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

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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`
This commit is contained in:
bors 2023-04-22 09:54:21 +00:00
commit f30fc0a5e2
3 changed files with 8 additions and 8 deletions
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4
tests/ui/option_if_let_else.fixed
View File

@ -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
});

4
tests/ui/option_if_let_else.rs
View File

@ -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
};
}

8
tests/ui/option_if_let_else.stderr
View File

@ -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 ~ });