Assert that non-extended temporaries and super let bindings have scopes
#147471
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dianne
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Oct 8, 2025
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| // 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). |
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I can't tell if the remarks on on Drop restrictions are out of date or just worded a bit confusingly.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.
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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 |
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I've clarified the PR description; thanks. Does it read better now? This is only about the temporary scopes assigned to expressions before |
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@bors r+ rollup |
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…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.
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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
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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.
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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
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This PR clarifies a point of confusion in the compiler: all bodies have an outer temporary drop scope, including
staticandconstitem 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 madeScopeTree::default_temporary_scopeandRvalueScopes::temporary_scopepanic 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 letbindings are dropped in their block's enclosing temporary scope, this applies to them as well: the enclosing temporary scope should exist.Footnotes
See https://github.com/rust-lang/rust/blob/master/compiler/rustc_hir_analysis/src/check/region.rs#L773-L778 for non-
fn/closure bodies. Thethis.cx.var_parent = None;enables lifetime extension to'staticlifetimes and theScopeData::Destructionscope 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. ↩