybe_create_entry_wrapper(
 
             let main_func_ref = m.declare_func_in_func(main_func_id, &mut bcx.func);
 
-            let result = if is_main_fn && ignore_lang_start_wrapper {
+            let result = if ignore_lang_start_wrapper {
-                // regular main fn, but ignoring #[lang = "start"] as we are running in the jit
+                // ignoring #[lang = "start"] as we are running in the jit
                 // FIXME set program arguments somehow
                 let call_inst = bcx.ins().call(main_func_ref, &[]);
                 let call_results = bcx.func.dfg.inst_results(call_inst).to_owned();
@@ -133,7 +131,8 @@ pub(crate) fn maybe_create_entry_wrapper(
                     types::I64 => bcx.ins().sextend(types::I64, res),
                     _ => unimplemented!("16bit systems are not yet supported"),
                 }
-            } else if is_main_fn {
+            } else {
+                // Regular main fn invoked via start lang item.
                 let start_def_id = tcx.require_lang_item(LangItem::Start, None);
                 let start_instance = Instance::expect_resolve(
                     tcx,
@@ -150,10 +149,6 @@ pub(crate) fn maybe_create_entry_wrapper(
                 let call_inst =
                     bcx.ins().call(func_ref, &[main_val, arg_argc, arg_argv, arg_sigpipe]);
                 bcx.inst_results(call_inst)[0]
-            } else {
-                // using user-defined start fn
-                let call_inst = bcx.ins().call(main_func_ref, &[arg_argc, arg_argv]);
-                bcx.inst_results(call_inst)[0]
             };
 
             bcx.ins().return_(&[result]);

compiler/rustc_codegen_gcc/build_system/src/test.rs

@@ -426,19 +426,6 @@ fn std_tests(env: &Env, args: &TestArg) -> Result<(), String> {
     run_command_with_env(&command, None, Some(env))?;
     maybe_run_command_in_vm(&[&cargo_target_dir.join("track-caller-attribute")], env, args)?;
 
-    // FIXME: create a function "display_if_not_quiet" or something along the line.
-    println!("[AOT] mod_bench");
-    let mut command = args.config_info.rustc_command_vec();
-    command.extend_from_slice(&[
-        &"example/mod_bench.rs",
-        &"--crate-type",
-        &"bin",
-        &"--target",
-        &args.config_info.target_triple,
-    ]);
-    run_command_with_env(&command, None, Some(env))?;
-    // FIXME: the compiled binary is not run.
-
     Ok(())
 }
 
@@ -696,19 +683,6 @@ fn test_libcore(env: &Env, args: &TestArg) -> Result<(), String> {
     Ok(())
 }
 
-// echo "[BENCH COMPILE] mod_bench"
-//
-// COMPILE_MOD_BENCH_INLINE="$RUSTC example/mod_bench.rs --crate-type bin -Zmir-opt-level=3 -O --crate-name mod_bench_inline"
-// COMPILE_MOD_BENCH_LLVM_0="rustc example/mod_bench.rs --crate-type bin -Copt-level=0 -o $cargo_target_dir/mod_bench_llvm_0 -Cpanic=abort"
-// COMPILE_MOD_BENCH_LLVM_1="rustc example/mod_bench.rs --crate-type bin -Copt-level=1 -o $cargo_target_dir/mod_bench_llvm_1 -Cpanic=abort"
-// COMPILE_MOD_BENCH_LLVM_2="rustc example/mod_bench.rs --crate-type bin -Copt-level=2 -o $cargo_target_dir/mod_bench_llvm_2 -Cpanic=abort"
-// COMPILE_MOD_BENCH_LLVM_3="rustc example/mod_bench.rs --crate-type bin -Copt-level=3 -o $cargo_target_dir/mod_bench_llvm_3 -Cpanic=abort"
-//
-// Use 100 runs, because a single compilations doesn't take more than ~150ms, so it isn't very slow
-// hyperfine --runs ${COMPILE_RUNS:-100} "$COMPILE_MOD_BENCH_INLINE" "$COMPILE_MOD_BENCH_LLVM_0" "$COMPILE_MOD_BENCH_LLVM_1" "$COMPILE_MOD_BENCH_LLVM_2" "$COMPILE_MOD_BENCH_LLVM_3"
-// echo "[BENCH RUN] mod_bench"
-// hyperfine --runs ${RUN_RUNS:-10} $cargo_target_dir/mod_bench{,_inline} $cargo_target_dir/mod_bench_llvm_*
-
 fn extended_rand_tests(env: &Env, args: &TestArg) -> Result<(), String> {
     if !args.is_using_gcc_master_branch() {
         println!("Not using GCC master branch. Skipping `extended_rand_tests`.");

compiler/rustc_codegen_gcc/example/alloc_example.rs

@@ -1,5 +1,6 @@
-#![feature(start, core_intrinsics, alloc_error_handler, lang_items)]
+#![feature(core_intrinsics, alloc_error_handler, lang_items)]
 #![no_std]
+#![no_main]
 #![allow(internal_features)]
 
 extern crate alloc;
@@ -37,8 +38,8 @@ unsafe extern "C" fn _Unwind_Resume() {
     core::intrinsics::unreachable();
 }
 
-#[start]
+#[no_mangle]
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(_argc: core::ffi::c_int, _argv: *const *const u8) -> core::ffi::c_int {
     let world: Box<&str> = Box::new("Hello World!\0");
     unsafe {
         puts(*world as *const str as *const u8);

compiler/rustc_codegen_gcc/example/mini_core_hello_world.rs

@@ -1,7 +1,7 @@
 // Adapted from https://github.com/sunfishcode/mir2cranelift/blob/master/rust-examples/nocore-hello-world.rs
 
 #![feature(
-    no_core, unboxed_closures, start, lang_items, never_type, linkage,
+    no_core, unboxed_closures, lang_items, never_type, linkage,
     extern_types, thread_local
 )]
 #![no_core]

compiler/rustc_codegen_gcc/example/mod_bench.rs

@@ -1,36 +0,0 @@
-#![feature(start, core_intrinsics, lang_items)]
-#![no_std]
-#![allow(internal_features)]
-
-#[link(name = "c")]
-extern "C" {}
-
-#[panic_handler]
-fn panic_handler(_: &core::panic::PanicInfo<'_>) -> ! {
-    core::intrinsics::abort();
-}
-
-#[lang="eh_personality"]
-fn eh_personality(){}
-
-// Required for rustc_codegen_llvm
-#[no_mangle]
-unsafe extern "C" fn _Unwind_Resume() {
-    core::intrinsics::unreachable();
-}
-
-#[start]
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
-    for i in 2..100_000_000 {
-        black_box((i + 1) % i);
-    }
-
-    0
-}
-
-#[inline(never)]
-fn black_box(i: u32) {
-    if i != 1 {
-        core::intrinsics::abort();
-    }
-}

compiler/rustc_codegen_gcc/src/context.rs

@@ -513,7 +513,6 @@ impl<'gcc, 'tcx> MiscCodegenMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         } else {
             // If the symbol already exists, it is an error: for example, the user wrote
             // #[no_mangle] extern "C" fn main(..) {..}
-            // instead of #[start]
             None
         }
     }

compiler/rustc_codegen_gcc/tests/run/abort1.rs

@@ -3,11 +3,12 @@
 // Run-time:
 //   status: signal
 
-#![feature(auto_traits, lang_items, no_core, start, intrinsics, rustc_attrs)]
+#![feature(auto_traits, lang_items, no_core, intrinsics, rustc_attrs)]
 #![allow(internal_features)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 /*
  * Core
@@ -49,7 +50,7 @@ fn test_fail() -> ! {
     unsafe { intrinsics::abort() };
 }
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     test_fail();
 }

compiler/rustc_codegen_gcc/tests/run/abort2.rs

@@ -3,11 +3,12 @@
 // Run-time:
 //   status: signal
 
-#![feature(auto_traits, lang_items, no_core, start, intrinsics, rustc_attrs)]
+#![feature(auto_traits, lang_items, no_core, intrinsics, rustc_attrs)]
 #![allow(internal_features)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 /*
  * Core
@@ -50,8 +51,8 @@ fn fail() -> i32 {
     0
 }
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     fail();
     0
 }

compiler/rustc_codegen_gcc/tests/run/array.rs

@@ -7,10 +7,11 @@
 //     5
 //     10
 
-#![feature(no_core, start)]
+#![feature(no_core)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 extern crate mini_core;
 
@@ -28,8 +29,8 @@ fn make_array() -> [u8; 3] {
     [42, 10, 5]
 }
 
-#[start]
+#[no_mangle]
-fn main(argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     let array = [42, 7, 5];
     let array2 = make_array();
     unsafe {

compiler/rustc_codegen_gcc/tests/run/assign.rs

@@ -6,10 +6,11 @@
 //     10
 
 #![allow(internal_features, unused_attributes)]
-#![feature(auto_traits, lang_items, no_core, start, intrinsics, rustc_attrs, track_caller)]
+#![feature(auto_traits, lang_items, no_core, intrinsics, rustc_attrs, track_caller)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 /*
  * Core
@@ -142,8 +143,8 @@ fn inc(num: isize) -> isize {
 }
 
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     argc = inc(argc);
     unsafe {
         libc::printf(b"%ld\n\0" as *const u8 as *const i8, argc);

compiler/rustc_codegen_gcc/tests/run/closure.rs

@@ -8,10 +8,11 @@
 //     Int argument: 2
 //     Both args: 11
 
-#![feature(no_core, start)]
+#![feature(no_core)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 extern crate mini_core;
 
@@ -22,8 +23,8 @@ mod libc {
     }
 }
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     let string = "Arg: %d\n\0";
     let mut closure = || {
         unsafe {

compiler/rustc_codegen_gcc/tests/run/condition.rs

@@ -5,10 +5,11 @@
 //   stdout: true
 //     1
 
-#![feature(no_core, start)]
+#![feature(no_core)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 extern crate mini_core;
 
@@ -19,8 +20,8 @@ mod libc {
     }
 }
 
-#[start]
+#[no_mangle]
-fn main(argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     unsafe {
         if argc == 1 {
             libc::printf(b"true\n\0" as *const u8 as *const i8);

compiler/rustc_codegen_gcc/tests/run/empty_main.rs

@@ -3,11 +3,12 @@
 // Run-time:
 //   status: 0
 
-#![feature(auto_traits, lang_items, no_core, start)]
+#![feature(auto_traits, lang_items, no_core)]
 #![allow(internal_features)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 /*
  * Core
@@ -34,7 +35,7 @@ pub(crate) unsafe auto trait Freeze {}
  * Code
  */
 
-#[start]
+#[no_mangle]
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     0
 }

compiler/rustc_codegen_gcc/tests/run/exit.rs

@@ -3,11 +3,12 @@
 // Run-time:
 //   status: 2
 
-#![feature(auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(auto_traits, lang_items, no_core, intrinsics)]
 #![allow(internal_features)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 mod libc {
     #[link(name = "c")]
@@ -41,8 +42,8 @@ pub(crate) unsafe auto trait Freeze {}
  * Code
  */
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     unsafe {
         libc::exit(2);
     }

compiler/rustc_codegen_gcc/tests/run/exit_code.rs

@@ -3,11 +3,12 @@
 // Run-time:
 //   status: 1
 
-#![feature(auto_traits, lang_items, no_core, start)]
+#![feature(auto_traits, lang_items, no_core)]
 #![allow(internal_features)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 /*
  * Core
@@ -34,7 +35,7 @@ pub(crate) unsafe auto trait Freeze {}
  * Code
  */
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     1
 }

compiler/rustc_codegen_gcc/tests/run/fun_ptr.rs

@@ -4,10 +4,11 @@
 //   status: 0
 //   stdout: 1
 
-#![feature(no_core, start)]
+#![feature(no_core)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 extern crate mini_core;
 
@@ -26,8 +27,8 @@ fn call_func(func: fn(i16) -> i8, param: i16) -> i8 {
     func(param)
 }
 
-#[start]
+#[no_mangle]
-fn main(argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     unsafe {
         let result = call_func(i16_as_i8, argc as i16) as isize;
         libc::printf(b"%ld\n\0" as *const u8 as *const i8, result);

compiler/rustc_codegen_gcc/tests/run/mut_ref.rs

@@ -8,10 +8,11 @@
 //     11
 
 #![allow(internal_features, unused_attributes)]
-#![feature(auto_traits, lang_items, no_core, start, intrinsics, rustc_attrs, track_caller)]
+#![feature(auto_traits, lang_items, no_core, intrinsics, rustc_attrs, track_caller)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 /*
  * Core
@@ -148,8 +149,8 @@ fn update_num(num: &mut isize) {
     *num = *num + 5;
 }
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     let mut test = test(argc);
     unsafe {
         libc::printf(b"%ld\n\0" as *const u8 as *const i8, test.field);

compiler/rustc_codegen_gcc/tests/run/operations.rs

@@ -6,10 +6,11 @@
 //     10
 
 #![allow(internal_features, unused_attributes)]
-#![feature(auto_traits, lang_items, no_core, start, intrinsics, arbitrary_self_types, rustc_attrs)]
+#![feature(auto_traits, lang_items, no_core, intrinsics, arbitrary_self_types, rustc_attrs)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 /*
  * Core
@@ -231,8 +232,8 @@ pub fn panic_const_mul_overflow() -> ! {
  * Code
  */
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     unsafe {
         libc::printf(b"%ld\n\0" as *const u8 as *const i8, 40 + argc);
         libc::printf(b"%ld\n\0" as *const u8 as *const i8, 40 - argc);

compiler/rustc_codegen_gcc/tests/run/ptr_cast.rs

@@ -4,10 +4,11 @@
 //   status: 0
 //   stdout: 1
 
-#![feature(no_core, start)]
+#![feature(no_core)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 extern crate mini_core;
 
@@ -24,8 +25,8 @@ fn make_array() -> [u8; 3] {
     [42, 10, 5]
 }
 
-#[start]
+#[no_mangle]
-fn main(argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     unsafe {
         let ptr = ONE as *mut usize;
         let value = ptr as usize;

compiler/rustc_codegen_gcc/tests/run/return-tuple.rs

@@ -6,11 +6,12 @@
 //     10
 //     42
 
-#![feature(auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(auto_traits, lang_items, no_core, intrinsics)]
 #![allow(internal_features)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 #[lang = "copy"]
 pub unsafe trait Copy {}
@@ -61,8 +62,8 @@ fn int_cast(a: u16, b: i16) -> (u8, u16, u32, usize, i8, i16, i32, isize, u8, u3
     )
 }
 
-#[start]
+#[no_mangle]
-fn main(argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     let (a, b, c, d, e, f, g, h, i, j) = int_cast(10, 42);
     unsafe {
         libc::printf(b"%d\n\0" as *const u8 as *const i8, c);

compiler/rustc_codegen_gcc/tests/run/slice.rs

@@ -4,10 +4,11 @@
 //   status: 0
 //   stdout: 5
 
-#![feature(no_core, start)]
+#![feature(no_core)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 extern crate mini_core;
 
@@ -26,8 +27,8 @@ fn index_slice(s: &[u32]) -> u32 {
     }
 }
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     let array = [42, 7, 5];
     unsafe {
         libc::printf(b"%ld\n\0" as *const u8 as *const i8, index_slice(&array));

compiler/rustc_codegen_gcc/tests/run/static.rs

@@ -9,11 +9,12 @@
 //      12
 //      1
 
-#![feature(auto_traits, lang_items, no_core, start, intrinsics, rustc_attrs)]
+#![feature(auto_traits, lang_items, no_core, intrinsics, rustc_attrs)]
 #![allow(internal_features)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 /*
  * Core
@@ -98,8 +99,8 @@ static mut WITH_REF: WithRef = WithRef {
     refe: unsafe { &TEST },
 };
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     unsafe {
         libc::printf(b"%ld\n\0" as *const u8 as *const i8, CONSTANT);
         libc::printf(b"%ld\n\0" as *const u8 as *const i8, TEST2.field);

compiler/rustc_codegen_gcc/tests/run/structs.rs

@@ -5,11 +5,12 @@
 //   stdout: 1
 //     2
 
-#![feature(auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(auto_traits, lang_items, no_core, intrinsics)]
 #![allow(internal_features)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 /*
  * Core
@@ -55,8 +56,8 @@ fn one() -> isize {
     1
 }
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     let test = Test {
         field: one(),
     };

compiler/rustc_codegen_gcc/tests/run/tuple.rs

@@ -4,11 +4,12 @@
 //   status: 0
 //   stdout: 3
 
-#![feature(auto_traits, lang_items, no_core, start, intrinsics)]
+#![feature(auto_traits, lang_items, no_core, intrinsics)]
 #![allow(internal_features)]
 
 #![no_std]
 #![no_core]
+#![no_main]
 
 /*
  * Core
@@ -42,8 +43,8 @@ mod libc {
  * Code
  */
 
-#[start]
+#[no_mangle]
-fn main(mut argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(argc: i32, _argv: *const *const u8) -> i32 {
     let test: (isize, isize, isize) = (3, 1, 4);
     unsafe {
         libc::printf(b"%ld\n\0" as *const u8 as *const i8, test.0);

compiler/rustc_codegen_llvm/src/context.rs

@@ -751,7 +751,6 @@ impl<'ll, 'tcx> MiscCodegenMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         } else {
             // If the symbol already exists, it is an error: for example, the user wrote
             // #[no_mangle] extern "C" fn main(..) {..}
-            // instead of #[start]
             None
         }
     }

compiler/rustc_codegen_ssa/messages.ftl

@@ -211,7 +211,7 @@ codegen_ssa_msvc_missing_linker = the msvc targets depend on the msvc linker but
 codegen_ssa_multiple_external_func_decl = multiple declarations of external function `{$function}` from library `{$library_name}` have different calling conventions
 
 codegen_ssa_multiple_main_functions = entry symbol `main` declared multiple times
-    .help = did you use `#[no_mangle]` on `fn main`? Use `#[start]` instead
+    .help = did you use `#[no_mangle]` on `fn main`? Use `#![no_main]` to suppress the usual Rust-generated entry point
 
 codegen_ssa_no_field = no field `{$name}`
 

compiler/rustc_codegen_ssa/src/base.rs

@@ -490,8 +490,8 @@ pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         let ptr_ty = cx.type_ptr();
         let (arg_argc, arg_argv) = get_argc_argv(&mut bx);
 
-        let (start_fn, start_ty, args, instance) = if let EntryFnType::Main { sigpipe } = entry_type
+        let EntryFnType::Main { sigpipe } = entry_type;
-        {
+        let (start_fn, start_ty, args, instance) = {
             let start_def_id = cx.tcx().require_lang_item(LangItem::Start, None);
             let start_instance = ty::Instance::expect_resolve(
                 cx.tcx(),
@@ -512,10 +512,6 @@ pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
                 vec![rust_main, arg_argc, arg_argv, arg_sigpipe],
                 Some(start_instance),
             )
-        } else {
-            debug!("using user-defined start fn");
-            let start_ty = cx.type_func(&[isize_ty, ptr_ty], isize_ty);
-            (rust_main, start_ty, vec![arg_argc, arg_argv], None)
         };
 
         let result = bx.call(start_ty, None, None, start_fn, &args, None, instance);
@@ -530,7 +526,8 @@ pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     }
 }
 
-/// Obtain the `argc` and `argv` values to pass to the rust start function.
+/// Obtain the `argc` and `argv` values to pass to the rust start function
+/// (i.e., the "start" lang item).
 fn get_argc_argv<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(bx: &mut Bx) -> (Bx::Value, Bx::Value) {
     if bx.cx().sess().target.os.contains("uefi") {
         // Params for UEFI

compiler/rustc_error_codes/src/error_codes/E0132.md

@@ -1,32 +1,3 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 A function with the `start` attribute was declared with type parameters.
-
-Erroneous code example:
-
-```compile_fail,E0132
-#![feature(start)]
-
-#[start]
-fn f<T>() {}
-```
-
-It is not possible to declare type parameters on a function that has the `start`
-attribute. Such a function must have the following type signature (for more
-information, view [the unstable book][1]):
-
-[1]: https://doc.rust-lang.org/unstable-book/language-features/start.html
-
-```
-# let _:
-fn(isize, *const *const u8) -> isize;
-```
-
-Example:
-
-```
-#![feature(start)]
-
-#[start]
-fn my_start(argc: isize, argv: *const *const u8) -> isize {
-    0
-}
-```

compiler/rustc_error_codes/src/error_codes/E0138.md

@@ -1,25 +1,3 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 More than one function was declared with the `#[start]` attribute.
-
-Erroneous code example:
-
-```compile_fail,E0138
-#![feature(start)]
-
-#[start]
-fn foo(argc: isize, argv: *const *const u8) -> isize {}
-
-#[start]
-fn f(argc: isize, argv: *const *const u8) -> isize {}
-// error: multiple 'start' functions
-```
-
-This error indicates that the compiler found multiple functions with the
-`#[start]` attribute. This is an error because there must be a unique entry
-point into a Rust program. Example:
-
-```
-#![feature(start)]
-
-#[start]
-fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
-```

compiler/rustc_error_codes/src/error_codes/E0647.md

@@ -1,13 +1,3 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 The `start` function was defined with a where clause.
-
-Erroneous code example:
-
-```compile_fail,E0647
-#![feature(start)]
-
-#[start]
-fn start(_: isize, _: *const *const u8) -> isize where (): Copy {
-    //^ error: `#[start]` function is not allowed to have a where clause
-    0
-}
-```

compiler/rustc_error_codes/src/lib.rs

@@ -24,6 +24,10 @@
 //
 // Both columns are necessary because it's not possible in Rust to create a new identifier such as
 // `E0123` from an integer literal such as `0123`, unfortunately.
+//
+// Do *not* remove entries from this list. Instead, just add a note th the corresponding markdown
+// file saying that this error is not emitted by the compiler any more (see E0001.md for an
+// example), and remove all code examples that do not build any more.
 #[macro_export]
 macro_rules! error_codes {
     ($macro:path) => (

compiler/rustc_feature/src/builtin_attrs.rs

@@ -448,7 +448,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
 
     // Entry point:
-    ungated!(start, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No),
     ungated!(no_start, CrateLevel, template!(Word), WarnFollowing, EncodeCrossCrate::No),
     ungated!(no_main, CrateLevel, template!(Word), WarnFollowing, EncodeCrossCrate::No),
 

compiler/rustc_feature/src/removed.rs

@@ -220,8 +220,9 @@ declare_features! (
     (removed, rustc_diagnostic_macros, "1.38.0", None, None),
     /// Allows identifying crates that contain sanitizer runtimes.
     (removed, sanitizer_runtime, "1.17.0", None, None),
-    (removed, simd, "1.0.0", Some(27731),
+    (removed, simd, "1.0.0", Some(27731), Some("removed in favor of `#[repr(simd)]`")),
-     Some("removed in favor of `#[repr(simd)]`")),
+    /// Allows using `#[start]` on a function indicating that it is the program entrypoint.
+    (removed, start, "1.0.0", Some(29633), Some("not portable enough and never RFC'd")),
     /// Allows `#[link(kind = "static-nobundle", ...)]`.
     (removed, static_nobundle, "1.16.0", Some(37403),
      Some(r#"subsumed by `#[link(kind = "static", modifiers = "-bundle", ...)]`"#)),

compiler/rustc_feature/src/unstable.rs

@@ -300,8 +300,6 @@ declare_features! (
     (internal, rustdoc_internals, "1.58.0", Some(90418)),
     /// Allows using the `rustdoc::missing_doc_code_examples` lint
     (unstable, rustdoc_missing_doc_code_examples, "1.31.0", Some(101730)),
-    /// Allows using `#[start]` on a function indicating that it is the program entrypoint.
-    (unstable, start, "1.0.0", Some(29633)),
     /// Allows using `#[structural_match]` which indicates that a type is structurally matchable.
     /// FIXME: Subsumed by trait `StructuralPartialEq`, cannot move to removed until a library
     /// feature with the same name exists.

compiler/rustc_hir/src/lang_items.rs

@@ -332,6 +332,10 @@ language_item_table! {
     FallbackSurfaceDrop,     sym::fallback_surface_drop, fallback_surface_drop_fn, Target::Fn,             GenericRequirement::None;
     AllocLayout,             sym::alloc_layout,        alloc_layout,               Target::Struct,         GenericRequirement::None;
 
+    /// For all binary crates without `#![no_main]`, Rust will generate a "main" function.
+    /// The exact name and signature are target-dependent. The "main" function will invoke
+    /// this lang item, passing it the `argc` and `argv` (or null, if those don't exist
+    /// on the current target) as well as the user-defined `fn main` from the binary crate.
     Start,                   sym::start,               start_fn,                   Target::Fn,             GenericRequirement::Exact(1);
 
     EhPersonality,           sym::eh_personality,      eh_personality,             Target::Fn,             GenericRequirement::None;

compiler/rustc_hir_analysis/messages.ftl

@@ -489,21 +489,6 @@ hir_analysis_simd_ffi_highly_experimental = use of SIMD type{$snip} in FFI is hi
 hir_analysis_specialization_trait = implementing `rustc_specialization_trait` traits is unstable
     .help = add `#![feature(min_specialization)]` to the crate attributes to enable
 
-hir_analysis_start_function_parameters = `#[start]` function is not allowed to have type parameters
-    .label = `#[start]` function cannot have type parameters
-
-hir_analysis_start_function_where = `#[start]` function is not allowed to have a `where` clause
-    .label = `#[start]` function cannot have a `where` clause
-
-hir_analysis_start_not_async = `#[start]` function is not allowed to be `async`
-    .label = `#[start]` is not allowed to be `async`
-
-hir_analysis_start_not_target_feature = `#[start]` function is not allowed to have `#[target_feature]`
-    .label = `#[start]` function is not allowed to have `#[target_feature]`
-
-hir_analysis_start_not_track_caller = `#[start]` function is not allowed to be `#[track_caller]`
-    .label = `#[start]` function is not allowed to be `#[track_caller]`
-
 hir_analysis_static_specialize = cannot specialize on `'static` lifetime
 
 hir_analysis_tait_forward_compat = item constrains opaque type that is not in its signature

compiler/rustc_hir_analysis/src/check/entry.rs

@@ -5,7 +5,7 @@ use rustc_hir as hir;
 use rustc_hir::Node;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::span_bug;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypingMode};
+use rustc_middle::ty::{self, TyCtxt, TypingMode};
 use rustc_session::config::EntryFnType;
 use rustc_span::def_id::{CRATE_DEF_ID, DefId, LocalDefId};
 use rustc_span::{Span, sym};
@@ -18,7 +18,6 @@ use crate::errors;
 pub(crate) fn check_for_entry_fn(tcx: TyCtxt<'_>) {
     match tcx.entry_fn(()) {
         Some((def_id, EntryFnType::Main { .. })) => check_main_fn_ty(tcx, def_id),
-        Some((def_id, EntryFnType::Start)) => check_start_fn_ty(tcx, def_id),
         _ => {}
     }
 }
@@ -192,83 +191,3 @@ fn check_main_fn_ty(tcx: TyCtxt<'_>, main_def_id: DefId) {
         });
     }
 }
-
-fn check_start_fn_ty(tcx: TyCtxt<'_>, start_def_id: DefId) {
-    let start_def_id = start_def_id.expect_local();
-    let start_id = tcx.local_def_id_to_hir_id(start_def_id);
-    let start_span = tcx.def_span(start_def_id);
-    let start_t = tcx.type_of(start_def_id).instantiate_identity();
-    match start_t.kind() {
-        ty::FnDef(..) => {
-            if let Node::Item(it) = tcx.hir_node(start_id) {
-                if let hir::ItemKind::Fn { sig, generics, .. } = &it.kind {
-                    let mut error = false;
-                    if !generics.params.is_empty() {
-                        tcx.dcx().emit_err(errors::StartFunctionParameters { span: generics.span });
-                        error = true;
-                    }
-                    if generics.has_where_clause_predicates {
-                        tcx.dcx().emit_err(errors::StartFunctionWhere {
-                            span: generics.where_clause_span,
-                        });
-                        error = true;
-                    }
-                    if sig.header.asyncness.is_async() {
-                        let span = tcx.def_span(it.owner_id);
-                        tcx.dcx().emit_err(errors::StartAsync { span });
-                        error = true;
-                    }
-
-                    let attrs = tcx.hir().attrs(start_id);
-                    for attr in attrs {
-                        if attr.has_name(sym::track_caller) {
-                            tcx.dcx().emit_err(errors::StartTrackCaller {
-                                span: attr.span,
-                                start: start_span,
-                            });
-                            error = true;
-                        }
-                        if attr.has_name(sym::target_feature)
-                            // Calling functions with `#[target_feature]` is
-                            // not unsafe on WASM, see #84988
-                            && !tcx.sess.target.is_like_wasm
-                            && !tcx.sess.opts.actually_rustdoc
-                        {
-                            tcx.dcx().emit_err(errors::StartTargetFeature {
-                                span: attr.span,
-                                start: start_span,
-                            });
-                            error = true;
-                        }
-                    }
-
-                    if error {
-                        return;
-                    }
-                }
-            }
-
-            let expected_sig = ty::Binder::dummy(tcx.mk_fn_sig(
-                [tcx.types.isize, Ty::new_imm_ptr(tcx, Ty::new_imm_ptr(tcx, tcx.types.u8))],
-                tcx.types.isize,
-                false,
-                hir::Safety::Safe,
-                ExternAbi::Rust,
-            ));
-
-            let _ = check_function_signature(
-                tcx,
-                ObligationCause::new(
-                    start_span,
-                    start_def_id,
-                    ObligationCauseCode::StartFunctionType,
-                ),
-                start_def_id.into(),
-                expected_sig,
-            );
-        }
-        _ => {
-            span_bug!(start_span, "start has a non-function type: found `{}`", start_t);
-        }
-    }
-}

compiler/rustc_hir_analysis/src/errors.rs

@@ -619,48 +619,6 @@ pub(crate) struct TargetFeatureOnMain {
     pub main: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(hir_analysis_start_not_track_caller)]
-pub(crate) struct StartTrackCaller {
-    #[primary_span]
-    pub span: Span,
-    #[label]
-    pub start: Span,
-}
-
-#[derive(Diagnostic)]
-#[diag(hir_analysis_start_not_target_feature)]
-pub(crate) struct StartTargetFeature {
-    #[primary_span]
-    pub span: Span,
-    #[label]
-    pub start: Span,
-}
-
-#[derive(Diagnostic)]
-#[diag(hir_analysis_start_not_async, code = E0752)]
-pub(crate) struct StartAsync {
-    #[primary_span]
-    #[label]
-    pub span: Span,
-}
-
-#[derive(Diagnostic)]
-#[diag(hir_analysis_start_function_where, code = E0647)]
-pub(crate) struct StartFunctionWhere {
-    #[primary_span]
-    #[label]
-    pub span: Span,
-}
-
-#[derive(Diagnostic)]
-#[diag(hir_analysis_start_function_parameters, code = E0132)]
-pub(crate) struct StartFunctionParameters {
-    #[primary_span]
-    #[label]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(hir_analysis_main_function_return_type_generic, code = E0131)]
 pub(crate) struct MainFunctionReturnTypeGeneric {

compiler/rustc_middle/src/traits/mod.rs

@@ -345,9 +345,6 @@ pub enum ObligationCauseCode<'tcx> {
     /// `main` has wrong type
     MainFunctionType,
 
-    /// `start` has wrong type
-    StartFunctionType,
-
     /// language function has wrong type
     LangFunctionType(Symbol),
 

compiler/rustc_passes/messages.ftl

@@ -502,11 +502,6 @@ passes_multiple_rustc_main =
     .first = first `#[rustc_main]` function
     .additional = additional `#[rustc_main]` function
 
-passes_multiple_start_functions =
-    multiple `start` functions
-    .label = multiple `start` functions
-    .previous = previous `#[start]` function here
-
 passes_must_not_suspend =
     `must_not_suspend` attribute should be applied to a struct, enum, union, or trait
     .label = is not a struct, enum, union, or trait

compiler/rustc_passes/src/check_attr.rs

@@ -275,7 +275,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     | sym::lang
                     | sym::needs_allocator
                     | sym::default_lib_allocator
-                    | sym::start
                     | sym::custom_mir,
                     ..
                 ] => {}
@@ -2655,7 +2654,6 @@ fn check_invalid_crate_level_attr(tcx: TyCtxt<'_>, attrs: &[Attribute]) {
         sym::repr,
         sym::path,
         sym::automatically_derived,
-        sym::start,
         sym::rustc_main,
         sym::derive,
         sym::test,

compiler/rustc_passes/src/entry.rs

@@ -10,9 +10,7 @@ use rustc_session::RemapFileNameExt;
 use rustc_session::config::{CrateType, EntryFnType, RemapPathScopeComponents, sigpipe};
 use rustc_span::{Span, Symbol, sym};
 
-use crate::errors::{
+use crate::errors::{AttrOnlyInFunctions, ExternMain, MultipleRustcMain, NoMainErr};
-    AttrOnlyInFunctions, ExternMain, MultipleRustcMain, MultipleStartFunctions, NoMainErr,
-};
 
 struct EntryContext<'tcx> {
     tcx: TyCtxt<'tcx>,
@@ -20,9 +18,6 @@ struct EntryContext<'tcx> {
     /// The function has the `#[rustc_main]` attribute.
     rustc_main_fn: Option<(LocalDefId, Span)>,
 
-    /// The function that has the attribute `#[start]` on it.
-    start_fn: Option<(LocalDefId, Span)>,
-
     /// The functions that one might think are `main` but aren't, e.g.
     /// main functions not defined at the top level. For diagnostics.
     non_main_fns: Vec<Span>,
@@ -40,8 +35,7 @@ fn entry_fn(tcx: TyCtxt<'_>, (): ()) -> Option<(DefId, EntryFnType)> {
         return None;
     }
 
-    let mut ctxt =
+    let mut ctxt = EntryContext { tcx, rustc_main_fn: None, non_main_fns: Vec::new() };
-        EntryContext { tcx, rustc_main_fn: None, start_fn: None, non_main_fns: Vec::new() };
 
     for id in tcx.hir().items() {
         check_and_search_item(id, &mut ctxt);
@@ -57,7 +51,7 @@ fn attr_span_by_symbol(ctxt: &EntryContext<'_>, id: ItemId, sym: Symbol) -> Opti
 
 fn check_and_search_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
     if !matches!(ctxt.tcx.def_kind(id.owner_id), DefKind::Fn) {
-        for attr in [sym::start, sym::rustc_main] {
+        for attr in [sym::rustc_main] {
             if let Some(span) = attr_span_by_symbol(ctxt, id, attr) {
                 ctxt.tcx.dcx().emit_err(AttrOnlyInFunctions { span, attr });
             }
@@ -91,24 +85,11 @@ fn check_and_search_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
                 });
             }
         }
-        EntryPointType::Start => {
-            if ctxt.start_fn.is_none() {
-                ctxt.start_fn = Some((id.owner_id.def_id, ctxt.tcx.def_span(id.owner_id)));
-            } else {
-                ctxt.tcx.dcx().emit_err(MultipleStartFunctions {
-                    span: ctxt.tcx.def_span(id.owner_id),
-                    labeled: ctxt.tcx.def_span(id.owner_id.to_def_id()),
-                    previous: ctxt.start_fn.unwrap().1,
-                });
-            }
-        }
     }
 }
 
 fn configure_main(tcx: TyCtxt<'_>, visitor: &EntryContext<'_>) -> Option<(DefId, EntryFnType)> {
-    if let Some((def_id, _)) = visitor.start_fn {
+    if let Some((local_def_id, _)) = visitor.rustc_main_fn {
-        Some((def_id.to_def_id(), EntryFnType::Start))
-    } else if let Some((local_def_id, _)) = visitor.rustc_main_fn {
         let def_id = local_def_id.to_def_id();
         Some((def_id, EntryFnType::Main { sigpipe: sigpipe(tcx) }))
     } else {

compiler/rustc_passes/src/errors.rs

@@ -1313,17 +1313,6 @@ pub(crate) struct MultipleRustcMain {
     pub additional: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(passes_multiple_start_functions, code = E0138)]
-pub(crate) struct MultipleStartFunctions {
-    #[primary_span]
-    pub span: Span,
-    #[label]
-    pub labeled: Span,
-    #[label(passes_previous)]
-    pub previous: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(passes_extern_main)]
 pub(crate) struct ExternMain {

compiler/rustc_session/src/config.rs

@@ -1268,7 +1268,6 @@ pub enum EntryFnType {
         /// and an `include!()`.
         sigpipe: u8,
     },
-    Start,
 }
 
 #[derive(Copy, PartialEq, PartialOrd, Clone, Ord, Eq, Hash, Debug, Encodable, Decodable)]

compiler/rustc_trait_selection/messages.ftl

@@ -261,7 +261,6 @@ trait_selection_oc_fn_lang_correct_type = {$lang_item_name ->
         *[lang_item_name] lang item `{$lang_item_name}`
     } function has wrong type
 trait_selection_oc_fn_main_correct_type = `main` function has wrong type
-trait_selection_oc_fn_start_correct_type = `#[start]` function has wrong type
 trait_selection_oc_generic = mismatched types
 
 trait_selection_oc_if_else_different = `if` and `else` have incompatible types
@@ -396,7 +395,6 @@ trait_selection_subtype = ...so that the {$requirement ->
     [if_else_different] `if` and `else` have incompatible types
     [no_else] `if` missing an `else` returns `()`
     [fn_main_correct_type] `main` function has the correct type
-    [fn_start_correct_type] `#[start]` function has the correct type
     [fn_lang_correct_type] lang item function has the correct type
     [intrinsic_correct_type] intrinsic has the correct type
     [method_correct_type] method receiver has the correct type
@@ -410,7 +408,6 @@ trait_selection_subtype_2 = ...so that {$requirement ->
     [if_else_different] `if` and `else` have incompatible types
     [no_else] `if` missing an `else` returns `()`
     [fn_main_correct_type] `main` function has the correct type
-    [fn_start_correct_type] `#[start]` function has the correct type
     [fn_lang_correct_type] lang item function has the correct type
     [intrinsic_correct_type] intrinsic has the correct type
     [method_correct_type] method receiver has the correct type

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

@@ -2318,7 +2318,6 @@ impl<'tcx> ObligationCause<'tcx> {
             | ObligationCauseCode::MatchExpressionArm(_)
             | ObligationCauseCode::IfExpression { .. }
             | ObligationCauseCode::LetElse
-            | ObligationCauseCode::StartFunctionType
             | ObligationCauseCode::LangFunctionType(_)
             | ObligationCauseCode::IntrinsicType
             | ObligationCauseCode::MethodReceiver => FailureCode::Error0308,
@@ -2376,9 +2375,6 @@ impl<'tcx> ObligationCause<'tcx> {
             ObligationCauseCode::MainFunctionType => {
                 ObligationCauseFailureCode::FnMainCorrectType { span }
             }
-            ObligationCauseCode::StartFunctionType => {
-                ObligationCauseFailureCode::FnStartCorrectType { span, subdiags }
-            }
             &ObligationCauseCode::LangFunctionType(lang_item_name) => {
                 ObligationCauseFailureCode::FnLangCorrectType { span, subdiags, lang_item_name }
             }
@@ -2421,7 +2417,6 @@ impl<'tcx> ObligationCause<'tcx> {
                 "const is compatible with trait"
             }
             ObligationCauseCode::MainFunctionType => "`main` function has the correct type",
-            ObligationCauseCode::StartFunctionType => "`#[start]` function has the correct type",
             ObligationCauseCode::LangFunctionType(_) => "lang item function has the correct type",
             ObligationCauseCode::IntrinsicType => "intrinsic has the correct type",
             ObligationCauseCode::MethodReceiver => "method receiver has the correct type",
@@ -2442,7 +2437,6 @@ impl IntoDiagArg for ObligationCauseAsDiagArg<'_> {
                 "const_compat"
             }
             ObligationCauseCode::MainFunctionType => "fn_main_correct_type",
-            ObligationCauseCode::StartFunctionType => "fn_start_correct_type",
             ObligationCauseCode::LangFunctionType(_) => "fn_lang_correct_type",
             ObligationCauseCode::IntrinsicType => "intrinsic_correct_type",
             ObligationCauseCode::MethodReceiver => "method_correct_type",

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -2740,7 +2740,6 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             | ObligationCauseCode::IfExpression { .. }
             | ObligationCauseCode::IfExpressionWithNoElse
             | ObligationCauseCode::MainFunctionType
-            | ObligationCauseCode::StartFunctionType
             | ObligationCauseCode::LangFunctionType(_)
             | ObligationCauseCode::IntrinsicType
             | ObligationCauseCode::MethodReceiver

compiler/rustc_trait_selection/src/errors.rs

@@ -1695,13 +1695,6 @@ pub enum ObligationCauseFailureCode {
         #[primary_span]
         span: Span,
     },
-    #[diag(trait_selection_oc_fn_start_correct_type, code = E0308)]
-    FnStartCorrectType {
-        #[primary_span]
-        span: Span,
-        #[subdiagnostic]
-        subdiags: Vec<TypeErrorAdditionalDiags>,
-    },
     #[diag(trait_selection_oc_fn_lang_correct_type, code = E0308)]
     FnLangCorrectType {
         #[primary_span]

src/doc/rustc/src/platform-support/nto-qnx.md

@@ -78,7 +78,7 @@ extern "C" {
 }
 
 #[no_mangle]
-pub extern "C" fn main(_argc: isize, _argv: *const *const u8) -> isize {
+pub extern "C" fn main(_argc: core::ffi::c_int, _argv: *const *const u8) -> core::ffi::c_int {
     const HELLO: &'static str = "Hello World, the answer is %d\n\0";
     unsafe {
         printf(HELLO.as_ptr() as *const _, 42);

src/doc/unstable-book/src/language-features/lang-items.md

@@ -46,14 +46,15 @@ allocation. A freestanding program that uses the `Box` sugar for dynamic
 allocations via `malloc` and `free`:
 
 ```rust,ignore (libc-is-finicky)
-#![feature(lang_items, start, core_intrinsics, rustc_private, panic_unwind, rustc_attrs)]
+#![feature(lang_items, core_intrinsics, rustc_private, panic_unwind, rustc_attrs)]
 #![allow(internal_features)]
 #![no_std]
+#![no_main]
 
 extern crate libc;
 extern crate unwind;
 
-use core::ffi::c_void;
+use core::ffi::{c_int, c_void};
 use core::intrinsics;
 use core::panic::PanicInfo;
 use core::ptr::NonNull;
@@ -91,8 +92,8 @@ unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
     p
 }
 
-#[start]
+#[no_mangle]
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(_argc: c_int, _argv: *const *const u8) -> c_int {
     let _x = Box::new(1);
 
     0

src/doc/unstable-book/src/language-features/start.md

@@ -1,59 +0,0 @@
-# `start`
-
-The tracking issue for this feature is: [#29633]
-
-[#29633]: https://github.com/rust-lang/rust/issues/29633
-
-------------------------
-
-Allows you to mark a function as the entry point of the executable, which is
-necessary in `#![no_std]` environments.
-
-The function marked `#[start]` is passed the command line parameters in the same
-format as the C main function (aside from the integer types being used).
-It has to be non-generic and have the following signature:
-
-```rust,ignore (only-for-syntax-highlight)
-# let _:
-fn(isize, *const *const u8) -> isize
-# ;
-```
-
-This feature should not be confused with the `start` *lang item* which is
-defined by the `std` crate and is written `#[lang = "start"]`.
-
-## Usage together with the `std` crate
-
-`#[start]` can be used in combination with the `std` crate, in which case the
-normal `main` function (which would get called from the `std` crate) won't be
-used as an entry point.
-The initialization code in `std` will be skipped this way.
-
-Example:
-
-```rust
-#![feature(start)]
-
-#[start]
-fn start(_argc: isize, _argv: *const *const u8) -> isize {
-    0
-}
-```
-
-Unwinding the stack past the `#[start]` function is currently considered
-Undefined Behavior (for any unwinding implementation):
-
-```rust,ignore (UB)
-#![feature(start)]
-
-#[start]
-fn start(_argc: isize, _argv: *const *const u8) -> isize {
-    std::panic::catch_unwind(|| {
-        panic!(); // panic safely gets caught or safely aborts execution
-    });
-
-    panic!(); // UB!
-
-    0
-}
-```

src/tools/clippy/tests/ui/borrow_as_ptr_no_std.fixed

@@ -1,10 +1,9 @@
 #![warn(clippy::borrow_as_ptr)]
-#![feature(lang_items, start, libc)]
 #![no_std]
+#![crate_type = "lib"]
 
 #[clippy::msrv = "1.75"]
-#[start]
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
     let val = 1;
     let _p = core::ptr::addr_of!(val);
 
@@ -12,11 +11,3 @@ fn main(_argc: isize, _argv: *const *const u8) -> isize {
     let _p_mut = core::ptr::addr_of_mut!(val_mut);
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

src/tools/clippy/tests/ui/borrow_as_ptr_no_std.rs

@@ -1,10 +1,9 @@
 #![warn(clippy::borrow_as_ptr)]
-#![feature(lang_items, start, libc)]
 #![no_std]
+#![crate_type = "lib"]
 
 #[clippy::msrv = "1.75"]
-#[start]
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
     let val = 1;
     let _p = &val as *const i32;
 
@@ -12,11 +11,3 @@ fn main(_argc: isize, _argv: *const *const u8) -> isize {
     let _p_mut = &mut val_mut as *mut i32;
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

src/tools/clippy/tests/ui/borrow_as_ptr_no_std.stderr

@@ -1,5 +1,5 @@
 error: borrow as raw pointer
-  --> tests/ui/borrow_as_ptr_no_std.rs:9:14
+  --> tests/ui/borrow_as_ptr_no_std.rs:8:14
    |
 LL |     let _p = &val as *const i32;
    |              ^^^^^^^^^^^^^^^^^^ help: try: `core::ptr::addr_of!(val)`
@@ -8,7 +8,7 @@ LL |     let _p = &val as *const i32;
    = help: to override `-D warnings` add `#[allow(clippy::borrow_as_ptr)]`
 
 error: borrow as raw pointer
-  --> tests/ui/borrow_as_ptr_no_std.rs:12:18
+  --> tests/ui/borrow_as_ptr_no_std.rs:11:18
    |
 LL |     let _p_mut = &mut val_mut as *mut i32;
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::ptr::addr_of_mut!(val_mut)`

src/tools/clippy/tests/ui/box_default_no_std.rs

@@ -1,6 +1,6 @@
-#![feature(lang_items, start, libc)]
 #![warn(clippy::box_default)]
 #![no_std]
+#![crate_type = "lib"]
 
 pub struct NotBox<T> {
     _value: T,
@@ -18,16 +18,7 @@ impl<T: Default> Default for NotBox<T> {
     }
 }
 
-#[start]
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
     let _p = NotBox::new(isize::default());
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

src/tools/clippy/tests/ui/crashes/ice-7410.rs

@@ -1,7 +1,7 @@
 //@compile-flags: -Clink-arg=-nostartfiles
 //@ignore-target: apple windows
 
-#![feature(lang_items, start, libc)]
+#![crate_type = "lib"]
 #![no_std]
 #![allow(clippy::if_same_then_else)]
 #![allow(clippy::redundant_pattern_matching)]
@@ -15,18 +15,9 @@ impl Drop for S {
     fn drop(&mut self) {}
 }
 
-#[start]
+pub fn main(argc: isize, argv: *const *const u8) -> isize {
-fn main(argc: isize, argv: *const *const u8) -> isize {
     if let Some(_) = Some(S) {
     } else {
     }
     0
 }
-
-#[panic_handler]
-fn panic(_info: &PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

src/tools/clippy/tests/ui/crate_level_checks/no_std_main_recursion.rs

@@ -1,32 +0,0 @@
-//@compile-flags: -Clink-arg=-nostartfiles
-//@ignore-target: apple
-
-#![feature(lang_items, start, libc)]
-#![no_std]
-
-use core::panic::PanicInfo;
-use core::sync::atomic::{AtomicUsize, Ordering};
-
-static N: AtomicUsize = AtomicUsize::new(0);
-
-#[warn(clippy::main_recursion)]
-#[start]
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
-    let x = N.load(Ordering::Relaxed);
-    N.store(x + 1, Ordering::Relaxed);
-
-    if x < 3 {
-        main(_argc, _argv);
-    }
-
-    0
-}
-
-#[allow(clippy::empty_loop)]
-#[panic_handler]
-fn panic(_info: &PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

src/tools/clippy/tests/ui/crate_level_checks/no_std_swap.fixed

@@ -1,11 +1,10 @@
 #![no_std]
-#![feature(lang_items, start, libc)]
 #![crate_type = "lib"]
 
 use core::panic::PanicInfo;
 
 #[warn(clippy::all)]
-fn main() {
+pub fn main() {
     let mut a = 42;
     let mut b = 1337;
 

src/tools/clippy/tests/ui/crate_level_checks/no_std_swap.rs

@@ -1,11 +1,10 @@
 #![no_std]
-#![feature(lang_items, start, libc)]
 #![crate_type = "lib"]
 
 use core::panic::PanicInfo;
 
 #[warn(clippy::all)]
-fn main() {
+pub fn main() {
     let mut a = 42;
     let mut b = 1337;
 

src/tools/clippy/tests/ui/crate_level_checks/no_std_swap.stderr

@@ -1,5 +1,5 @@
 error: this looks like you are trying to swap `a` and `b`
-  --> tests/ui/crate_level_checks/no_std_swap.rs:12:5
+  --> tests/ui/crate_level_checks/no_std_swap.rs:11:5
    |
 LL | /     a = b;
 ...  |

src/tools/clippy/tests/ui/def_id_nocore.rs

@@ -1,6 +1,6 @@
 //@ignore-target: apple
 
-#![feature(no_core, lang_items, start)]
+#![feature(no_core, lang_items)]
 #![no_core]
 #![allow(clippy::missing_safety_doc)]
 

src/tools/clippy/tests/ui/empty_loop_no_std.rs

@@ -2,27 +2,11 @@
 //@ignore-target: apple
 
 #![warn(clippy::empty_loop)]
-#![feature(lang_items, start, libc)]
+#![crate_type = "lib"]
 #![no_std]
 
-use core::panic::PanicInfo;
+pub fn main(argc: isize, argv: *const *const u8) -> isize {
-
-#[start]
-fn main(argc: isize, argv: *const *const u8) -> isize {
     // This should trigger the lint
     loop {}
     //~^ ERROR: empty `loop {}` wastes CPU cycles
 }
-
-#[panic_handler]
-fn panic(_info: &PanicInfo) -> ! {
-    // This should NOT trigger the lint
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {
-    // This should also trigger the lint
-    loop {}
-    //~^ ERROR: empty `loop {}` wastes CPU cycles
-}

src/tools/clippy/tests/ui/empty_loop_no_std.stderr

@@ -1,5 +1,5 @@
 error: empty `loop {}` wastes CPU cycles
-  --> tests/ui/empty_loop_no_std.rs:13:5
+  --> tests/ui/empty_loop_no_std.rs:10:5
    |
 LL |     loop {}
    |     ^^^^^^^
@@ -8,13 +8,5 @@ LL |     loop {}
    = note: `-D clippy::empty-loop` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::empty_loop)]`
 
-error: empty `loop {}` wastes CPU cycles
+error: aborting due to 1 previous error
-  --> tests/ui/empty_loop_no_std.rs:26:5
-   |
-LL |     loop {}
-   |     ^^^^^^^
-   |
-   = help: you should either use `panic!()` or add a call pausing or sleeping the thread to the loop body
-
-error: aborting due to 2 previous errors
 

src/tools/clippy/tests/ui/floating_point_arithmetic_nostd.rs

@@ -1,4 +1,4 @@
-#![feature(lang_items, start)]
+#![crate_type = "lib"]
 #![warn(clippy::imprecise_flops)]
 #![warn(clippy::suboptimal_flops)]
 #![no_std]
@@ -17,15 +17,6 @@ fn fake_abs1(num: f64) -> f64 {
     if num >= 0.0 { num } else { -num }
 }
 
-#[start]
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

src/tools/clippy/tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -6,7 +6,6 @@
 //@aux-build:../auxiliary/proc_macros.rs
 
 #![warn(clippy::missing_const_for_fn)]
-#![feature(start)]
 #![feature(type_alias_impl_trait)]
 
 extern crate helper;
@@ -71,15 +70,6 @@ mod with_test_fn {
     }
 }
 
-// Allowing on this function, because it would lint, which we don't want in this case.
-// if we have `#[start]` and `#[test]` check `is_entrypoint_fn(cx, def_id.to_def_id())` is stopped
-// working
-#[allow(clippy::missing_const_for_fn)]
-#[start]
-fn init(num: isize, something: *const *const u8) -> isize {
-    1
-}
-
 trait Foo {
     // This should not be suggested to be made const
     // (rustc doesn't allow const trait methods)

src/tools/clippy/tests/ui/missing_spin_loop_no_std.fixed

@@ -1,22 +1,13 @@
 #![warn(clippy::missing_spin_loop)]
-#![feature(lang_items, start, libc)]
+#![crate_type = "lib"]
 #![no_std]
 
 use core::sync::atomic::{AtomicBool, Ordering};
 
-#[start]
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
     // This should trigger the lint
     let b = AtomicBool::new(true);
     // This should lint with `core::hint::spin_loop()`
     while b.load(Ordering::Acquire) { core::hint::spin_loop() }
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

src/tools/clippy/tests/ui/missing_spin_loop_no_std.rs

@@ -1,22 +1,13 @@
 #![warn(clippy::missing_spin_loop)]
-#![feature(lang_items, start, libc)]
+#![crate_type = "lib"]
 #![no_std]
 
 use core::sync::atomic::{AtomicBool, Ordering};
 
-#[start]
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
     // This should trigger the lint
     let b = AtomicBool::new(true);
     // This should lint with `core::hint::spin_loop()`
     while b.load(Ordering::Acquire) {}
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

src/tools/clippy/tests/ui/missing_spin_loop_no_std.stderr

@@ -1,5 +1,5 @@
 error: busy-waiting loop should at least have a spin loop hint
-  --> tests/ui/missing_spin_loop_no_std.rs:12:37
+  --> tests/ui/missing_spin_loop_no_std.rs:11:37
    |
 LL |     while b.load(Ordering::Acquire) {}
    |                                     ^^ help: try: `{ core::hint::spin_loop() }`

src/tools/clippy/tests/ui/result_unit_error_no_std.rs

@@ -1,5 +1,6 @@
-#![feature(lang_items, start, libc)]
+#![feature(lang_items, libc)]
 #![no_std]
+#![no_main]
 #![warn(clippy::result_unit_err)]
 
 #[clippy::msrv = "1.80"]
@@ -12,8 +13,8 @@ pub fn returns_unit_error_lint() -> Result<u32, ()> {
     Err(())
 }
 
-#[start]
+#[no_mangle]
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
+extern "C" fn main(_argc: core::ffi::c_int, _argv: *const *const u8) -> core::ffi::c_int {
     0
 }
 

src/tools/clippy/tests/ui/result_unit_error_no_std.stderr

@@ -1,5 +1,5 @@
 error: this returns a `Result<_, ()>`
-  --> tests/ui/result_unit_error_no_std.rs:11:1
+  --> tests/ui/result_unit_error_no_std.rs:12:1
    |
 LL | pub fn returns_unit_error_lint() -> Result<u32, ()> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/tools/clippy/tests/ui/zero_ptr_no_std.fixed

@@ -1,19 +1,10 @@
-#![feature(lang_items, start, libc)]
+#![crate_type = "lib"]
 #![no_std]
 #![deny(clippy::zero_ptr)]
 
-#[start]
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
     let _ = core::ptr::null::<usize>();
     let _ = core::ptr::null_mut::<f64>();
     let _: *const u8 = core::ptr::null();
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

src/tools/clippy/tests/ui/zero_ptr_no_std.rs

@@ -1,19 +1,10 @@
-#![feature(lang_items, start, libc)]
+#![crate_type = "lib"]
 #![no_std]
 #![deny(clippy::zero_ptr)]
 
-#[start]
+pub fn main(_argc: isize, _argv: *const *const u8) -> isize {
-fn main(_argc: isize, _argv: *const *const u8) -> isize {
     let _ = 0 as *const usize;
     let _ = 0 as *mut f64;
     let _: *const u8 = 0 as *const _;
     0
 }
-
-#[panic_handler]
-fn panic(_info: &core::panic::PanicInfo) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"]
-extern "C" fn eh_personality() {}

src/tools/clippy/tests/ui/zero_ptr_no_std.stderr

@@ -1,5 +1,5 @@
 error: `0 as *const _` detected
-  --> tests/ui/zero_ptr_no_std.rs:7:13
+  --> tests/ui/zero_ptr_no_std.rs:6:13
    |
 LL |     let _ = 0 as *const usize;
    |             ^^^^^^^^^^^^^^^^^ help: try: `core::ptr::null::<usize>()`
@@ -11,13 +11,13 @@ LL | #![deny(clippy::zero_ptr)]
    |         ^^^^^^^^^^^^^^^^
 
 error: `0 as *mut _` detected
-  --> tests/ui/zero_ptr_no_std.rs:8:13
+  --> tests/ui/zero_ptr_no_std.rs:7:13
    |
 LL |     let _ = 0 as *mut f64;
    |             ^^^^^^^^^^^^^ help: try: `core::ptr::null_mut::<f64>()`
 
 error: `0 as *const _` detected
-  --> tests/ui/zero_ptr_no_std.rs:9:24
+  --> tests/ui/zero_ptr_no_std.rs:8:24
    |
 LL |     let _: *const u8 = 0 as *const _;
    |                        ^^^^^^^^^^^^^ help: try: `core::ptr::null()`

src/tools/miri/src/bin/miri.rs

@@ -33,7 +33,7 @@ use std::sync::atomic::{AtomicI32, Ordering};
 use std::sync::{Arc, Once};
 
 use miri::{
-    BacktraceStyle, BorrowTrackerMethod, MiriConfig, ProvenanceMode, RetagFields, ValidationMode,
+    BacktraceStyle, BorrowTrackerMethod, MiriConfig, MiriEntryFnType,ProvenanceMode, RetagFields, ValidationMode,
 };
 use rustc_abi::ExternAbi;
 use rustc_data_structures::sync;
@@ -51,7 +51,7 @@ use rustc_middle::query::LocalCrate;
 use rustc_middle::traits::{ObligationCause, ObligationCauseCode};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::util::Providers;
-use rustc_session::config::{CrateType, EntryFnType, ErrorOutputType, OptLevel};
+use rustc_session::config::{CrateType, ErrorOutputType, OptLevel};
 use rustc_session::search_paths::PathKind;
 use rustc_session::{CtfeBacktrace, EarlyDiagCtxt};
 use rustc_span::def_id::DefId;
@@ -73,9 +73,9 @@ impl MiriCompilerCalls {
     }
 }
 
-fn entry_fn(tcx: TyCtxt<'_>) -> (DefId, EntryFnType) {
+fn entry_fn(tcx: TyCtxt<'_>) -> (DefId, MiriEntryFnType) {
-    if let Some(entry_def) = tcx.entry_fn(()) {
+    if let Some((def_id, entry_type)) = tcx.entry_fn(()) {
-        return entry_def;
+        return (def_id, MiriEntryFnType::Rustc(entry_type));
     }
     // Look for a symbol in the local crate named `miri_start`, and treat that as the entry point.
     let sym = tcx.exported_symbols(LOCAL_CRATE).iter().find_map(|(sym, _)| {
@@ -102,7 +102,7 @@ fn entry_fn(tcx: TyCtxt<'_>) -> (DefId, EntryFnType) {
         .is_ok();
 
         if correct_func_sig {
-            (*id, EntryFnType::Start)
+            (*id, MiriEntryFnType::MiriStart)
         } else {
             tcx.dcx().fatal(
                 "`miri_start` must have the following signature:\n\

src/tools/miri/src/eval.rs

@@ -19,6 +19,12 @@ use crate::diagnostics::report_leaks;
 use crate::shims::tls;
 use crate::*;
 
+#[derive(Copy, Clone, Debug)]
+pub enum MiriEntryFnType {
+    MiriStart,
+    Rustc(EntryFnType),
+}
+
 /// When the main thread would exit, we will yield to any other thread that is ready to execute.
 /// But we must only do that a finite number of times, or a background thread running `loop {}`
 /// will hang the program.
@@ -272,7 +278,7 @@ impl<'tcx> MainThreadState<'tcx> {
 pub fn create_ecx<'tcx>(
     tcx: TyCtxt<'tcx>,
     entry_id: DefId,
-    entry_type: EntryFnType,
+    entry_type: MiriEntryFnType,
     config: &MiriConfig,
 ) -> InterpResult<'tcx, InterpCx<'tcx, MiriMachine<'tcx>>> {
     let typing_env = ty::TypingEnv::fully_monomorphized();
@@ -300,7 +306,7 @@ pub fn create_ecx<'tcx>(
     // Setup first stack frame.
     let entry_instance = ty::Instance::mono(tcx, entry_id);
 
-    // First argument is constructed later, because it's skipped if the entry function uses #[start].
+    // First argument is constructed later, because it's skipped for `miri_start.`
 
     // Second argument (argc): length of `config.args`.
     let argc =
@@ -373,11 +379,9 @@ pub fn create_ecx<'tcx>(
     // Call start function.
 
     match entry_type {
-        EntryFnType::Main { .. } => {
+        MiriEntryFnType::Rustc(EntryFnType::Main { .. }) => {
             let start_id = tcx.lang_items().start_fn().unwrap_or_else(|| {
-                tcx.dcx().fatal(
+                tcx.dcx().fatal("could not find start lang item");
-                    "could not find start function. Make sure the entry point is marked with `#[start]`."
-                );
             });
             let main_ret_ty = tcx.fn_sig(entry_id).no_bound_vars().unwrap().output();
             let main_ret_ty = main_ret_ty.no_bound_vars().unwrap();
@@ -413,7 +417,7 @@ pub fn create_ecx<'tcx>(
                 StackPopCleanup::Root { cleanup: true },
             )?;
         }
-        EntryFnType::Start => {
+        MiriEntryFnType::MiriStart => {
             ecx.call_function(
                 entry_instance,
                 ExternAbi::Rust,
@@ -434,7 +438,7 @@ pub fn create_ecx<'tcx>(
 pub fn eval_entry<'tcx>(
     tcx: TyCtxt<'tcx>,
     entry_id: DefId,
-    entry_type: EntryFnType,
+    entry_type: MiriEntryFnType,
     config: MiriConfig,
 ) -> Option<i32> {
     // Copy setting before we move `config`.

src/tools/miri/src/lib.rs

@@ -135,8 +135,8 @@ pub use crate::diagnostics::{
     EvalContextExt as _, NonHaltingDiagnostic, TerminationInfo, report_error,
 };
 pub use crate::eval::{
-    AlignmentCheck, BacktraceStyle, IsolatedOp, MiriConfig, RejectOpWith, ValidationMode,
+    AlignmentCheck, BacktraceStyle, IsolatedOp, MiriConfig, MiriEntryFnType, RejectOpWith,
-    create_ecx, eval_entry,
+    ValidationMode, create_ecx, eval_entry,
 };
 pub use crate::helpers::{AccessKind, EvalContextExt as _};
 pub use crate::intrinsics::EvalContextExt as _;

src/tools/miri/test-cargo-miri/no-std-smoke/src/main.rs

@@ -1,6 +1,5 @@
 // Copied from tests/pass/no-std.rs
 
-#![feature(start)]
 #![no_std]
 
 // Plumbing to let us use `writeln!` to host stdout:
@@ -22,8 +21,8 @@ impl Write for Host {
     }
 }
 
-#[start]
+#[no_mangle]
-fn start(_: isize, _: *const *const u8) -> isize {
+fn miri_start(_: isize, _: *const *const u8) -> isize {
     writeln!(Host, "hello, world!").unwrap();
     0
 }

src/tools/miri/tests/fail/alloc/alloc_error_handler_custom.rs

@@ -1,8 +1,9 @@
 //@compile-flags: -Cpanic=abort
-#![feature(start, core_intrinsics)]
+#![feature(core_intrinsics)]
 #![feature(alloc_error_handler)]
 #![feature(allocator_api)]
 #![no_std]
+#![no_main]
 
 extern crate alloc;
 
@@ -43,7 +44,7 @@ mod plumbing {
     static GLOBAL: NoAlloc = NoAlloc;
 }
 
-#[start]
+#[no_mangle]
-fn start(_: isize, _: *const *const u8) -> isize {
+fn miri_start(_argc: isize, _argv: *const *const u8) -> isize {
     handle_alloc_error(Layout::for_value(&0));
 }

src/tools/miri/tests/fail/alloc/alloc_error_handler_custom.stderr

@@ -16,7 +16,7 @@ LL | fn alloc_error_handler(layout: Layout) -> ! {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: inside `alloc::alloc::handle_alloc_error::rt_error` at RUSTLIB/alloc/src/alloc.rs:LL:CC
    = note: inside `alloc::alloc::handle_alloc_error` at RUSTLIB/alloc/src/alloc.rs:LL:CC
-note: inside `start`
+note: inside `miri_start`
   --> tests/fail/alloc/alloc_error_handler_custom.rs:LL:CC
    |
 LL |     handle_alloc_error(Layout::for_value(&0));

src/tools/miri/tests/fail/alloc/alloc_error_handler_no_std.rs

@@ -1,8 +1,9 @@
 //@compile-flags: -Cpanic=abort
-#![feature(start, core_intrinsics)]
+#![feature(core_intrinsics)]
 #![feature(alloc_error_handler)]
 #![feature(allocator_api)]
 #![no_std]
+#![no_main]
 
 extern crate alloc;
 
@@ -41,7 +42,7 @@ mod plumbing {
     static GLOBAL: NoAlloc = NoAlloc;
 }
 
-#[start]
+#[no_mangle]
-fn start(_: isize, _: *const *const u8) -> isize {
+fn miri_start(_argc: isize, _argv: *const *const u8) -> isize {
     handle_alloc_error(Layout::for_value(&0));
 }

src/tools/miri/tests/fail/alloc/alloc_error_handler_no_std.stderr

@@ -12,7 +12,7 @@ LL |     core::intrinsics::abort();
    = note: inside `alloc::alloc::__alloc_error_handler::__rdl_oom` at RUSTLIB/alloc/src/alloc.rs:LL:CC
    = note: inside `alloc::alloc::handle_alloc_error::rt_error` at RUSTLIB/alloc/src/alloc.rs:LL:CC
    = note: inside `alloc::alloc::handle_alloc_error` at RUSTLIB/alloc/src/alloc.rs:LL:CC
-note: inside `start`
+note: inside `miri_start`
   --> tests/fail/alloc/alloc_error_handler_no_std.rs:LL:CC
    |
 LL |     handle_alloc_error(Layout::for_value(&0));

src/tools/miri/tests/fail/alloc/no_global_allocator.rs

@@ -2,15 +2,15 @@
 //@normalize-stderr-test: "OS `.*`" -> "$$OS"
 // Make sure we pretend the allocation symbols don't exist when there is no allocator
 
-#![feature(start)]
 #![no_std]
+#![no_main]
 
 extern "Rust" {
     fn __rust_alloc(size: usize, align: usize) -> *mut u8;
 }
 
-#[start]
+#[no_mangle]
-fn start(_: isize, _: *const *const u8) -> isize {
+fn miri_start(_argc: isize, _argv: *const *const u8) -> isize {
     unsafe {
         __rust_alloc(1, 1); //~ERROR: unsupported operation: can't call foreign function `__rust_alloc`
     }

src/tools/miri/tests/fail/alloc/no_global_allocator.stderr

@@ -7,7 +7,7 @@ LL |         __rust_alloc(1, 1);
    = help: if this is a basic API commonly used on this target, please report an issue with Miri
    = help: however, note that Miri does not aim to support every FFI function out there; for instance, we will not support APIs for things such as GUIs, scripting languages, or databases
    = note: BACKTRACE:
-   = note: inside `start` at tests/fail/alloc/no_global_allocator.rs:LL:CC
+   = note: inside `miri_start` at tests/fail/alloc/no_global_allocator.rs:LL:CC
 
 error: aborting due to 1 previous error
 

src/tools/miri/tests/fail/panic/no_std.rs

@@ -1,14 +1,15 @@
 //@compile-flags: -Cpanic=abort
-#![feature(start, core_intrinsics)]
+#![feature(core_intrinsics)]
 #![no_std]
+#![no_main]
 
 use core::fmt::Write;
 
 #[path = "../../utils/mod.no_std.rs"]
 mod utils;
 
-#[start]
+#[no_mangle]
-fn start(_: isize, _: *const *const u8) -> isize {
+fn miri_start(_argc: isize, _argv: *const *const u8) -> isize {
     panic!("blarg I am dead")
 }
 

src/tools/miri/tests/fail/panic/no_std.stderr

@@ -8,7 +8,7 @@ LL |     core::intrinsics::abort();
    |
    = note: BACKTRACE:
    = note: inside `panic_handler` at tests/fail/panic/no_std.rs:LL:CC
-note: inside `start`
+note: inside `miri_start`
   --> tests/fail/panic/no_std.rs:LL:CC
    |
 LL |     panic!("blarg I am dead")

src/tools/miri/tests/pass/alloc-access-tracking.rs

@@ -1,5 +1,5 @@
-#![feature(start)]
 #![no_std]
+#![no_main]
 //@compile-flags: -Zmiri-track-alloc-id=21 -Zmiri-track-alloc-accesses -Cpanic=abort
 //@normalize-stderr-test: "id 21" -> "id $$ALLOC"
 //@only-target: linux # alloc IDs differ between OSes (due to extern static allocations)
@@ -9,8 +9,8 @@ extern "Rust" {
     fn miri_dealloc(ptr: *mut u8, size: usize, align: usize);
 }
 
-#[start]
+#[no_mangle]
-fn start(_: isize, _: *const *const u8) -> isize {
+fn miri_start(_argc: isize, _argv: *const *const u8) -> isize {
     unsafe {
         let ptr = miri_alloc(123, 1);
         *ptr = 42; // Crucially, only a write is printed here, no read!

src/tools/miri/tests/pass/alloc-access-tracking.stderr

@@ -5,7 +5,7 @@ LL |         let ptr = miri_alloc(123, 1);
    |                   ^^^^^^^^^^^^^^^^^^ created Miri bare-metal heap allocation of 123 bytes (alignment ALIGN bytes) with id $ALLOC
    |
    = note: BACKTRACE:
-   = note: inside `start` at tests/pass/alloc-access-tracking.rs:LL:CC
+   = note: inside `miri_start` at tests/pass/alloc-access-tracking.rs:LL:CC
 
 note: tracking was triggered
   --> tests/pass/alloc-access-tracking.rs:LL:CC
@@ -14,7 +14,7 @@ LL |         *ptr = 42; // Crucially, only a write is printed here, no read!
    |         ^^^^^^^^^ write access to allocation with id $ALLOC
    |
    = note: BACKTRACE:
-   = note: inside `start` at tests/pass/alloc-access-tracking.rs:LL:CC
+   = note: inside `miri_start` at tests/pass/alloc-access-tracking.rs:LL:CC
 
 note: tracking was triggered
   --> tests/pass/alloc-access-tracking.rs:LL:CC
@@ -23,7 +23,7 @@ LL |         assert_eq!(*ptr, 42);
    |         ^^^^^^^^^^^^^^^^^^^^ read access to allocation with id $ALLOC
    |
    = note: BACKTRACE:
-   = note: inside `start` at RUSTLIB/core/src/macros/mod.rs:LL:CC
+   = note: inside `miri_start` at RUSTLIB/core/src/macros/mod.rs:LL:CC
    = note: this note originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: tracking was triggered
@@ -33,5 +33,5 @@ LL |         miri_dealloc(ptr, 123, 1);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^ freed allocation with id $ALLOC
    |
    = note: BACKTRACE:
-   = note: inside `start` at tests/pass/alloc-access-tracking.rs:LL:CC
+   = note: inside `miri_start` at tests/pass/alloc-access-tracking.rs:LL:CC
 

src/tools/miri/tests/pass/miri-alloc.rs

@@ -1,5 +1,5 @@
-#![feature(start)]
 #![no_std]
+#![no_main]
 //@compile-flags: -Cpanic=abort
 // windows tls dtors go through libstd right now, thus this test
 // cannot pass. When windows tls dtors go through the special magic
@@ -11,8 +11,8 @@ extern "Rust" {
     fn miri_dealloc(ptr: *mut u8, size: usize, align: usize);
 }
 
-#[start]
+#[no_mangle]
-fn start(_: isize, _: *const *const u8) -> isize {
+fn miri_start(_argc: isize, _argv: *const *const u8) -> isize {
     unsafe {
         let ptr = miri_alloc(123, 1);
         core::ptr::write_bytes(ptr, 0u8, 123);

src/tools/miri/tests/pass/miri_start.stdout

@@ -1 +0,0 @@
-Hello from miri_start!

src/tools/miri/tests/pass/no_std.rs

@@ -1,19 +0,0 @@
-//@compile-flags: -Cpanic=abort
-#![feature(start)]
-#![no_std]
-
-use core::fmt::Write;
-
-#[path = "../utils/mod.no_std.rs"]
-mod utils;
-
-#[start]
-fn start(_: isize, _: *const *const u8) -> isize {
-    writeln!(utils::MiriStdout, "hello, world!").unwrap();
-    0
-}
-
-#[panic_handler]
-fn panic_handler(_: &core::panic::PanicInfo) -> ! {
-    loop {}
-}

src/tools/miri/tests/pass/no_std_miri_start.rs

@@ -1,6 +1,6 @@
 //@compile-flags: -Cpanic=abort
-#![no_main]
 #![no_std]
+#![no_main]
 
 use core::fmt::Write;
 
@@ -9,7 +9,7 @@ mod utils;
 
 #[no_mangle]
 fn miri_start(_argc: isize, _argv: *const *const u8) -> isize {
-    writeln!(utils::MiriStdout, "Hello from miri_start!").unwrap();
+    writeln!(utils::MiriStdout, "hello, world!").unwrap();
     0
 }
 

src/tools/miri/tests/pass/start.rs

@@ -1,8 +0,0 @@
-#![feature(start)]
-
-#[start]
-fn start(_: isize, _: *const *const u8) -> isize {
-    println!("Hello from start!");
-
-    0
-}

src/tools/miri/tests/pass/start.stdout

@@ -1 +0,0 @@
-Hello from start!

src/tools/tidy/src/issues.txt

@@ -183,7 +183,6 @@ ui/async-await/issue-67252-unnamed-future.rs
 ui/async-await/issue-67651.rs
 ui/async-await/issue-67765-async-diagnostic.rs
 ui/async-await/issue-68112.rs
-ui/async-await/issue-68523-start.rs
 ui/async-await/issue-68523.rs
 ui/async-await/issue-69446-fnmut-capture.rs
 ui/async-await/issue-70594.rs
@@ -2395,7 +2394,6 @@ ui/issues/issue-50618.rs
 ui/issues/issue-5062.rs
 ui/issues/issue-5067.rs
 ui/issues/issue-50688.rs
-ui/issues/issue-50714-1.rs
 ui/issues/issue-50714.rs
 ui/issues/issue-50761.rs
 ui/issues/issue-50781.rs
@@ -2630,7 +2628,6 @@ ui/issues/issue-9259.rs
 ui/issues/issue-92741.rs
 ui/issues/issue-9382.rs
 ui/issues/issue-9446.rs
-ui/issues/issue-9575.rs
 ui/issues/issue-9719.rs
 ui/issues/issue-9725.rs
 ui/issues/issue-9737.rs
@@ -2645,7 +2642,6 @@ ui/issues/issue-9968.rs
 ui/issues/issue-99838.rs
 ui/iterators/issue-28098.rs
 ui/iterators/issue-58952-filter-type-length.rs
-ui/lang-items/issue-19660.rs
 ui/lang-items/issue-83471.rs
 ui/lang-items/issue-87573.rs
 ui/late-bound-lifetimes/issue-36381.rs
@@ -3702,7 +3698,6 @@ ui/rfcs/rfc-2005-default-binding-mode/issue-44912-or.rs
 ui/rfcs/rfc-2008-non-exhaustive/uninhabited/issue-65157-repeated-match-arm.rs
 ui/rfcs/rfc-2093-infer-outlives/issue-54467.rs
 ui/rfcs/rfc-2396-target_feature-11/issue-108645-target-feature-on-main.rs
-ui/rfcs/rfc-2396-target_feature-11/issue-108645-target-feature-on-start.rs
 ui/rfcs/rfc-2396-target_feature-11/issue-108655-inline-always-closure.rs
 ui/rfcs/rfc-2396-target_feature-11/issue-99876.rs
 ui/rfcs/rfc-2497-if-let-chains/issue-88498.rs

src/tools/tidy/src/ui_tests.rs

@@ -17,7 +17,7 @@ use ignore::Walk;
 const ENTRY_LIMIT: u32 = 901;
 // FIXME: The following limits should be reduced eventually.
 
-const ISSUES_ENTRY_LIMIT: u32 = 1667;
+const ISSUES_ENTRY_LIMIT: u32 = 1663;
 
 const EXPECTED_TEST_FILE_EXTENSIONS: &[&str] = &[
     "rs",     // test source files

tests/codegen-units/item-collection/cross-crate-closures.rs

@@ -3,14 +3,14 @@
 //@ compile-flags:-Zprint-mono-items=eager -Zinline-mir=no
 
 #![deny(dead_code)]
-#![feature(start)]
+#![no_main]
 
 //@ aux-build:cgu_extern_closures.rs
 extern crate cgu_extern_closures;
 
-//~ MONO_ITEM fn start @@ cross_crate_closures-cgu.0[Internal]
+//~ MONO_ITEM fn main @@ cross_crate_closures-cgu.0[External]
-#[start]
+#[no_mangle]
-fn start(_: isize, _: *const *const u8) -> isize {
+extern "C" fn main(_: core::ffi::c_int, _: *const *const u8) -> core::ffi::c_int {
     //~ MONO_ITEM fn cgu_extern_closures::inlined_fn @@ cross_crate_closures-cgu.0[Internal]
     //~ MONO_ITEM fn cgu_extern_closures::inlined_fn::{closure#0} @@ cross_crate_closures-cgu.0[Internal]
     let _ = cgu_extern_closures::inlined_fn(1, 2);