Auto merge of #104844 - cjgillot:mention-eval-place, r=jackh726,RalfJung

Evaluate place expression in `PlaceMention`

https://github.com/rust-lang/rust/pull/102256 introduces a `PlaceMention(place)` MIR statement which keep trace of `let _ = place` statements from surface rust, but without semantics.

This PR proposes to change the behaviour of `let _ =` patterns with respect to the borrow-checker to verify that the bound place is live.

Specifically, consider this code:
```rust
let _ = {
    let a = 5;
    &a
};
```

This passes borrowck without error on stable. Meanwhile, replacing `_` by `_: _` or `_p` errors with "error[E0597]: `a` does not live long enough", [see playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c448d25a7c205dc95a0967fe96bccce8).

This PR *does not* change how `_` patterns behave with respect to initializedness: it remains ok to bind a moved-from place to `_`.

The relevant test is `tests/ui/borrowck/let_underscore_temporary.rs`. Crater check found no regression.

For consistency, this PR changes miri to evaluate the place found in `PlaceMention`, and report eventual dangling pointers found within it.

r? `@RalfJung`

compiler/rustc_const_eval/src/interpret/step.rs

@@ -113,8 +113,14 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
             Intrinsic(box intrinsic) => self.emulate_nondiverging_intrinsic(intrinsic)?,
 
-            // Statements we do not track.
-            PlaceMention(..) | AscribeUserType(..) => {}
+            // Evaluate the place expression, without reading from it.
+            PlaceMention(box place) => {
+                let _ = self.eval_place(*place)?;
+            }
+
+            // This exists purely to guide borrowck lifetime inference, and does not have
+            // an operational effect.
+            AscribeUserType(..) => {}
 
             // Currently, Miri discards Coverage statements. Coverage statements are only injected
             // via an optional compile time MIR pass and have no side effects. Since Coverage