Auto merge of #111440 - cjgillot:refprop-debuginfo, r=oli-obk

Allow MIR debuginfo to point to a variable's address

MIR optimizations currently do not to operate on borrowed locals.

When enabling #106285, many borrows will be left as-is because they are used in debuginfo. This pass allows to replace this pattern directly in MIR debuginfo:
```rust
a => _1
_1 = &raw? mut? _2
```
becomes
```rust
a => &_2
// No statement to borrow _2.
```

This pass is implemented as a drive-by in ReferencePropagation MIR pass.

This transformation allows following following MIR opts to treat _2 as an unborrowed local, and optimize it as such, even in builds with debuginfo.

In codegen, when encountering `a => &..&_2`, we create a list of allocas:
```llvm
store ptr %_2.dbg.spill, ptr %a.ref0.dbg.spill
store ptr %a.ref0.dbg.spill, ptr %a.ref1.dbg.spill
...
call void `@llvm.dbg.declare(metadata` ptr %a.ref{n}.dbg.spill, /* ... */)
```

Caveat: this transformation looses the exact type, we do not differentiate `a` as a immutable, mutable reference or a raw pointer. Everything is declared to `*mut` to codegen. I'm not convinced this is a blocker.

1 files changed, 3 insertions, 12 deletions

diff --git a/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.diff b/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.diff
index abb89b91dd3..73b9ea46c44 100644
--- a/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.diff
+++ b/tests/mir-opt/issue_76432.test.SimplifyComparisonIntegral.diff
@@ -21,9 +21,9 @@
           let _13: &T;                     // in scope 1 at $DIR/issue_76432.rs:+3:18: +3:24
           let _14: &T;                     // in scope 1 at $DIR/issue_76432.rs:+3:26: +3:32
           scope 2 {
-              debug v1 => _12;             // in scope 2 at $DIR/issue_76432.rs:+3:10: +3:16
-              debug v2 => _13;             // in scope 2 at $DIR/issue_76432.rs:+3:18: +3:24
-              debug v3 => _14;             // in scope 2 at $DIR/issue_76432.rs:+3:26: +3:32
+              debug v1 => &(*_2)[0 of 3];  // in scope 2 at $DIR/issue_76432.rs:+3:10: +3:16
+              debug v2 => &(*_2)[1 of 3];  // in scope 2 at $DIR/issue_76432.rs:+3:18: +3:24
+              debug v3 => &(*_2)[2 of 3];  // in scope 2 at $DIR/issue_76432.rs:+3:26: +3:32
           }
       }
   
@@ -52,15 +52,6 @@
       }
   
       bb2: {
-          StorageLive(_12);                // scope 1 at $DIR/issue_76432.rs:+3:10: +3:16
-          _12 = &(*_2)[0 of 3];            // scope 1 at $DIR/issue_76432.rs:+3:10: +3:16
-          StorageLive(_13);                // scope 1 at $DIR/issue_76432.rs:+3:18: +3:24
-          _13 = &(*_2)[1 of 3];            // scope 1 at $DIR/issue_76432.rs:+3:18: +3:24
-          StorageLive(_14);                // scope 1 at $DIR/issue_76432.rs:+3:26: +3:32
-          _14 = &(*_2)[2 of 3];            // scope 1 at $DIR/issue_76432.rs:+3:26: +3:32
-          StorageDead(_14);                // scope 1 at $DIR/issue_76432.rs:+3:84: +3:85
-          StorageDead(_13);                // scope 1 at $DIR/issue_76432.rs:+3:84: +3:85
-          StorageDead(_12);                // scope 1 at $DIR/issue_76432.rs:+3:84: +3:85
           StorageDead(_5);                 // scope 0 at $DIR/issue_76432.rs:+6:1: +6:2
           StorageDead(_2);                 // scope 0 at $DIR/issue_76432.rs:+6:1: +6:2
           return;                          // scope 0 at $DIR/issue_76432.rs:+6:2: +6:2