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Remove object lifetime cast #564

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Merged
merged 3 commits into from
Mar 31, 2025
Merged

Remove object lifetime cast #564

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6906ff6
Remove object lifetime cast
BoxyUwU Mar 13, 2025
355da55
Make lifetimes explicit and document safety
BoxyUwU Mar 27, 2025
cd742cb
Merge branch 'main' into remove_object_lifetime_cast
tobz Mar 29, 2025
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8 changes: 7 additions & 1 deletion metrics/src/recorder/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -140,10 +140,16 @@ pub struct LocalRecorderGuard<'a> {
impl<'a> LocalRecorderGuard<'a> {
/// Creates a new `LocalRecorderGuard` and sets the thread-local recorder.
fn new(recorder: &'a dyn Recorder) -> Self {
// SAFETY: Why this it okay to extend the lifetime of the object type from `'a` to `'static`?
let recorder_ptr = unsafe {
std::mem::transmute::<*const (dyn Recorder + '_), *mut (dyn Recorder + 'static)>(
recorder as &dyn Recorder,
)
};
// SAFETY: While we take a lifetime-less pointer to the given reference, the reference we derive _from_ the
// pointer is given the same lifetime of the reference used to construct the guard -- captured in the guard type
// itself -- and so derived references never outlive the source reference.
let recorder_ptr = unsafe { NonNull::new_unchecked(recorder as *const _ as *mut _) };
let recorder_ptr = unsafe { NonNull::new_unchecked(recorder_ptr) };

let prev_recorder =
LOCAL_RECORDER.with(|local_recorder| local_recorder.replace(Some(recorder_ptr)));
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