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Specify temporary lifetime extension through expressions #2051

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e856ddf
Specify lifetime extension through expressions
dianne Oct 12, 2025
596d282
Reframe lifetime extension to not be specific to `let` statements
dianne Nov 2, 2025
71178bb
Reorganize, remove hacks, clarify, and add examples
dianne Nov 3, 2025
a0ae157
Add a couple examples where the spec didn't change
dianne Nov 3, 2025
e37d803
Adjust other references to extending expressions
dianne Nov 3, 2025
6536b3b
Add a correction and an example
dianne Nov 14, 2025
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162 changes: 108 additions & 54 deletions src/destructors.md
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Expand Up @@ -375,11 +375,8 @@ r[destructors.scope.lifetime-extension]
> [!NOTE]
> The exact rules for temporary lifetime extension are subject to change. This is describing the current behavior only.

r[destructors.scope.lifetime-extension.let]
The temporary scopes for expressions in `let` statements are sometimes
*extended* to the scope of the block containing the `let` statement. This is
done when the usual temporary scope would be too small, based on certain
syntactic rules. For example:
r[destructors.scope.lifetime-extension.intro]
The temporary scopes for expressions are sometimes *extended*. This is done when the usual temporary scope would be too small, based on certain syntactic rules. For example:

```rust
let x = &mut 0;
Expand All @@ -388,21 +385,27 @@ let x = &mut 0;
println!("{}", x);
```

r[destructors.scope.lifetime-extension.static]
Lifetime extension also applies to `static` and `const` items, where it
makes temporaries live until the end of the program. For example:
r[destructors.scope.lifetime-extension.sub-expressions]
If a [borrow], [dereference][dereference expression], [field][field expression], or [tuple indexing expression] has an extended temporary scope, then so does its operand. If an [indexing expression] has an extended temporary scope, then the indexed expression also has an extended temporary scope.
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@dianne dianne Nov 4, 2025 •
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I think this may need a terminology update, a carve-out in destructors.scope.lifetime-extension.exprs, or at least clarification on what "extended temporary scope" means. In particular, if you have an expression like

{ &*&temp() }

it's important that this rule takes precedence, so that temp() lives past the block. destructors.scope.lifetime-extension.exprs.borrows currently is worded to be the definitive lifetime of borrow operators' operands, but if only that was considered, temp() would be dropped at the end of the tail expression.

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@dianne dianne Nov 14, 2025

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This is partially addressed by the new example in 6536b3b. I think it'll still also want some reformulation or clarification of lifetime-extension.sub-expressions to make really make sense, though.

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@traviscross traviscross Nov 14, 2025 •
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Doesn't it almost feel as though the things in this rule should be rolled in to "extending based on expressions"? E.g.:

The extended scope of an expression is defined in terms of extending expressions and their extending parents. An extending expression is an expression which is one of the following... the indexed expression of a tuple indexing expression, the extending parent of which is the tuple indexing expression.

Or is there some important distinction we're trying to encode here in these being handled separately?

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@dianne dianne Nov 14, 2025 •
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There's a couple distinctions.

First, except for borrow expression operands, none of those subexpressions are extending. For instance,

let x = (&PrintOnDrop("temporary dropped")).0;
println!("next statement");

prints "temporary dropped", then "next statement". Intuitively, since the result of indexing is evaluated and then copied into x, the temporary doesn't need to be extended past its use, so we don't extend it. The purpose of lifetime-extension.sub-expressions is to handle reborrows, borrows of projections, etc.: if we'd instead written

let x = &(&PrintOnDrop("temporary dropped")).0;
println!("next statement");

we'd be reborrowing the temporary's field rather than evaluating it, so we would extend the temporary, so it'd print "next statement", then "temporary dropped". Extending subexpressions generally should only be those where if you substituted a borrow expression in, it's syntactically obvious that the borrow will need to be live to use the result of evaluating the extending parent expression. Maybe there's a way to express that intuition productively in an admonition somehow?

Second, lifetime-extension.sub-expressions also applies when extending based on patterns:

let ref x = PrintOnDrop("temporary dropped").0;
println!("next statement");

prints "next statement", then "temporary dropped". Because x is borrowing from a place based on the temporary, the entire temporary is extended. The example in lifetime-extension.sub-expressions illustrates this in a bit more detail.

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@traviscross traviscross Nov 15, 2025 •
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I don't fully understand how to apply this rule in the same way that rustc does. I put together a series of examples and mechanically analyzed them according to the rules in this PR. Can you explain the behavior of the last two examples in light of the behavior of the ones that precede them?

Playground link

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@traviscross traviscross Nov 15, 2025 •
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Setting that aside, and setting aside the matter of the interaction with "extending based on patterns" (which as we discussed on Zulip, might end up needing to be restructured eventually given how it would interact with a binding version of super let or super let in), it feels to me as though these are properly a kind of extending expression but that, as we work outward, we're carrying a flag indicating whether we last stepped through a "full" extending expression or through a more "limited" extending expression, and the value of that flag affects our behavior, e.g. when we get out to the let initializer.

(To the degree that's not true, maybe it's worth considering whether it should be.)

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@dianne dianne Nov 15, 2025 •
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My notes on each example, per the mental model I was aiming for with that change (I'll address the limited extending model after):

  1. let _ = (&e.log(1)).0; //~ Not extended.
    Our analyses agree here.
  2. let _ = &(&e.log(2)).0; //~ Extended.
    Our analyses agree here.
  3. let _ = &(&(e.log(2),).0).0; //~ Extended.
    e.log(2) here isn't extended; instead, it's immediately moved into the tuple temporary, which is extended. I believe its scope is that of the tuple expression, per destructors.scope.operands. I'd also add another step: the reason (&(e.log(2),).0).0 uses its extended scope as its temporary scope is because it's the operand of a borrow expression. Otherwise, our analyses agree.
  4. let _ = &[(e.log(2),).0][0].0; //~ Extended.
    In this case, the temporary that gets extended is the array given by evaluating [(e.log(2),).0]. A place based on it, array[0].0, is borrowed, and since we want that borrow to be live for the block, we extend the array's lifetime.
  5. let _ = &[(e.log(2),).0]; //~ Extended.
    As with 4, the temporary that gets extended is the array given by evaluating [(e.log(2),).0].
  6. let _ = &[(&e.log(1)).0]; //~ Not extended.
    As with 4 and 5, the temporary being extended is the array. The key here is that we're not borrowing from the e.log(1) temporary in the final value; we evaluate (&e.log(1)).0 to something like LogDrop(_e1, 1) where _e1 is a reborrow of e and then move that value into the array. As such, e.log(1) does not need to be extended; this is why tuple indexing expressions are not extending parents.
  7. let _ = &(&e.log(1),).0; //~ Not extended.
    This one we should extend according to the new model we're discussing. e.log(1) should have the tuple expression's temporary scope, which should be extended. Indeed, it's currently an error to use the result because it borrows from the dropped e.log(1) temporary. I don't think my compiler PR handles this yet either, but I'd like for it to work.
    The way rustc does this is when it extends (&e.log(1),).0 it applies lifetime-extension.sub-expressions to extend the place(s) it's based on as well (so (&e.log(1),)) and then stops.

For the limited extending model, I think there's a conceptual reason to keep them distinct. Extending parent expressions represent things like constructors and conditionals that build (or select) a value and are guaranteed to contain (or evaluate to) their extending subexpressions. Projections like .0 are sort of a dual to that: if we want to extend the lifetime of a projected place, we have to extend the base place too, thus we extend the indexed operand. Actually, I think under this PR's model, we can clean it up and get & out of the "sub-expressions" list so it's just the base places of place expressions. I'll experiment with that on the compiler side of things and update this PR if it works out.

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I wonder what the right thing to do is in cases like

let x = [&temp(), local][1];
let y = &[&temp(), local][1];

I see a few options:

  1. Under rustc's current model (including under Temporary lifetime extension for blocks rust#146098 currently), neither let statement extends its temp(). This is safest in a way: this means we're not extending unused temporaries to the end of the block. Of course, it's still possible today to create unused extended temporaries, e.g. with let _ = &temp();.
  2. Under this PR after the latest update, I think only the definition of y would extend its temp(), since the array would have an extended temporary scope and temp() would have the array's temporary scope as its temporary scope. In the case of x, the array wouldn't have an extended temporary scope because the result of indexing is used as a value.
  3. Under a model where indexed expressions are limited-extending, I could possibly see both statements extending their temp()s, because in either case we can see that the temp() could end up referenced in the final value. I'd have to think through what the exact rules would be to end up with that result.

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@traviscross traviscross Nov 15, 2025 •
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Makes sense. Regarding 3..=6... oops, yes, of course -- I must have been staring at too many variations for too long. Interesting about 7.

Revised analysis of the interesting cases:

Playground link

Regarding the sub-expressions rule:

If a [borrow], [dereference][dereference expression], [field][field expression], or [tuple indexing expression] has an extended temporary scope, then so does its operand. If an [indexing expression] has an extended temporary scope, then the indexed expression also has an extended temporary scope.

In "extending based on expressions", we're defining expressions to have an extended scope even if that extended scope is never used. E.g.:

r[destructors.scope.lifetime-extension.exprs.let]
The extended scope of the initializer expression of a let statement is the scope of the block containing the let statement.

For sub-expressions, then, it seems we have to account for that somehow.

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In "extending based on expressions", we're defining expressions to have an extended scope even if that extended scope is never used.

For sub-expressions, then, it seems we have to account for that somehow.

This is a terminology problem. I couldn't think of a better term than "extended scope" at the time, but there's meant to be a distinction. Every expression has an extended scope. Borrow expressions' operands use their extended scope as their temporary scope; as such, they're said to have an extended temporary scope (though it looks like I forgot to add that definition). The initializer expression of a let statement with an extending pattern also has an extended temporary scope.

I can think of a few things that might help:

  • Actually defining "extended temporary scope". That term is a holdover from the stable version of the temporary lifetime extension section, where it was more obvious what it meant. It might still be obvious enough here if we can get rid of the other too-similar terms, and under the definition I proposed above it might be confusing that the "extended temporary scope" of an expression can sometimes be its normal temporary scope.
  • Removing the term "extended temporary scope". sub-expressions is e.g. saying that the tuple operand expression of a tuple indexing expression has the same temporary scope as the tuple indexing expression. We don't need a notion of "extended temporary scope" to express that.
  • Renaming the term "extended scope". The term I used in the compiler is "extended_parent scope" to match with the notions of "parent scope" and "var_parent scope" used there. That feels too close to "extending parent" though, maybe something like "extended ancestor scope" would be fine? I'm not sure.
  • Removing the term "extended scope". e.g. a previous iteration of this PR defined the "extending ancestor" of an expression as the closest non-extending expression above that expression, then defined the scopes of borrow expressions' operands and super temporaries in terms of its temporary scope. It felt messier, so I ended making rules to determine its temporary scope directly (as the "extended scope" of an expression).

With this in mind, there isn't intended to be any ambiguity or a need to determine precedence between exprs and sub-expressions. The rules should be written in a way that makes everything clear. They just haven't quite gotten there yet.


```rust
const C: &Vec<i32> = &Vec::new();
// Usually this would be a dangling reference as the `Vec` would only
// exist inside the initializer expression of `C`, but instead the
// borrow gets lifetime-extended so it effectively has `'static` lifetime.
println!("{:?}", C);
# use core::sync::atomic::{AtomicU64, Ordering::Relaxed};
# static X: AtomicU64 = AtomicU64::new(0);
# struct PrintOnDrop(&'static str);
# impl Drop for PrintOnDrop {
# fn drop(&mut self) {
# X.fetch_add(1, Relaxed);
# println!("{}", self.0);
# }
# }
let x = &(0, PrintOnDrop("tuple 1 dropped")).0;
let ref y = (0, PrintOnDrop("tuple 2 dropped")).0;
// Though only its first field is borrowed, the temporary for the entire tuple
// lives to the end of the block in both cases.
println!("{x}, {y}");
# assert_eq!(0, X.load(Relaxed));
```

r[destructors.scope.lifetime-extension.sub-expressions]
If a [borrow], [dereference][dereference expression], [field][field expression], or [tuple indexing expression] has an extended temporary scope, then so does its operand. If an [indexing expression] has an extended temporary scope, then the indexed expression also has an extended temporary scope.

r[destructors.scope.lifetime-extension.patterns]
#### Extending based on patterns

Expand Down Expand Up @@ -445,7 +448,7 @@ So `ref x`, `V(ref x)` and `[ref x, y]` are all extending patterns, but `x`, `&r

r[destructors.scope.lifetime-extension.patterns.let]
If the pattern in a `let` statement is an extending pattern then the temporary
scope of the initializer expression is extended.
scope of the initializer expression is extended to the scope of the block containing the `let` statement.

```rust
# fn temp() {}
Expand Down Expand Up @@ -473,37 +476,102 @@ let &ref x = &*&temp(); // OK
r[destructors.scope.lifetime-extension.exprs]
#### Extending based on expressions

r[destructors.scope.lifetime-extension.exprs.borrows]
The [temporary scope] of the operand of a [borrow] expression is the *extended scope* of the operand expression, defined below.

r[destructors.scope.lifetime-extension.exprs.super-macros]
The [scope][temporary scope] of each [super temporary] of a [super macro call] expression is the extended scope of the super macro call expression.
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Two things here:

  • I think it's helpful to start destructors.scope.lifetime-extension.exprs with a straightforward explanation of what all the definitions are for. Otherwise, I found that either too many definitions are needed before any of them can be explained, or the explanations have to refer to terms that haven't been defined yet. For the sake of directness, I moved destructors.scope.lifetime-extension.exprs.borrows to the start, but that meant having to refer to terms that are defined later. A possible alternative would be writing a new destructors.scope.lifetime-extension.exprs.intro rule, but when I tried that it felt redundant.
  • I don't love the term "extended scope", but I wasn't able to think of anything better, either in terms of naming or in terms of reframing to avoid defining it at all. I feel like it could probably also use some additional clarification but I'm still thinking of a good admonition to add for it.


r[destructors.scope.lifetime-extension.exprs.extending]
For a let statement with an initializer, an *extending expression* is an
expression which is one of the following:
The extended scope of an expression is defined in terms of *extending expressions* and their *extending parents*. An extending expression is an expression which is one of the following:
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Three things here:

  • To simplify the definitions of extending expressions and extended scopes, I've made let initializers, static/const items, and const block tails no longer extending. This might hurt the intuition of extending expressions a bit though.
  • I added the definition of "extending parent" when reworking the upwards walk. This keeps it rigorous without hacks (the parent of an expression isn't defined elsewhere, and going through the parent of a scope to save words is confusing). It also also makes it clear that extending expressions are all subexpressions and thus have parent expressions. But spending words on it feels a bit awkward too, either adding bloat or making things convoluted (or both).
  • Extending expressions are necessary to define extended scopes, but extended scopes are necessary to justify the definition of extending expressions. As such, I felt a need to connect this to the surrounding sections, but I don't love how it turned out. Maybe there's a better way to handle that.

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Following up on making "extending parents" make more sense and making it clear that "extending expression" isn't a classification of expressions but a classification of subexpressions, maybe it would be worth renaming "extending expression" to "extending subexpression"? I think it makes more sense from a spec point of view, but it's more awkward to read and write (and it means having to adapt language elsewhere), so I haven't gone through with it.

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@traviscross traviscross Nov 5, 2025 •
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It has some appeal, but I similarly see the problems you mention. If one says "extending subexpression", then one kind of wants the parent to be an "extending expression" (in the same way as "header", "subheader", etc.), but that's probably confusing.


* The initializer expression.
* The operand of an extending [borrow] expression.
* The [super operands] of an extending [super macro call] expression.
* The operand(s) of an extending [array][array expression], [cast][cast
* The operand of a [borrow] expression, the extending parent of which is the borrow expression.
* The [super operands] of a [super macro call] expression, the extending parent of which is the macro call expression.
* The operand(s) of an [array][array expression], [cast][cast
expression], [braced struct][struct expression], or [tuple][tuple expression]
expression.
* The arguments to an extending [tuple struct] or [tuple enum variant] constructor expression.
* The final expression of an extending [block expression] except for an [async block expression].
* The final expression of an extending [`if`] expression's consequent, `else if`, or `else` block.
* An arm expression of an extending [`match`] expression.
expression, the extending parent of which is the array, cast, braced struct, or tuple expression.
* The arguments to a [tuple struct] or [tuple enum variant] constructor expression, the extending parent of which is the constructor expression.
* The final expression of a plain [block expression] or [`unsafe` block expression], the extending parent of which is the block expression.
* The final expression of an [`if`] expression's consequent, `else if`, or `else` block, the extending parent of which is the `if` expression.
* An arm expression of a [`match`] expression, the extending parent of which is the `match` expression.
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@traviscross traviscross Nov 15, 2025 •
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It's maybe not what we want to do right now, but it occurred to me that these and the other rules could probably be usefully presented in the manner of inference rules, e.g. (and making no assertions about correctness of this sketch):

enum Scope {
    Block,
    Stmt,
    // ...
}
struct S(scope: Scope, ext_scope: Scope);

/// Borrow expressions
S(x, x) ⊢  $expr
----------------
S(s, x) ⊢ &$expr

/// Array expressions
S(s, x) ⊢  $expr
----------------------
S(s, x) ⊢ [$expr, ...]

/// Function calls
S(s, s) ⊢       $expr
---------------------------
S(s, x) ⊢ $path($expr, ...)

/// Let initializers
S(Stmt, Block) ⊢           &$expr
---------------------------------
S(Stmt, Block) ⊢ let ... = &$expr

// Etc.

When @Nadrieril and I were working on match ergonomics, we ended up finding it rather helpful for analysis to present the rules in this manner. See, e.g. this document for a discussion of notation.

@Nadrieril ended up writing a cool web tool for doing analysis and comparisons based on this kind of notation.

It's similarly been occurring to me here that it would be helpful to have a tool that encodes the formal rules in software and walks through the analysis of an expression. (Of course, one option is always to do something like that in the compiler with an approach similar to #[rustc_capture_analysis]).

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I'm pretty familiar with inference rules; if it would help to discuss them in that way, I could write them out for different variants of lifetime extension (e.g. the stable version, the compiler PR version where exprs.other uses the enclosing scope of the extended parent, the version we've been talking about where exprs.other uses the temporary scope of the extended parent, etc.).

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That would definitely be helpful. I could see including that directly in the Reference as well.


> [!NOTE]
> The desugaring of a [destructuring assignment] makes its assigned value operand (the RHS) an extending expression within a newly-introduced block. For details, see [expr.assign.destructure.tmp-ext].

So the borrow expressions in `&mut 0`, `(&1, &mut 2)`, and `Some(&mut 3)`
> [!NOTE]
> `rustc` does not treat [array repeat operands] of [array] expressions as extending expressions. Whether it should is an open question.
>
> For details, see [Rust issue #146092](https://github.com/rust-lang/rust/issues/146092).

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.

r[destructors.scope.lifetime-extension.exprs.borrows]
The operand of an extending [borrow] expression has its [temporary scope] [extended].
r[destructors.scope.lifetime-extension.exprs.parent]
The extended scope of an extending expression is the extended scope of its extending parent.

r[destructors.scope.lifetime-extension.exprs.super-macros]
The [super temporaries] of an extending [super macro call] expression have their [scopes][temporary scopes] [extended].
r[destructors.scope.lifetime-extension.exprs.let]
The extended scope of the initializer expression of a `let` statement is the scope of the block containing the `let` statement.

> [!EXAMPLE]
> In this example, the temporary value holding the result of `temp()` is extended to the end of the block in which `x` is declared:
>
> ```rust,edition2024
> # fn temp() {}
> let x = { &temp() };
> println!("{x:?}");
> ```
>
> `temp()` is the operand of a borrow expression, so its temporary scope is its extended scope.
> To determine its extended scope, look outward:
>
> * Since borrow expressions' operands are extending, the extended scope of `temp()` is the extended scope of its extending parent, the borrow expression.
> * `&temp()` is the final expression of a plain block. Since the final expressions of plain blocks are extending, the extended temporary scope of `&temp()` is the extended scope of its extending parent, the block expression.
> * `{ &temp() }` is the initializer expression of a `let` statement, so its extended scope is the scope of the block containg that `let` statement.
>
> If not for temporary lifetime extension, the result of `temp()` would be dropped after evaluating the tail expression of the block `{ &temp() }` ([destructors.scope.temporary.enclosing]).

r[destructors.scope.lifetime-extension.exprs.static]
The extended scope of the body expression of a [static][static item] or [constant item], and of the final expression of a [const block expression], is the entire program. This prevents destructors from being run.

```rust
# #[derive(Debug)] struct PanicOnDrop;
# impl Drop for PanicOnDrop { fn drop(&mut self) { panic!() } }
# impl PanicOnDrop { const fn new() -> PanicOnDrop { PanicOnDrop } }
const C: &PanicOnDrop = &PanicOnDrop::new();
// Usually this would be a dangling reference as the result of
// `PanicOnDrop::new()` would only exist inside the initializer expression of
// `C`, but instead the borrow gets lifetime-extended so it effectively has
// a `'static` lifetime and its destructor is never run.
println!("{:?}", C);
// `const` blocks may likewise extend temporaries to the end of the program:
// the result of `PanicOnDrop::new()` is not dropped.
println!("{:?}", const { &PanicOnDrop::new() });
```

r[destructors.scope.lifetime-extension.exprs.other]
The extended scope of any other expression is its [temporary scope].

> [!NOTE]
> `rustc` does not treat [array repeat operands] of extending [array] expressions as extending expressions. Whether it should is an open question.
> In this case, the expression is not extending, meaning it cannot be a borrow expression or a [super operand][super operands] to a [super macro call] expression, so its temporary scope is given by [destructors.scope.temporary.enclosing].

> [!EXAMPLE]
> In this example, the temporary value holding the result of `temp()` is extended to the end of the statement:
>
> For details, see [Rust issue #146092](https://github.com/rust-lang/rust/issues/146092).
> ```rust,edition2024
> # fn temp() {}
> # fn use_temp(_: &()) {}
> use_temp({ &temp() });
> ```
>
> `temp()` is the operand of a borrow expression, so its temporary scope is its extended scope.
> To determine its extended scope, look outward:
>
> * Since borrow expressions' operands are extending, the extended scope of `temp()` is the extended scope of its extending parent, the borrow expression.
> * `&temp()` is the final expression of a plain block. Since the final expressions of plain blocks are extending, the extended scope of `&temp()` is the extended scope of its extending parent, the block expression.
> * `{ &temp() }` is the argument of a call expression, which is not extending. Since no other cases apply, its extended scope is its temporary scope.
> * Per [destructors.scope.temporary.enclosing], the temporary scope of `{ &temp() }`, and thus the extended scope of `temp()`, is the scope of the statement.
>
> If not for temporary lifetime extension, the result of `temp()` would be dropped after evaluating the tail expression of the block `{ &temp() }` ([destructors.scope.temporary.enclosing]).

#### Examples

Expand Down Expand Up @@ -606,22 +674,6 @@ let x = 'a: { break 'a &temp() }; // ERROR
# x;
```

```rust,edition2024,compile_fail,E0716
# use core::pin::pin;
# fn temp() {}
// The argument to `pin!` is only an extending expression if the call
// is an extending expression. Since it's not, the inner block is not
// an extending expression, so the temporaries in its trailing
// expression are dropped immediately.
pin!({ &temp() }); // ERROR
```

```rust,edition2024,compile_fail,E0716
# fn temp() {}
// As above.
format_args!("{:?}", { &temp() }); // ERROR
```

r[destructors.forget]
## Not running destructors

Expand All @@ -647,6 +699,7 @@ There is one additional case to be aware of: when a panic reaches a [non-unwindi
[Assignment]: expressions/operator-expr.md#assignment-expressions
[binding modes]: patterns.md#binding-modes
[closure]: types/closure.md
[constant item]: items/constant-items.md
[destructors]: destructors.md
[destructuring assignment]: expr.assign.destructure
[expression]: expressions.md
Expand All @@ -660,6 +713,7 @@ There is one additional case to be aware of: when a panic reaches a [non-unwindi
[promoted]: destructors.md#constant-promotion
[scrutinee]: glossary.md#scrutinee
[statement]: statements.md
[static item]: items/static-items.md
[temporary]: expressions.md#temporaries
[unwinding]: panic.md#unwinding
[variable]: variables.md
Expand All @@ -681,22 +735,22 @@ There is one additional case to be aware of: when a panic reaches a [non-unwindi

[array expression]: expressions/array-expr.md#array-expressions
[array repeat operands]: expr.array.repeat-operand
[async block expression]: expr.block.async
[block expression]: expressions/block-expr.md
[borrow]: expr.operator.borrow
[cast expression]: expressions/operator-expr.md#type-cast-expressions
[const block expression]: expr.block.const
[dereference expression]: expressions/operator-expr.md#the-dereference-operator
[extended]: destructors.scope.lifetime-extension
[field expression]: expressions/field-expr.md
[indexing expression]: expressions/array-expr.md#array-and-slice-indexing-expressions
[struct expression]: expressions/struct-expr.md
[super macro call]: expr.super-macros
[super operands]: expr.super-macros
[super temporaries]: expr.super-macros
[super temporary]: expr.super-macros
[temporary scope]: destructors.scope.temporary
[temporary scopes]: destructors.scope.temporary
[tuple expression]: expressions/tuple-expr.md#tuple-expressions
[tuple indexing expression]: expressions/tuple-expr.md#tuple-indexing-expressions
[`unsafe` block expression]: expr.block.unsafe

[`for`]: expressions/loop-expr.md#iterator-loops
[`if let`]: expressions/if-expr.md#if-let-patterns
Expand Down
12 changes: 6 additions & 6 deletions src/expressions.md
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Expand Up @@ -262,7 +262,7 @@ r[expr.super-macros.intro]
Certain built-in macros may create [temporaries] whose [scopes][temporary scopes] may be [extended]. These temporaries are *super temporaries* and these macros are *super macros*. [Invocations][macro invocations] of these macros are *super macro call expressions*. Arguments to these macros may be *super operands*.

> [!NOTE]
> When a super macro call expression is an [extending expression], its super operands are [extending expressions] and the [scopes][temporary scopes] of the super temporaries are [extended]. See [destructors.scope.lifetime-extension.exprs].
> The super operands of a super macro call are [extending expressions] and the [scopes][temporary scopes] of the super temporaries are [extended]. See [destructors.scope.lifetime-extension.exprs].

r[expr.super-macros.format_args]
#### `format_args!`
Expand All @@ -272,10 +272,11 @@ Except for the format string argument, all arguments passed to [`format_args!`]

```rust,edition2024
# fn temp() -> String { String::from("") }
// Due to the call being an extending expression and the argument
// being a super operand, the inner block is an extending expression,
// so the scope of the temporary created in its trailing expression
// is extended.
// Due to the argument being a super operand, the inner block is an
// extending expression, so the scope of the temporary created in its
// trailing expression is extended to the extended scope of the call.
// Since the call is the initializer of a `let` statement, this
// extends it to the end of the surrounding block.
let _ = format_args!("{}", { &temp() }); // OK
```

Expand Down Expand Up @@ -406,7 +407,6 @@ They are never allowed before:
[destructors]: destructors.md
[drop scope]: destructors.md#drop-scopes
[extended]: destructors.scope.lifetime-extension
[extending expression]: destructors.scope.lifetime-extension.exprs
[extending expressions]: destructors.scope.lifetime-extension.exprs
[field]: expressions/field-expr.md
[functional update]: expressions/struct-expr.md#functional-update-syntax
Expand Down
6 changes: 3 additions & 3 deletions src/expressions/operator-expr.md
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Expand Up @@ -900,9 +900,9 @@ r[expr.assign.destructure.tmp-scopes]

r[expr.assign.destructure.tmp-ext]
> [!NOTE]
> Due to the desugaring, the assigned value operand (the RHS) of a destructuring assignment is an [extending expression] within a newly-introduced block.
> Due to the desugaring, the assigned value operand (the RHS) of a destructuring assignment is the initializer expression of a `let` statement within a newly-introduced block.
>
> Below, because the [temporary scope] is extended to the end of this introduced block, the assignment is allowed.
> Below, because the [temporary scope] is [extended] to the end of this introduced block, the assignment is allowed.
>
> ```rust
> # fn temp() {}
Expand Down Expand Up @@ -1089,7 +1089,7 @@ As with normal assignment expressions, compound assignment expressions always pr
[dropping]: ../destructors.md
[eval order test]: https://github.com/rust-lang/rust/blob/1.58.0/src/test/ui/expr/compound-assignment/eval-order.rs
[explicit discriminants]: ../items/enumerations.md#explicit-discriminants
[extending expression]: destructors.scope.lifetime-extension.exprs
[extended]: destructors.scope.lifetime-extension.exprs
[field-less enums]: ../items/enumerations.md#field-less-enum
[grouped expression]: grouped-expr.md
[literal expression]: literal-expr.md#integer-literal-expressions
Expand Down
4 changes: 2 additions & 2 deletions src/items/constant-items.md
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Expand Up @@ -90,7 +90,7 @@ const _: &mut u8 = unsafe { &mut S }; // ERROR.
> // the program.
> ```
>
> Here, the value `0` is a temporary whose scope is extended to the end of the program (see [destructors.scope.lifetime-extension.static]). Such temporaries cannot be mutably borrowed in constant expressions (see [const-eval.const-expr.borrows]).
> Here, the value `0` is a temporary whose scope is extended to the end of the program (see [destructors.scope.lifetime-extension.exprs.static]). Such temporaries cannot be mutably borrowed in constant expressions (see [const-eval.const-expr.borrows]).
>
> To allow this, we'd have to decide whether each use of the constant creates a new `u8` value or whether each use shares the same lifetime-extended temporary. The latter choice, though closer to how `rustc` thinks about this today, would break the conceptual model that, in most cases, the constant initializer can be thought of as being inlined wherever the constant is used. Since we haven't decided, and due to the other problem mentioned, this is not allowed.

Expand Down Expand Up @@ -175,7 +175,7 @@ const _: &&mut u8 = unsafe { &S }; // OK.
> const _: &AtomicU8 = &AtomicU8::new(0); // ERROR.
> ```
>
> Here, the `AtomicU8` is a temporary whose scope is extended to the end of the program (see [destructors.scope.lifetime-extension.static]). Such temporaries with interior mutability cannot be borrowed in constant expressions (see [const-eval.const-expr.borrows]).
> Here, the `AtomicU8` is a temporary whose scope is extended to the end of the program (see [destructors.scope.lifetime-extension.exprs.static]). Such temporaries with interior mutability cannot be borrowed in constant expressions (see [const-eval.const-expr.borrows]).
>
> To allow this, we'd have to decide whether each use of the constant creates a new `AtomicU8` or whether each use shares the same lifetime-extended temporary. The latter choice, though closer to how `rustc` thinks about this today, would break the conceptual model that, in most cases, the constant initializer can be thought of as being inlined wherever the constant is used. Since we haven't decided, this is not allowed.

Expand Down
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