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

Evaluate place expression in `PlaceMention`

rust-lang#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`

Author: bors

tests/ui/option_if_let_else.fixed

@@ -25,7 +25,7 @@ fn else_if_option(string: Option<&str>) -> Option<(bool, &str)> {
 fn unop_bad(string: &Option<&str>, mut num: Option<i32>) {
     let _ = string.map_or(0, |s| s.len());
     let _ = num.as_ref().map_or(&0, |s| s);
-    let _ = num.as_mut().map_or(&mut 0, |s| {
+    let _ = num.as_mut().map_or(&0, |s| {
         *s += 1;
         s
     });
@@ -34,7 +34,7 @@ fn unop_bad(string: &Option<&str>, mut num: Option<i32>) {
         s += 1;
         s
     });
-    let _ = num.as_mut().map_or(&mut 0, |s| {
+    let _ = num.as_mut().map_or(&0, |s| {
         *s += 1;
         s
     });

tests/ui/option_if_let_else.rs

@@ -33,7 +33,7 @@ fn unop_bad(string: &Option<&str>, mut num: Option<i32>) {
         *s += 1;
         s
     } else {
-        &mut 0
+        &0
     };
     let _ = if let Some(ref s) = num { s } else { &0 };
     let _ = if let Some(mut s) = num {
@@ -46,7 +46,7 @@ fn unop_bad(string: &Option<&str>, mut num: Option<i32>) {
         *s += 1;
         s
     } else {
-        &mut 0
+        &0
     };
 }
 

tests/ui/option_if_let_else.stderr

@@ -30,13 +30,13 @@ LL |       let _ = if let Some(s) = &mut num {
 LL | |         *s += 1;
 LL | |         s
 LL | |     } else {
-LL | |         &mut 0
+LL | |         &0
 LL | |     };
    | |_____^
    |
 help: try
    |
-LL ~     let _ = num.as_mut().map_or(&mut 0, |s| {
+LL ~     let _ = num.as_mut().map_or(&0, |s| {
 LL +         *s += 1;
 LL +         s
 LL ~     });
@@ -76,13 +76,13 @@ LL |       let _ = if let Some(ref mut s) = num {
 LL | |         *s += 1;
 LL | |         s
 LL | |     } else {
-LL | |         &mut 0
+LL | |         &0
 LL | |     };
    | |_____^
    |
 help: try
    |
-LL ~     let _ = num.as_mut().map_or(&mut 0, |s| {
+LL ~     let _ = num.as_mut().map_or(&0, |s| {
 LL +         *s += 1;
 LL +         s
 LL ~     });