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, 31 insertions, 0 deletions

diff --git a/tests/mir-opt/dead-store-elimination/ref.rs b/tests/mir-opt/dead-store-elimination/ref.rs
new file mode 100644
index 00000000000..2d3200edab9
--- /dev/null
+++ b/tests/mir-opt/dead-store-elimination/ref.rs
@@ -0,0 +1,31 @@
+//@ test-mir-pass: DeadStoreElimination-initial
+
+pub struct Foo {
+    a: i32,
+    b: i64,
+    c: i32,
+}
+
+// EMIT_MIR ref.tuple.DeadStoreElimination-initial.diff
+pub fn tuple(v: (i32, &Foo)) -> i32 {
+    // CHECK-LABEL: fn tuple
+    // CHECK: debug _dead => [[dead:_[0-9]+]];
+    // CHECK: bb0:
+    // CHECK: DBG: [[dead]] = &((*_3).2: i32)
+    let _dead = &v.1.c;
+    v.1.a
+}
+
+// EMIT_MIR ref.dead_first.DeadStoreElimination-initial.diff
+pub fn dead_first(v: &Foo) -> &i32 {
+    // CHECK-LABEL: fn dead_first
+    // CHECK: debug a => [[var_a:_[0-9]+]];
+    // CHECK: bb0:
+    // CHECK: DBG: [[var_a]] = &((*_1).2: i32)
+    // CHECK: [[tmp_4:_[0-9]+]] = &((*_1).0: i32)
+    // CHECK: [[tmp_3:_[0-9]+]] = &(*[[tmp_4]])
+    // CHECK: [[var_a]] = move [[tmp_3]]
+    let mut a = &v.c;
+    a = &v.a;
+    a
+}