Auto merge of #142771 - dianqk:mir-stmt-debuginfo, r=cjgillot

Introduce debuginfo to statements in MIR

The PR introduces support for debug information within dead statements. Currently, only the reference statement is supported, which is sufficient to fix rust-lang/rust#128081.

I don't modify Stable MIR, as I don't think we need debug information when using it.

This PR represents the debug information for the dead reference statement via `#dbg_value`. For example, `let _foo_b = &foo.b` becomes `#dbg_value(ptr %foo, !22, !DIExpression(DW_OP_plus_uconst, 4, DW_OP_stack_value), !26)`. You can see this here: https://rust.godbolt.org/z/d43js6adv.

The general principle for handling debug information is to never provide less debug information than the optimized LLVM IR.

The current rules for dropping debug information in this PR are:

- If the LLVM IR cannot represent a reference address, it's replaced with poison or simply dropped. For example, see: https://rust.godbolt.org/z/shGqPec8W. I'm using poison in all such cases now.
- All debuginfos is dropped when merging multiple successor BBs. An example is available here: https://rust.godbolt.org/z/TE1q3Wq6M.

I doesn't drop debuginfos in `MatchBranchSimplification`, because LLVM also pick one branch for it.

1 files changed, 8 insertions, 8 deletions

diff --git a/tests/mir-opt/pre-codegen/clone_as_copy.rs b/tests/mir-opt/pre-codegen/clone_as_copy.rs
index f5ff1854d38..00f24754d59 100644
--- a/tests/mir-opt/pre-codegen/clone_as_copy.rs
+++ b/tests/mir-opt/pre-codegen/clone_as_copy.rs
@@ -25,19 +25,19 @@ enum Enum1 {
 // EMIT_MIR clone_as_copy.clone_as_copy.PreCodegen.after.mir
 fn clone_as_copy(v: &NestCopy) -> NestCopy {
     // CHECK-LABEL: fn clone_as_copy(
-    // CHECK-NOT: = AllCopy { {{.*}} };
-    // CHECK-NOT: = NestCopy { {{.*}} };
-    // CHECK: _0 = copy (*_1);
-    // CHECK: return;
+    // CHECK: let [[DEAD_VAR:_.*]]: &AllCopy;
+    // CHECK: bb0: {
+    // CHECK-NEXT: DBG: [[DEAD_VAR]] = &((*_1).1: AllCopy)
+    // CHECK-NEXT: _0 = copy (*_1);
+    // CHECK-NEXT: return;
     v.clone()
 }
 
-// FIXME: We can merge into exactly one assignment statement.
 // EMIT_MIR clone_as_copy.enum_clone_as_copy.PreCodegen.after.mir
 fn enum_clone_as_copy(v: &Enum1) -> Enum1 {
     // CHECK-LABEL: fn enum_clone_as_copy(
-    // CHECK-NOT: = Enum1::
-    // CHECK: _0 = copy (*_1);
-    // CHECK: _0 = copy (*_1);
+    // CHECK: bb0: {
+    // CHECK-NEXT: _0 = copy (*_1);
+    // CHECK-NEXT: return;
     v.clone()
 }