improper_ctypes: extern "C" fns

src/libpanic_abort/lib.rs

@@ -21,6 +21,7 @@
 // Rust's "try" function, but if we're aborting on panics we just call the
 // function as there's nothing else we need to do here.
 #[rustc_std_internal_symbol]
+#[allow(improper_ctypes)]
 pub unsafe extern fn __rust_maybe_catch_panic(f: fn(*mut u8),
                                               data: *mut u8,
                                               _data_ptr: *mut usize,

src/libpanic_unwind/lib.rs

@@ -69,6 +69,7 @@ mod dwarf;
 // hairy and tightly coupled, for more information see the compiler's
 // implementation of this.
 #[no_mangle]
+#[allow(improper_ctypes)]
 pub unsafe extern "C" fn __rust_maybe_catch_panic(f: fn(*mut u8),
                                                   data: *mut u8,
                                                   data_ptr: *mut usize,

src/librustc_codegen_llvm/back/write.rs

@@ -239,6 +239,7 @@ impl<'a> Drop for DiagnosticHandlers<'a> {
     }
 }
 
+#[allow(improper_ctypes)]
 unsafe extern "C" fn report_inline_asm(cgcx: &CodegenContext<LlvmCodegenBackend>,
                                        msg: &str,
                                        cookie: c_uint) {

src/librustc_codegen_llvm/llvm/mod.rs

@@ -87,6 +87,7 @@ pub struct RustString {
 }
 
 /// Appending to a Rust string -- used by RawRustStringOstream.
+#[allow(improper_ctypes)]
 #[no_mangle]
 pub unsafe extern "C" fn LLVMRustStringWriteImpl(sr: &RustString,
                                                  ptr: *const c_char,

src/librustc_lint/types.rs

@@ -4,7 +4,7 @@ use rustc::hir::{ExprKind, Node};
 use crate::hir::def_id::DefId;
 use rustc::hir::lowering::is_range_literal;
 use rustc::ty::subst::SubstsRef;
-use rustc::ty::{self, AdtKind, ParamEnv, Ty, TyCtxt};
+use rustc::ty::{self, AdtKind, ParamEnv, Ty, TyCtxt, TypeFoldable};
 use rustc::ty::layout::{self, IntegerExt, LayoutOf, VariantIdx, SizeSkeleton};
 use rustc::{lint, util};
 use rustc_index::vec::Idx;
@@ -835,16 +835,13 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             ty::Array(ty, _) => self.check_type_for_ffi(cache, ty),
 
             ty::FnPtr(sig) => {
-                match sig.abi() {
-                    Abi::Rust | Abi::RustIntrinsic | Abi::PlatformIntrinsic | Abi::RustCall => {
-                        return FfiUnsafe {
-                            ty,
-                            reason: "this function pointer has Rust-specific calling convention",
-                            help: Some("consider using an `extern fn(...) -> ...` \
-                                        function pointer instead"),
-                        }
-                    }
-                    _ => {}
+                if self.is_internal_abi(sig.abi()) {
+                    return FfiUnsafe {
+                        ty,
+                        reason: "this function pointer has Rust-specific calling convention",
+                        help: Some("consider using an `extern fn(...) -> ...` \
+                                    function pointer instead"),
+                    };
                 }
 
                 let sig = cx.erase_late_bound_regions(&sig);
@@ -871,7 +868,10 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
             ty::Foreign(..) => FfiSafe,
 
-            ty::Param(..) |
+            // `extern "C" fn` functions can have type parameters, which may or may not be FFI-safe,
+            //  so they are currently ignored for the purposes of this lint, see #65134.
+            ty::Param(..) | ty::Projection(..) => FfiSafe,
+
             ty::Infer(..) |
             ty::Bound(..) |
             ty::Error |
@@ -880,7 +880,6 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             ty::GeneratorWitness(..) |
             ty::Placeholder(..) |
             ty::UnnormalizedProjection(..) |
-            ty::Projection(..) |
             ty::Opaque(..) |
             ty::FnDef(..) => bug!("unexpected type in foreign function: {:?}", ty),
         }
@@ -912,8 +911,6 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
     }
 
     fn check_for_opaque_ty(&mut self, sp: Span, ty: Ty<'tcx>) -> bool {
-        use crate::rustc::ty::TypeFoldable;
-
         struct ProhibitOpaqueTypes<'tcx> {
             ty: Option<Ty<'tcx>>,
         };
@@ -952,10 +949,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             return;
         }
 
-        // it is only OK to use this function because extern fns cannot have
-        // any generic types right now:
-        let ty = self.cx.tcx.normalize_erasing_regions(ParamEnv::reveal_all(), ty);
-
+        let ty = self.cx.tcx.normalize_erasing_regions(self.cx.param_env, ty);
         match self.check_type_for_ffi(&mut FxHashSet::default(), ty) {
             FfiResult::FfiSafe => {}
             FfiResult::FfiPhantom(ty) => {
@@ -989,15 +983,21 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
         let ty = self.cx.tcx.type_of(def_id);
         self.check_type_for_ffi_and_report_errors(span, ty);
     }
+
+    fn is_internal_abi(&self, abi: Abi) -> bool {
+        if let Abi::Rust | Abi::RustCall | Abi::RustIntrinsic | Abi::PlatformIntrinsic = abi {
+            true
+        } else {
+            false
+        }
+    }
 }
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ImproperCTypes {
     fn check_foreign_item(&mut self, cx: &LateContext<'_, '_>, it: &hir::ForeignItem) {
         let mut vis = ImproperCTypesVisitor { cx };
         let abi = cx.tcx.hir().get_foreign_abi(it.hir_id);
-        if let Abi::Rust | Abi::RustCall | Abi::RustIntrinsic | Abi::PlatformIntrinsic = abi {
-            // Don't worry about types in internal ABIs.
-        } else {
+        if !vis.is_internal_abi(abi) {
             match it.kind {
                 hir::ForeignItemKind::Fn(ref decl, _, _) => {
                     vis.check_foreign_fn(it.hir_id, decl);
@@ -1009,6 +1009,29 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for ImproperCTypes {
             }
         }
     }
+
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'a, 'tcx>,
+        kind: hir::intravisit::FnKind<'tcx>,
+        decl: &'tcx hir::FnDecl,
+        _: &'tcx hir::Body,
+        _: Span,
+        hir_id: hir::HirId,
+    ) {
+        use hir::intravisit::FnKind;
+
+        let abi = match kind {
+            FnKind::ItemFn(_, _, header, ..) => (header.abi),
+            FnKind::Method(_, sig, ..) => (sig.header.abi),
+            _ => return,
+        };
+
+        let mut vis = ImproperCTypesVisitor { cx };
+        if !vis.is_internal_abi(abi) {
+            vis.check_foreign_fn(hir_id, decl);
+        }
+    }
 }
 
 declare_lint_pass!(VariantSizeDifferences => [VARIANT_SIZE_DIFFERENCES]);

src/test/ui/abi/abi-sysv64-register-usage.rs

@@ -33,6 +33,7 @@ pub extern "sysv64" fn all_the_registers(rdi: i64, rsi: i64, rdx: i64,
 
 // this struct contains 8 i64's, while only 6 can be passed in registers.
 #[cfg(target_arch = "x86_64")]
+#[repr(C)]
 #[derive(PartialEq, Eq, Debug)]
 pub struct LargeStruct(i64, i64, i64, i64, i64, i64, i64, i64);
 

src/test/ui/align-with-extern-c-fn.rs

@@ -10,6 +10,7 @@
 #[repr(align(16))]
 pub struct A(i64);
 
+#[allow(improper_ctypes)]
 pub extern "C" fn foo(x: A) {}
 
 fn main() {

src/test/ui/issues/issue-16441.rs

@@ -5,6 +5,7 @@
 struct Empty;
 
 // This used to cause an ICE
+#[allow(improper_ctypes)]
 extern "C" fn ice(_a: Empty) {}
 
 fn main() {

src/test/ui/issues/issue-26997.rs

@@ -6,6 +6,7 @@ pub struct Foo {
 }
 
 impl Foo {
+    #[allow(improper_ctypes)]
     pub extern fn foo_new() -> Foo {
         Foo { x: 21, y: 33 }
     }

src/test/ui/issues/issue-28600.rs

@@ -4,6 +4,7 @@
 struct Test;
 
 impl Test {
+    #[allow(improper_ctypes)]
     #[allow(dead_code)]
     #[allow(unused_variables)]
     pub extern fn test(val: &str) {

src/test/ui/issues/issue-38763.rs

@@ -4,6 +4,7 @@
 #[repr(C)]
 pub struct Foo(i128);
 
+#[allow(improper_ctypes)]
 #[no_mangle]
 pub extern "C" fn foo(x: Foo) -> Foo { x }
 

src/test/ui/issues/issue-51907.rs

@@ -6,7 +6,9 @@ trait Foo {
 
 struct Bar;
 impl Foo for Bar {
+    #[allow(improper_ctypes)]
     extern fn borrow(&self) {}
+    #[allow(improper_ctypes)]
     extern fn take(self: Box<Self>) {}
 }
 

src/test/ui/lint/lint-ctypes-fn.rs

@@ -0,0 +1,186 @@
+#![feature(rustc_private)]
+
+#![allow(private_in_public)]
+#![deny(improper_ctypes)]
+
+extern crate libc;
+
+use std::default::Default;
+use std::marker::PhantomData;
+
+trait Mirror { type It: ?Sized; }
+
+impl<T: ?Sized> Mirror for T { type It = Self; }
+
+#[repr(C)]
+pub struct StructWithProjection(*mut <StructWithProjection as Mirror>::It);
+
+#[repr(C)]
+pub struct StructWithProjectionAndLifetime<'a>(
+    &'a mut <StructWithProjectionAndLifetime<'a> as Mirror>::It
+);
+
+pub type I32Pair = (i32, i32);
+
+#[repr(C)]
+pub struct ZeroSize;
+
+pub type RustFn = fn();
+
+pub type RustBadRet = extern fn() -> Box<u32>;
+
+pub type CVoidRet = ();
+
+pub struct Foo;
+
+#[repr(transparent)]
+pub struct TransparentI128(i128);
+
+#[repr(transparent)]
+pub struct TransparentStr(&'static str);
+
+#[repr(transparent)]
+pub struct TransparentBadFn(RustBadRet);
+
+#[repr(transparent)]
+pub struct TransparentInt(u32);
+
+#[repr(transparent)]
+pub struct TransparentRef<'a>(&'a TransparentInt);
+
+#[repr(transparent)]
+pub struct TransparentLifetime<'a>(*const u8, PhantomData<&'a ()>);
+
+#[repr(transparent)]
+pub struct TransparentUnit<U>(f32, PhantomData<U>);
+
+#[repr(transparent)]
+pub struct TransparentCustomZst(i32, ZeroSize);
+
+#[repr(C)]
+pub struct ZeroSizeWithPhantomData(PhantomData<i32>);
+
+pub extern "C" fn ptr_type1(size: *const Foo) { }
+//~^ ERROR: uses type `Foo`
+
+pub extern "C" fn ptr_type2(size: *const Foo) { }
+//~^ ERROR: uses type `Foo`
+
+pub extern "C" fn slice_type(p: &[u32]) { }
+//~^ ERROR: uses type `[u32]`
+
+pub extern "C" fn str_type(p: &str) { }
+//~^ ERROR: uses type `str`
+
+pub extern "C" fn box_type(p: Box<u32>) { }
+//~^ ERROR uses type `std::boxed::Box<u32>`
+
+pub extern "C" fn char_type(p: char) { }
+//~^ ERROR uses type `char`
+
+pub extern "C" fn i128_type(p: i128) { }
+//~^ ERROR uses type `i128`
+
+pub extern "C" fn u128_type(p: u128) { }
+//~^ ERROR uses type `u128`
+
+pub extern "C" fn tuple_type(p: (i32, i32)) { }
+//~^ ERROR uses type `(i32, i32)`
+
+pub extern "C" fn tuple_type2(p: I32Pair) { }
+//~^ ERROR uses type `(i32, i32)`
+
+pub extern "C" fn zero_size(p: ZeroSize) { }
+//~^ ERROR uses type `ZeroSize`
+
+pub extern "C" fn zero_size_phantom(p: ZeroSizeWithPhantomData) { }
+//~^ ERROR uses type `ZeroSizeWithPhantomData`
+
+pub extern "C" fn zero_size_phantom_toplevel() -> PhantomData<bool> {
+//~^ ERROR uses type `std::marker::PhantomData<bool>`
+    Default::default()
+}
+
+pub extern "C" fn fn_type(p: RustFn) { }
+//~^ ERROR uses type `fn()`
+
+pub extern "C" fn fn_type2(p: fn()) { }
+//~^ ERROR uses type `fn()`
+
+pub extern "C" fn fn_contained(p: RustBadRet) { }
+//~^ ERROR: uses type `std::boxed::Box<u32>`
+
+pub extern "C" fn transparent_i128(p: TransparentI128) { }
+//~^ ERROR: uses type `i128`
+
+pub extern "C" fn transparent_str(p: TransparentStr) { }
+//~^ ERROR: uses type `str`
+
+pub extern "C" fn transparent_fn(p: TransparentBadFn) { }
+//~^ ERROR: uses type `std::boxed::Box<u32>`
+
+pub extern "C" fn good3(fptr: Option<extern fn()>) { }
+
+pub extern "C" fn good4(aptr: &[u8; 4 as usize]) { }
+
+pub extern "C" fn good5(s: StructWithProjection) { }
+
+pub extern "C" fn good6(s: StructWithProjectionAndLifetime) { }
+
+pub extern "C" fn good7(fptr: extern fn() -> ()) { }
+
+pub extern "C" fn good8(fptr: extern fn() -> !) { }
+
+pub extern "C" fn good9() -> () { }
+
+pub extern "C" fn good10() -> CVoidRet { }
+
+pub extern "C" fn good11(size: isize) { }
+
+pub extern "C" fn good12(size: usize) { }
+
+pub extern "C" fn good13(n: TransparentInt) { }
+
+pub extern "C" fn good14(p: TransparentRef) { }
+
+pub extern "C" fn good15(p: TransparentLifetime) { }
+
+pub extern "C" fn good16(p: TransparentUnit<ZeroSize>) { }
+
+pub extern "C" fn good17(p: TransparentCustomZst) { }
+
+#[allow(improper_ctypes)]
+pub extern "C" fn good18(_: &String) { }
+
+#[cfg(not(target_arch = "wasm32"))]
+pub extern "C" fn good1(size: *const libc::c_int) { }
+
+#[cfg(not(target_arch = "wasm32"))]
+pub extern "C" fn good2(size: *const libc::c_uint) { }
+
+pub extern "C" fn unused_generic1<T>(size: *const Foo) { }
+//~^ ERROR: uses type `Foo`
+
+pub extern "C" fn unused_generic2<T>() -> PhantomData<bool> {
+//~^ ERROR uses type `std::marker::PhantomData<bool>`
+    Default::default()
+}
+
+pub extern "C" fn used_generic1<T>(x: T) { }
+
+pub extern "C" fn used_generic2<T>(x: T, size: *const Foo) { }
+//~^ ERROR: uses type `Foo`
+
+pub extern "C" fn used_generic3<T: Default>() -> T {
+    Default::default()
+}
+
+pub extern "C" fn used_generic4<T>(x: Vec<T>) { }
+//~^ ERROR: uses type `std::vec::Vec<T>`
+
+pub extern "C" fn used_generic5<T>() -> Vec<T> {
+//~^ ERROR: uses type `std::vec::Vec<T>`
+    Default::default()
+}
+
+fn main() {}

src/test/ui/lint/lint-ctypes-fn.stderr

@@ -0,0 +1,247 @@
+error: `extern` block uses type `Foo`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:63:35
+   |
+LL | pub extern "C" fn ptr_type1(size: *const Foo) { }
+   |                                   ^^^^^^^^^^ not FFI-safe
+   |
+note: lint level defined here
+  --> $DIR/lint-ctypes-fn.rs:4:9
+   |
+LL | #![deny(improper_ctypes)]
+   |         ^^^^^^^^^^^^^^^
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+note: type defined here
+  --> $DIR/lint-ctypes-fn.rs:34:1
+   |
+LL | pub struct Foo;
+   | ^^^^^^^^^^^^^^^
+
+error: `extern` block uses type `Foo`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:66:35
+   |
+LL | pub extern "C" fn ptr_type2(size: *const Foo) { }
+   |                                   ^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+note: type defined here
+  --> $DIR/lint-ctypes-fn.rs:34:1
+   |
+LL | pub struct Foo;
+   | ^^^^^^^^^^^^^^^
+
+error: `extern` block uses type `[u32]`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:69:33
+   |
+LL | pub extern "C" fn slice_type(p: &[u32]) { }
+   |                                 ^^^^^^ not FFI-safe
+   |
+   = help: consider using a raw pointer instead
+   = note: slices have no C equivalent
+
+error: `extern` block uses type `str`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:72:31
+   |
+LL | pub extern "C" fn str_type(p: &str) { }
+   |                               ^^^^ not FFI-safe
+   |
+   = help: consider using `*const u8` and a length instead
+   = note: string slices have no C equivalent
+
+error: `extern` block uses type `std::boxed::Box<u32>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:75:31
+   |
+LL | pub extern "C" fn box_type(p: Box<u32>) { }
+   |                               ^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+
+error: `extern` block uses type `char`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:78:32
+   |
+LL | pub extern "C" fn char_type(p: char) { }
+   |                                ^^^^ not FFI-safe
+   |
+   = help: consider using `u32` or `libc::wchar_t` instead
+   = note: the `char` type has no C equivalent
+
+error: `extern` block uses type `i128`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:81:32
+   |
+LL | pub extern "C" fn i128_type(p: i128) { }
+   |                                ^^^^ not FFI-safe
+   |
+   = note: 128-bit integers don't currently have a known stable ABI
+
+error: `extern` block uses type `u128`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:84:32
+   |
+LL | pub extern "C" fn u128_type(p: u128) { }
+   |                                ^^^^ not FFI-safe
+   |
+   = note: 128-bit integers don't currently have a known stable ABI
+
+error: `extern` block uses type `(i32, i32)`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:87:33
+   |
+LL | pub extern "C" fn tuple_type(p: (i32, i32)) { }
+   |                                 ^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider using a struct instead
+   = note: tuples have unspecified layout
+
+error: `extern` block uses type `(i32, i32)`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:90:34
+   |
+LL | pub extern "C" fn tuple_type2(p: I32Pair) { }
+   |                                  ^^^^^^^ not FFI-safe
+   |
+   = help: consider using a struct instead
+   = note: tuples have unspecified layout
+
+error: `extern` block uses type `ZeroSize`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:93:32
+   |
+LL | pub extern "C" fn zero_size(p: ZeroSize) { }
+   |                                ^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a member to this struct
+   = note: this struct has no fields
+note: type defined here
+  --> $DIR/lint-ctypes-fn.rs:26:1
+   |
+LL | pub struct ZeroSize;
+   | ^^^^^^^^^^^^^^^^^^^^
+
+error: `extern` block uses type `ZeroSizeWithPhantomData`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:96:40
+   |
+LL | pub extern "C" fn zero_size_phantom(p: ZeroSizeWithPhantomData) { }
+   |                                        ^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = note: composed only of `PhantomData`
+note: type defined here
+  --> $DIR/lint-ctypes-fn.rs:61:1
+   |
+LL | pub struct ZeroSizeWithPhantomData(PhantomData<i32>);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `extern` block uses type `std::marker::PhantomData<bool>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:99:51
+   |
+LL | pub extern "C" fn zero_size_phantom_toplevel() -> PhantomData<bool> {
+   |                                                   ^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = note: composed only of `PhantomData`
+
+error: `extern` block uses type `fn()`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:104:30
+   |
+LL | pub extern "C" fn fn_type(p: RustFn) { }
+   |                              ^^^^^^ not FFI-safe
+   |
+   = help: consider using an `extern fn(...) -> ...` function pointer instead
+   = note: this function pointer has Rust-specific calling convention
+
+error: `extern` block uses type `fn()`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:107:31
+   |
+LL | pub extern "C" fn fn_type2(p: fn()) { }
+   |                               ^^^^ not FFI-safe
+   |
+   = help: consider using an `extern fn(...) -> ...` function pointer instead
+   = note: this function pointer has Rust-specific calling convention
+
+error: `extern` block uses type `std::boxed::Box<u32>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:110:35
+   |
+LL | pub extern "C" fn fn_contained(p: RustBadRet) { }
+   |                                   ^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+
+error: `extern` block uses type `i128`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:113:39
+   |
+LL | pub extern "C" fn transparent_i128(p: TransparentI128) { }
+   |                                       ^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = note: 128-bit integers don't currently have a known stable ABI
+
+error: `extern` block uses type `str`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:116:38
+   |
+LL | pub extern "C" fn transparent_str(p: TransparentStr) { }
+   |                                      ^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider using `*const u8` and a length instead
+   = note: string slices have no C equivalent
+
+error: `extern` block uses type `std::boxed::Box<u32>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:119:37
+   |
+LL | pub extern "C" fn transparent_fn(p: TransparentBadFn) { }
+   |                                     ^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+
+error: `extern` block uses type `Foo`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:161:44
+   |
+LL | pub extern "C" fn unused_generic1<T>(size: *const Foo) { }
+   |                                            ^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+note: type defined here
+  --> $DIR/lint-ctypes-fn.rs:34:1
+   |
+LL | pub struct Foo;
+   | ^^^^^^^^^^^^^^^
+
+error: `extern` block uses type `std::marker::PhantomData<bool>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:164:43
+   |
+LL | pub extern "C" fn unused_generic2<T>() -> PhantomData<bool> {
+   |                                           ^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = note: composed only of `PhantomData`
+
+error: `extern` block uses type `Foo`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:171:48
+   |
+LL | pub extern "C" fn used_generic2<T>(x: T, size: *const Foo) { }
+   |                                                ^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+note: type defined here
+  --> $DIR/lint-ctypes-fn.rs:34:1
+   |
+LL | pub struct Foo;
+   | ^^^^^^^^^^^^^^^
+
+error: `extern` block uses type `std::vec::Vec<T>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:178:39
+   |
+LL | pub extern "C" fn used_generic4<T>(x: Vec<T>) { }
+   |                                       ^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+
+error: `extern` block uses type `std::vec::Vec<T>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-fn.rs:181:41
+   |
+LL | pub extern "C" fn used_generic5<T>() -> Vec<T> {
+   |                                         ^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
+   = note: this struct has unspecified layout
+
+error: aborting due to 24 previous errors
+

src/test/ui/mir/mir_cast_fn_ret.rs

@@ -1,8 +1,10 @@
 // run-pass
+#[allow(improper_ctypes)]
 pub extern "C" fn tuple2() -> (u16, u8) {
     (1, 2)
 }
 
+#[allow(improper_ctypes)]
 pub extern "C" fn tuple3() -> (u8, u8, u8) {
     (1, 2, 3)
 }

src/test/ui/mir/mir_codegen_calls.rs

@@ -74,6 +74,7 @@ fn test8() -> isize {
     Two::two()
 }
 
+#[allow(improper_ctypes)]
 extern fn simple_extern(x: u32, y: (u32, u32)) -> u32 {
     x + y.0 * y.1
 }