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Rollup merge of rust-lang#96820 - r-raymond:master, r=cuviper
Make RwLockReadGuard covariant Hi, first time contributor here, if anything is not as expected, please let me know. `RwLockReadGoard`'s type constructor is invariant. Since it behaves like a smart pointer to an immutable reference, there is no reason that it should not be covariant. Take e.g. ``` fn test_read_guard_covariance() { fn do_stuff<'a>(_: RwLockReadGuard<'_, &'a i32>, _: &'a i32) {} let j: i32 = 5; let lock = RwLock::new(&j); { let i = 6; do_stuff(lock.read().unwrap(), &i); } drop(lock); } ``` where the compiler complains that &i doesn't live long enough. If `RwLockReadGuard` is covariant, then the above code is accepted because the lifetime can be shorter than `'a`. In order for `RwLockReadGuard` to be covariant, it can't contain a full reference to the `RwLock`, which can never be covariant (because it exposes a mutable reference to the underlying data structure). By reducing the data structure to the required pieces of `RwLock`, the rest falls in place. If there is a better way to do a test that tests successful compilation, please let me know. Fixes rust-lang#80392
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// compile-flags: -O -C no-prepopulate-passes -Z mutable-noalias=yes | ||
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#![crate_type = "lib"] | ||
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use std::sync::{RwLock, RwLockReadGuard}; | ||
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// Make sure that `RwLockReadGuard` does not get a `noalias` attribute, because | ||
// the `RwLock` might alias writes after it is dropped. | ||
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// CHECK-LABEL: @maybe_aliased( | ||
// CHECK-NOT: noalias | ||
// CHECK-SAME: %_data | ||
#[no_mangle] | ||
pub unsafe fn maybe_aliased(_: RwLockReadGuard<'_, i32>, _data: &RwLock<i32>) {} |