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).

2 files changed, 8 insertions, 2 deletions

diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs b/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
index f5d6fc6f080..842212ac05d 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
+++ b/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)
     }
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index 3bf4d496408..e84ab0aa538 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/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;