Auto merge of #130052 - khuey:clear-dilocation-after-const-emission, r=michaelwoerister

Don't leave debug locations for constants sitting on the builder indefinitely

Because constants are currently emitted *before* the prologue, leaving the debug location on the IRBuilder spills onto other instructions in the prologue and messes up both line numbers as well as the point LLVM chooses to be the prologue end.

Example LLVM IR (irrelevant IR elided):
Before:
```
define internal { i64, i64 } `@_ZN3tmp3Foo18var_return_opt_try17he02116165b0fc08cE(ptr` align 8 %self) !dbg !347 { start:
  %self.dbg.spill = alloca [8 x i8], align 8
  %_0 = alloca [16 x i8], align 8
  %residual.dbg.spill = alloca [0 x i8], align 1
    #dbg_declare(ptr %residual.dbg.spill, !353, !DIExpression(), !357)
  store ptr %self, ptr %self.dbg.spill, align 8, !dbg !357
    #dbg_declare(ptr %self.dbg.spill, !350, !DIExpression(), !358)
```
After:
```
define internal { i64, i64 } `@_ZN3tmp3Foo18var_return_opt_try17h00b17d08874ddd90E(ptr` align 8 %self) !dbg !347 { start:
  %self.dbg.spill = alloca [8 x i8], align 8
  %_0 = alloca [16 x i8], align 8
  %residual.dbg.spill = alloca [0 x i8], align 1
    #dbg_declare(ptr %residual.dbg.spill, !353, !DIExpression(), !357)
  store ptr %self, ptr %self.dbg.spill, align 8
    #dbg_declare(ptr %self.dbg.spill, !350, !DIExpression(), !358)
```
Note in particular how !357 from %residual.dbg.spill's dbg_declare no longer falls through onto the store to %self.dbg.spill. This fixes argument values at entry when the constant is a ZST (e.g. `<Option as Try>::Residual`). This fixes #130003 (but note that it does *not* fix issues with argument values and non-ZST constants, which emit their own stores that have debug info on them, like #128945).

r? `@michaelwoerister`

compiler/rustc_codegen_gcc/src/debuginfo.rs

@@ -48,6 +48,10 @@ impl<'a, 'gcc, 'tcx> DebugInfoBuilderMethods for Builder<'a, 'gcc, 'tcx> {
     fn set_dbg_loc(&mut self, dbg_loc: Self::DILocation) {
         self.location = Some(dbg_loc);
     }
+
+    fn clear_dbg_loc(&mut self) {
+        self.location = None;
+    }
 }
 
 /// Generate the `debug_context` in an MIR Body.

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -1,8 +1,8 @@
 #![doc = include_str!("doc.md")]
 
 use std::cell::{OnceCell, RefCell};
-use std::iter;
 use std::ops::Range;
+use std::{iter, ptr};
 
 use libc::c_uint;
 use rustc_codegen_ssa::debuginfo::type_names;
@@ -209,6 +209,12 @@ impl<'ll> DebugInfoBuilderMethods for Builder<'_, 'll, '_> {
         }
     }
 
+    fn clear_dbg_loc(&mut self) {
+        unsafe {
+            llvm::LLVMSetCurrentDebugLocation2(self.llbuilder, ptr::null());
+        }
+    }
+
     fn insert_reference_to_gdb_debug_scripts_section_global(&mut self) {
         gdb::insert_reference_to_gdb_debug_scripts_section_global(self)
     }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1041,7 +1041,7 @@ unsafe extern "C" {
     pub fn LLVMDisposeBuilder<'a>(Builder: &'a mut Builder<'a>);
 
     // Metadata
-    pub fn LLVMSetCurrentDebugLocation2<'a>(Builder: &Builder<'a>, Loc: &'a Metadata);
+    pub fn LLVMSetCurrentDebugLocation2<'a>(Builder: &Builder<'a>, Loc: *const Metadata);
 
     // Terminators
     pub fn LLVMBuildRetVoid<'a>(B: &Builder<'a>) -> &'a Value;

compiler/rustc_codegen_ssa/src/mir/debuginfo.rs

@@ -547,6 +547,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         self.set_debug_loc(bx, var.source_info);
                         let base =
                             Self::spill_operand_to_stack(operand, Some(var.name.to_string()), bx);
+                        bx.clear_dbg_loc();
 
                         bx.dbg_var_addr(
                             dbg_var,

compiler/rustc_codegen_ssa/src/traits/debuginfo.rs

@@ -80,6 +80,7 @@ pub trait DebugInfoBuilderMethods: BackendTypes {
         fragment: Option<Range<Size>>,
     );
     fn set_dbg_loc(&mut self, dbg_loc: Self::DILocation);
+    fn clear_dbg_loc(&mut self);
     fn insert_reference_to_gdb_debug_scripts_section_global(&mut self);
     fn set_var_name(&mut self, value: Self::Value, name: &str);
 }

tests/debuginfo/zst-interferes-with-prologue.rs

@@ -0,0 +1,72 @@
+//@ min-lldb-version: 310
+
+//@ compile-flags:-g
+
+// === GDB TESTS ===================================================================================
+
+// gdb-command:break zst_interferes_with_prologue::Foo::var_return_opt_try
+// gdb-command:run
+
+// gdb-command:print self
+// gdb-command:next
+// gdb-command:print self
+// gdb-command:print $1 == $2
+// gdb-check:true
+
+// === LLDB TESTS ==================================================================================
+
+// lldb-command:b "zst_interferes_with_prologue::Foo::var_return_opt_try"
+// lldb-command:run
+
+// lldb-command:expr self
+// lldb-command:next
+// lldb-command:expr self
+// lldb-command:print $0 == $1
+// lldb-check:true
+
+struct Foo {
+    a: usize,
+}
+
+impl Foo {
+    #[inline(never)]
+    fn get_a(&self) -> Option<usize> {
+        Some(self.a)
+    }
+
+    #[inline(never)]
+    fn var_return(&self) -> usize {
+        let r = self.get_a().unwrap();
+        r
+    }
+
+    #[inline(never)]
+    fn var_return_opt_unwrap(&self) -> Option<usize> {
+        let r = self.get_a().unwrap();
+        Some(r)
+    }
+
+    #[inline(never)]
+    fn var_return_opt_match(&self) -> Option<usize> {
+        let r = match self.get_a() {
+            None => return None,
+            Some(a) => a,
+        };
+        Some(r)
+    }
+
+    #[inline(never)]
+    fn var_return_opt_try(&self) -> Option<usize> {
+        let r = self.get_a()?;
+        Some(r)
+    }
+}
+
+fn main() {
+    let f1 = Foo{ a: 1 };
+    let f2 = Foo{ a: 1 };
+    f1.var_return();
+    f1.var_return_opt_unwrap();
+    f1.var_return_opt_match();
+    f2.var_return_opt_try();
+}