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49 lines
1.7 KiB
Rust
49 lines
1.7 KiB
Rust
// This is a reduction of a concrete test illustrating a case that was
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// annoying to Rust developer niconii (see comment thread on #21114).
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//
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// With resolving issue #54556, pnkfelix hopes that the new diagnostic
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// output produced by NLL helps to *explain* the semantic significance
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// of temp drop order, and thus why inserting a semi-colon after the
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// `if let` expression in `main` works.
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//@ revisions: edition2021 edition2024
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//@ [edition2021] edition: 2021
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//@ [edition2024] edition: 2024
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//@ [edition2024] compile-flags: -Z unstable-options
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//@ [edition2024] check-pass
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#![cfg_attr(edition2024, feature(if_let_rescope))]
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struct Mutex;
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struct MutexGuard<'a>(&'a Mutex);
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impl Drop for Mutex { fn drop(&mut self) { println!("Mutex::drop"); } }
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impl<'a> Drop for MutexGuard<'a> { fn drop(&mut self) { println!("MutexGuard::drop"); } }
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impl Mutex {
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fn lock(&self) -> Result<MutexGuard, ()> { Ok(MutexGuard(self)) }
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}
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fn main() {
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let counter = Mutex;
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if let Ok(_) = counter.lock() { }
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//[edition2021]~^ ERROR: does not live long enough
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// Up until Edition 2021:
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// With this code as written, the dynamic semantics here implies
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// that `Mutex::drop` for `counter` runs *before*
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// `MutexGuard::drop`, which would be unsound since `MutexGuard`
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// still has a reference to `counter`.
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//
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// The goal of #54556 is to explain that within a compiler
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// diagnostic.
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// From Edition 2024:
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// Now `MutexGuard::drop` runs *before* `Mutex::drop` because
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// the lifetime of the `MutexGuard` is shortened to cover only
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// from `if let` until the end of the consequent block.
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// Therefore, Niconii's issue is properly solved thanks to the new
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// temporary lifetime rule for `if let`s.
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}
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