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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
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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.


```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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@dianne dianne Nov 3, 2025

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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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@dianne dianne Nov 3, 2025

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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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@dianne dianne Nov 5, 2025

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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.

> [!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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