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Add new temporary lifetime extension rule #1813

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1ae4ee6
Update reference for https://github.com/rust-lang/rust/pull/140593
m-ou-se May 7, 2025
83cb1a0
Fix links.
m-ou-se May 7, 2025
ea11029
Updated outdated text.
m-ou-se May 8, 2025
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8 changes: 5 additions & 3 deletions src/destructors.md
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Expand Up @@ -374,10 +374,10 @@ expression which is one of the following:
expression], [braced struct][struct expression], or [tuple][tuple expression]
expression.
* The final expression of any extending [block expression].
* The arguments to [tuple struct][tuple struct] and [tuple variant][tuple variant] constructors.
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Suggested change
* The arguments to [tuple struct][tuple struct] and [tuple variant][tuple variant] constructors.
* The arguments to [tuple struct] and [tuple variant] constructors.

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We were uncertain about two things here:

  • What is an "extending ... expression"? Does that mean it is a subexpression of something else that is extending?
  • Related to that, do these need the adjective "extending"? That is, "extending tuple struct" and "extending tuple variant"? If so, would it make sense to merge this with the third bullet point, or at least to move this bullet point to be next to it?

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@ehuss ehuss May 8, 2025 •
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I think to answer my own question, I do believe the "extending" part is relevant. My understanding is that:

  • Some{0: &temp()} — ok
  • (Some{0: &temp()},) — ok
  • foo(Some{0: &temp()},) — ERROR

because in function call argument position it is not an "extending expression". The same seems to be true for tuple constructors in rust-lang/rust#140593:

  • Some(&temp()) — ok
  • (Some(&temp()),) — ok
  • foo(Some(&temp()),) — ERROR


So the borrow expressions in `&mut 0`, `(&1, &mut 2)`, and `Some { 0: &mut 3 }`
are all extending expressions. The borrows in `&0 + &1` and `Some(&mut 0)` are
not: the latter is syntactically a function call expression.
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.

The operand of any extending borrow expression has its temporary scope
extended.
Expand Down Expand Up @@ -467,6 +467,8 @@ There is one additional case to be aware of: when a panic reaches a [non-unwindi
[struct pattern]: patterns.md#struct-patterns
[tuple pattern]: patterns.md#tuple-patterns
[tuple struct pattern]: patterns.md#tuple-struct-patterns
[tuple struct]: types/struct.md#r-type.struct.tuple
[tuple variant]: types/enum.md#r-type.enum.declaration
Comment on lines +470 to +471
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Let's switch these out for the r identifiers.


[array expression]: expressions/array-expr.md#array-expressions
[block expression]: expressions/block-expr.md
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