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Improve some SAFETY comments following suggestions

This commit is contained in:
Alexis Bourget 2020-08-29 20:10:05 +02:00
parent a83b79ec31
commit 5d29954b2f
1 changed files with 17 additions and 6 deletions
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23
library/std/src/thread/local.rs
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@ -315,12 +315,23 @@ mod lazy {
// value (an aliasing violation). To avoid setting the "I'm running a
// destructor" flag we just use `mem::replace` which should sequence the
// operations a little differently and make this safe to call.
//
// `ptr` can be dereferenced safely since it was obtained from
// `UnsafeCell::get`, which should not return a non-aligned or NUL pointer.
// What's more a `LazyKeyInner` can only be created with `new`, which ensures
// `inner` is correctly initialized and all calls to methods on `LazyKeyInner`
// will leave `inner` initialized too.
unsafe {
let _ = mem::replace(&mut *ptr, Some(value));
}
// SAFETY: the *ptr operation is made safe by the `mem::replace`
// call above that made sure a valid value is present behind it.
// SAFETY: the `*ptr` operation is made safe by the `mem::replace`
// call above combined with `ptr` being correct from the beginning
// (see previous SAFETY: comment above).
//
// Plus, with the call to `mem::replace` it is guaranteed there is
// a `Some` behind `ptr`, not a `None` so `unreachable_unchecked`
// will never be reached.
unsafe {
// After storing `Some` we want to get a reference to the contents of
// what we just stored. While we could use `unwrap` here and it should
@ -337,8 +348,8 @@ mod lazy {
#[allow(unused)]
pub unsafe fn take(&mut self) -> Option<T> {
// SAFETY: The other methods hand out references while taking &self.
// As such, calling this method when such references are still alive
// will fail because it takes a &mut self, conflicting with them.
// As such, callers of this method must ensure no `&` and `&mut` are
// available and used at the same time.
unsafe { (*self.inner.get()).take() }
}
}
@ -451,9 +462,9 @@ pub mod fast {
// LLVM issue: https://bugs.llvm.org/show_bug.cgi?id=41722
#[inline(never)]
unsafe fn try_initialize<F: FnOnce() -> T>(&self, init: F) -> Option<&'static T> {
// SAFETY: See comment above.
// SAFETY: See comment above (this function doc).
if !mem::needs_drop::<T>() || unsafe { self.try_register_dtor() } {
// SAFETY: See comment above.
// SAFETY: See comment above (his function doc).
Some(unsafe { self.inner.initialize(init) })
} else {
None