InstCombine introduces an incorrect use of a local after its storage has ended

[tmiasko]

pub fn f<T>(a: &T) -> *const T {
    let b: &*const T = &(a as *const T);
    *b
}

--- mir_dump/a.f.005-003.InstCombine.before.mir
+++ mir_dump/a.f.005-003.InstCombine.after.mir
@@ -1,2 +1,2 @@
-// MIR for `f` before InstCombine
+// MIR for `f` after InstCombine
 
@@ -18,5 +18,5 @@
         _3 = &_4;                        // scope 0 at a.rs:2:24: 2:40
-        _2 = &(*_3);                     // scope 0 at a.rs:2:24: 2:40
+        _2 = _3;                         // scope 0 at a.rs:2:24: 2:40
         StorageDead(_3);                 // scope 0 at a.rs:2:40: 2:41
-        _0 = (*_2);                      // scope 1 at a.rs:3:5: 3:7
+        _0 = (*_3);                      // scope 1 at a.rs:3:5: 3:7
         StorageDead(_4);                 // scope 0 at a.rs:4:1: 4:2

Found by MIR validator #77369 & #78147.

cc @simonvandel #76683

[tmiasko]

I will open a PR to disable problematic part of InstCombine for now.

[LeSeulArtichaut]

Did you bisect the regression to #76683?

[spastorino]

I see that we've disabled the optimization, can't we also add a regression test for it? cc @tmiasko

EDIT (camelid): See the relevant discussion in wg-prioritization.

[spastorino]

Assigning P-critical as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.