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Rollup merge of rust-lang#77648 - fusion-engineering-forks:static-mut…
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…ex, r=dtolnay

Static mutex is static

StaticMutex is only ever used with as a static (as the name already suggests). So it doesn't have to be generic over a lifetime, but can simply assume 'static.

This 'static lifetime guarantees the object is never moved, so this is no longer a manually checked requirement for unsafe calls to lock().

@rustbot modify labels: +T-libs +A-concurrency +C-cleanup
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Dylan-DPC authored Oct 15, 2020
2 parents 143aa29 + 44a2af3 commit a0781d4
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Showing 3 changed files with 8 additions and 14 deletions.
2 changes: 1 addition & 1 deletion library/std/src/sys/unix/os.rs
Original file line number Diff line number Diff line change
Expand Up @@ -470,7 +470,7 @@ pub unsafe fn environ() -> *mut *const *const c_char {
&mut environ
}

pub unsafe fn env_lock() -> StaticMutexGuard<'static> {
pub unsafe fn env_lock() -> StaticMutexGuard {
// It is UB to attempt to acquire this mutex reentrantly!
static ENV_LOCK: StaticMutex = StaticMutex::new();
ENV_LOCK.lock()
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2 changes: 1 addition & 1 deletion library/std/src/sys/vxworks/os.rs
Original file line number Diff line number Diff line change
Expand Up @@ -212,7 +212,7 @@ pub unsafe fn environ() -> *mut *const *const c_char {
&mut environ
}

pub unsafe fn env_lock() -> StaticMutexGuard<'static> {
pub unsafe fn env_lock() -> StaticMutexGuard {
// It is UB to attempt to acquire this mutex reentrantly!
static ENV_LOCK: StaticMutex = StaticMutex::new();
ENV_LOCK.lock()
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18 changes: 6 additions & 12 deletions library/std/src/sys_common/mutex.rs
Original file line number Diff line number Diff line change
Expand Up @@ -3,8 +3,7 @@ use crate::sys::mutex as imp;
/// An OS-based mutual exclusion lock, meant for use in static variables.
///
/// This mutex has a const constructor ([`StaticMutex::new`]), does not
/// implement `Drop` to cleanup resources, and causes UB when moved or used
/// reentrantly.
/// implement `Drop` to cleanup resources, and causes UB when used reentrantly.
///
/// This mutex does not implement poisoning.
///
Expand All @@ -16,31 +15,26 @@ unsafe impl Sync for StaticMutex {}

impl StaticMutex {
/// Creates a new mutex for use.
///
/// Behavior is undefined if the mutex is moved after it is
/// first used with any of the functions below.
/// Also, the behavior is undefined if this mutex is ever used reentrantly,
/// i.e., `lock` is called by the thread currently holding the lock.
pub const fn new() -> Self {
Self(imp::Mutex::new())
}

/// Calls raw_lock() and then returns an RAII guard to guarantee the mutex
/// will be unlocked.
///
/// It is undefined behaviour to call this function while locked, or if the
/// mutex has been moved since the last time this was called.
/// It is undefined behaviour to call this function while locked by the
/// same thread.
#[inline]
pub unsafe fn lock(&self) -> StaticMutexGuard<'_> {
pub unsafe fn lock(&'static self) -> StaticMutexGuard {
self.0.lock();
StaticMutexGuard(&self.0)
}
}

#[must_use]
pub struct StaticMutexGuard<'a>(&'a imp::Mutex);
pub struct StaticMutexGuard(&'static imp::Mutex);

impl Drop for StaticMutexGuard<'_> {
impl Drop for StaticMutexGuard {
#[inline]
fn drop(&mut self) {
unsafe {
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