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specify lifetime extension of match arms and if consequent/else block tail expressions #1981

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specify lifetime extension of match arms and if consequent/else block tail expressions #1981

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0a25dea
clean up and properly test temporary lifetime extension in doctests
dianne Aug 26, 2025
e5ff66d
specify lifetime extension of `match` arms and `if` tail expressions
dianne Aug 26, 2025
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16 changes: 14 additions & 2 deletions src/destructors.md
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Original file line number Diff line number Diff line change
Expand Up @@ -435,6 +435,8 @@ expression which is one of the following:
expression.
* The arguments to an extending [tuple struct] or [tuple variant] constructor expression.
* The final expression of any extending [block expression].
* The arm(s) of an extending [`match`] expression.
* The final expressions of an extending [`if`] expression's consequent, `else if`, and `else` blocks.

So the borrow expressions in `&mut 0`, `(&1, &mut 2)`, and `Some(&mut 3)`
are all extending expressions. The borrows in `&0 + &1` and `f(&mut 0)` are not.
Expand All @@ -448,15 +450,22 @@ Here are some examples where expressions have extended temporary scopes:

```rust
# fn temp() {}
# trait Use { fn use_temp(&self) -> &Self { self } }
# impl Use for () {}
// The temporary that stores the result of `temp()` lives in the same scope
// as x in these cases.
let x = &temp();
# x;
let x = &temp() as &dyn Send;
# x;
let x = (&*&temp(),);
# x;
let x = { [Some(&temp()) ] };
# x;
let x = match () { _ => &temp() };
# x;
let x = if true { &temp() } else { &temp() };
# x;
let ref x = temp();
# x;
let ref x = *&temp();
# x;
```
Expand All @@ -471,8 +480,11 @@ Here are some examples where expressions don't have extended temporary scopes:
// end of the let statement in these cases.

let x = std::convert::identity(&temp()); // ERROR
# x;
let x = (&temp()).use_temp(); // ERROR
# x;
let x = match &temp() { x => x }; // ERROR
# x;
```

r[destructors.forget]
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