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Document temporary scoping for destructuring assignments #1992

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45 changes: 45 additions & 0 deletions src/expressions/operator-expr.md
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Expand Up @@ -862,6 +862,49 @@ r[expr.assign.destructure.discard-value]
r[expr.assign.destructure.default-binding]
Note that default binding modes do not apply for the desugared expression.

> [!NOTE]
> Although basic assignment expressions are not [temporary scopes], the desugaring of destructuring assignments restricts the temporary scope of its assigned value operand.
> For example:
>
> ```rust
> # fn temp() {}
> use std::convert::identity;
>
> let x;
> // In a basic assignment, `temp()` is dropped at the end of the
> // enclosing temporary scope, so `x` can be assigned and used
> // within the same temporary scope.
> (x = identity(&temp()), x);
> ```
>
> ```rust,compile_fail,E0716
> # fn temp() {}
> # use std::convert::identity;
> let x;
> // In a destructuring assignment, `temp()` is dropped at the end of
> // the `let` statement in the desugaring, so `x` cannot be assigned.
> [x] = [identity(&temp())]; // ERROR
> ```
>
> Additionally, [temporary lifetime extension] applies to the `let` statement in a desugared destructuring assignment.
>
> ```rust
> # fn temp() {}
> # use std::convert::identity;
> let x;
> // The temporary scope of `temp()` is extended to the end of the
> // block in the desugaring, so `x` may be assigned, but it may not
> // be used.
> [x] = [&temp()];
> ```
>
> ```rust,compile_fail,E0716
> # fn temp() {}
> # use std::convert::identity;
> let x;
> ([x] = [&temp()], x); // ERROR
> ```

r[expr.compound-assign]
## Compound assignment expressions

Expand Down Expand Up @@ -1025,6 +1068,8 @@ As with normal assignment expressions, compound assignment expressions always pr
[unit]: ../types/tuple.md
[Unit-only enums]: ../items/enumerations.md#unit-only-enum
[value expression]: ../expressions.md#place-expressions-and-value-expressions
[temporary lifetime extension]: ../destructors.md#temporary-lifetime-extension
[temporary scopes]: ../destructors.md#temporary-scopes
[temporary value]: ../expressions.md#temporaries
[float-float]: https://github.com/rust-lang/rust/issues/15536
[Function pointer]: ../types/function-pointer.md
Expand Down