Rollup merge of rust-lang#107373 - michaelwoerister:dont-merge-vtables-when-debuginfo, r=WaffleLapkin

Don't merge vtables when full debuginfo is enabled.

This PR makes the compiler not emit the `unnamed_addr` attribute for vtables when full debuginfo is enabled, so that they don't get merged even if they have the same contents. This allows debuggers to more reliably map from a dyn pointer to the self-type of a trait object by looking at the vtable's debuginfo.

The PR only changes the behavior of the LLVM backend as other backends don't emit vtable debuginfo (as far as I can tell).

The performance impact of this change should be small as [measured](rust-lang#103514 (comment)) in a previous PR.

Author: matthiaskrgr

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -1499,6 +1499,11 @@ pub fn create_vtable_di_node<'ll, 'tcx>(
         return;
     }
 
+    // When full debuginfo is enabled, we want to try and prevent vtables from being
+    // merged. Otherwise debuggers will have a hard time mapping from dyn pointer
+    // to concrete type.
+    llvm::SetUnnamedAddress(vtable, llvm::UnnamedAddr::No);
+
     let vtable_name =
         compute_debuginfo_vtable_name(cx.tcx, ty, poly_trait_ref, VTableNameKind::GlobalVariable);
     let vtable_type_di_node = build_vtable_type_di_node(cx, ty, poly_trait_ref);

tests/codegen/debug-vtable.rs

@@ -9,6 +9,14 @@
 // compile-flags: -Cdebuginfo=2 -Copt-level=0 -Csymbol-mangling-version=v0
 // ignore-tidy-linelength
 
+// Make sure that vtables don't have the unnamed_addr attribute when debuginfo is enabled.
+// This helps debuggers more reliably map from dyn pointer to concrete type.
+// CHECK: @vtable.0 = private constant <{
+// CHECK: @vtable.1 = private constant <{
+// CHECK: @vtable.2 = private constant <{
+// CHECK: @vtable.3 = private constant <{
+// CHECK: @vtable.4 = private constant <{
+
 // NONMSVC: ![[USIZE:[0-9]+]] = !DIBasicType(name: "usize"
 // MSVC: ![[USIZE:[0-9]+]] = !DIDerivedType(tag: DW_TAG_typedef, name: "usize"
 // NONMSVC: ![[PTR:[0-9]+]] = !DIDerivedType(tag: DW_TAG_pointer_type, name: "*const ()"