Assert that non-extended temporaries and `super let` bindings have scopes by dianne · Pull Request #147471 · rust-lang/rust

dianne · 2025-10-08T10:50:44Z

This PR clarifies a point of confusion in the compiler: all bodies have an outer temporary drop scope, including static and const item bodies¹. Whenever a temporary should be dropped in its enclosing temporary scope, it should have a temporary scope to be dropped in so that its drop can be scheduled². As such, I've updated some relevant comments and made ScopeTree::default_temporary_scope and RvalueScopes::temporary_scope panic when an enclosing temporary scope isn't found instead of allowing potential bugs where potentially-drop-sensitive temporaries are effectively given static lifetimes.

Since non-extended super let bindings are dropped in their block's enclosing temporary scope, this applies to them as well: the enclosing temporary scope should exist.

See https://github.com/rust-lang/rust/blob/master/compiler/rustc_hir_analysis/src/check/region.rs#L773-L778 for non-fn/closure bodies. The this.cx.var_parent = None; enables lifetime extension to 'static lifetimes and the ScopeData::Destruction scope ensures non-extended temporaries are dropped in the body expression's scope. ↩
For certain borrowed temporaries, drops that don't require running destructors may later be removed by constant promotion. That is unrelated to this PR. ↩

rustbot · 2025-10-08T10:50:49Z

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dianne · 2025-10-08T10:59:28Z

compiler/rustc_hir_analysis/src/check/region.rs

                // Similarly, `const X: ... = &f();` would have the result of `f()`
                // live for `'static`, implying (if Drop restrictions on constants
                // ever get lifted) that the value *could* have a destructor, but
                // it'd get leaked instead of the destructor running during the
                // evaluation of `X` (if at all allowed by CTFE).


I can't tell if the remarks on on Drop restrictions are out of date or just worded a bit confusingly. I'm also not sure whether this PR is the right place to update that.

theemathas · 2025-10-08T13:25:19Z

The only way a temporary should be extended to have a 'static lifetime is through lifetime extension

I'm not sure what this means. But I just wanted to note that constant-promotion in consts/statics can have a similar effect to lifetime extension, and has different behavior from constant-promotion outside of consts/statics. See #124328

dianne · 2025-10-08T14:26:37Z

I've clarified the PR description; thanks. Does it read better now? This is only about the temporary scopes assigned to expressions before PromoteTemps runs and is of most concern for non-promotable expressions (e.g. those that produce drop-sensitive temporaries).

nnethercote

This is stuff I know very little about, but the change is small enough and well explained so I'm happy with it.

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nnethercote · 2025-10-13T10:04:14Z

@bors r+ rollup

bors · 2025-10-13T10:04:16Z

📌 Commit 444eb4c has been approved by nnethercote

It is now in the queue for this repository.

…nethercote Assert that non-extended temporaries and `super let` bindings have scopes This PR clarifies a point of confusion in the compiler: all bodies have an outer temporary drop scope, including `static` and `const` item bodies[^1]. Whenever a temporary should be dropped in its enclosing temporary scope, it should have a temporary scope to be dropped in so that its drop can be scheduled[^2]. As such, I've updated some relevant comments and made `ScopeTree::default_temporary_scope` and `RvalueScopes::temporary_scope` panic when an enclosing temporary scope isn't found instead of allowing potential bugs where potentially-drop-sensitive temporaries are effectively given static lifetimes. Since non-extended `super let` bindings are dropped in their block's enclosing temporary scope, this applies to them as well: the enclosing temporary scope should exist. [^1]: See https://github.com/rust-lang/rust/blob/master/compiler/rustc_hir_analysis/src/check/region.rs#L773-L778 for non-`fn`/closure bodies. The `this.cx.var_parent = None;` enables [lifetime extension to `'static` lifetimes](https://doc.rust-lang.org/stable/reference/destructors.html#r-destructors.scope.lifetime-extension.static) and the `ScopeData::Destruction` scope ensures non-extended temporaries are dropped in the body expression's scope. [^2]: For certain borrowed temporaries, drops that don't require running destructors may later be removed by constant promotion. That is unrelated to this PR.

Rollup of 7 pull requests Successful merges: - #144266 (Supress swapping lhs and rhs in equality suggestion in extern macro ) - #147471 (Assert that non-extended temporaries and `super let` bindings have scopes) - #147533 (Renumber return local after state transform) - #147566 (rewrite outlives placeholder constraints to outlives static when handling opaque types) - #147613 (Make logging filters work again by moving EnvFilter into its own layer) - #147615 (reduce calls to attr.span() in old doc attr parsing) - #147636 (miri subtree update) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #147471 - dianne:assert-temporary-scope, r=nnethercote Assert that non-extended temporaries and `super let` bindings have scopes This PR clarifies a point of confusion in the compiler: all bodies have an outer temporary drop scope, including `static` and `const` item bodies[^1]. Whenever a temporary should be dropped in its enclosing temporary scope, it should have a temporary scope to be dropped in so that its drop can be scheduled[^2]. As such, I've updated some relevant comments and made `ScopeTree::default_temporary_scope` and `RvalueScopes::temporary_scope` panic when an enclosing temporary scope isn't found instead of allowing potential bugs where potentially-drop-sensitive temporaries are effectively given static lifetimes. Since non-extended `super let` bindings are dropped in their block's enclosing temporary scope, this applies to them as well: the enclosing temporary scope should exist. [^1]: See https://github.com/rust-lang/rust/blob/master/compiler/rustc_hir_analysis/src/check/region.rs#L773-L778 for non-`fn`/closure bodies. The `this.cx.var_parent = None;` enables [lifetime extension to `'static` lifetimes](https://doc.rust-lang.org/stable/reference/destructors.html#r-destructors.scope.lifetime-extension.static) and the `ScopeData::Destruction` scope ensures non-extended temporaries are dropped in the body expression's scope. [^2]: For certain borrowed temporaries, drops that don't require running destructors may later be removed by constant promotion. That is unrelated to this PR.

Rollup of 7 pull requests Successful merges: - rust-lang/rust#144266 (Supress swapping lhs and rhs in equality suggestion in extern macro ) - rust-lang/rust#147471 (Assert that non-extended temporaries and `super let` bindings have scopes) - rust-lang/rust#147533 (Renumber return local after state transform) - rust-lang/rust#147566 (rewrite outlives placeholder constraints to outlives static when handling opaque types) - rust-lang/rust#147613 (Make logging filters work again by moving EnvFilter into its own layer) - rust-lang/rust#147615 (reduce calls to attr.span() in old doc attr parsing) - rust-lang/rust#147636 (miri subtree update) r? `@ghost` `@rustbot` modify labels: rollup

…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#144266 (Supress swapping lhs and rhs in equality suggestion in extern macro ) - rust-lang#147471 (Assert that non-extended temporaries and `super let` bindings have scopes) - rust-lang#147533 (Renumber return local after state transform) - rust-lang#147566 (rewrite outlives placeholder constraints to outlives static when handling opaque types) - rust-lang#147613 (Make logging filters work again by moving EnvFilter into its own layer) - rust-lang#147615 (reduce calls to attr.span() in old doc attr parsing) - rust-lang#147636 (miri subtree update) r? `@ghost` `@rustbot` modify labels: rollup

rustbot assigned nnethercote Oct 8, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 8, 2025

dianne commented Oct 8, 2025

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dianne mentioned this pull request Oct 8, 2025

Further specify temporary scoping for statics and consts rust-lang/reference#2041

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assert that non-extended temporaries have scopes

444eb4c

dianne force-pushed the assert-temporary-scope branch from af03a85 to 444eb4c Compare October 8, 2025 13:14

nnethercote approved these changes Oct 13, 2025

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 13, 2025

matthiaskrgr mentioned this pull request Oct 13, 2025

Rollup of 7 pull requests #147640

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bors merged commit 2a10082 into rust-lang:master Oct 13, 2025
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rustbot added this to the 1.92.0 milestone Oct 13, 2025

dianne mentioned this pull request Oct 14, 2025

Temporary lifetime extension for blocks #146098

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dianne deleted the assert-temporary-scope branch November 13, 2025 11:07

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Assert that non-extended temporaries and `super let` bindings have scopes#147471

Assert that non-extended temporaries and `super let` bindings have scopes#147471
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