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).

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();
+}