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Add SingleBox::leak to provide access to a &'static mut #8

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Add SingleBox::leak to provide access to a &'static mut #8

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19 changes: 19 additions & 0 deletions src/alloc_single.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -97,6 +97,21 @@ impl<'a, T> SingleBox<'a, T> {
fn as_ptr(&self) -> *mut T {
self.single.storage.get()
}

/// Consumes and leaks the SingleBox, returning a mutable reference, &'b mut T.
///
/// If the underlying AllocSingle has static lifetime, this can return a &'static mut.
///
/// This will prevent T's destructor from being called.
pub fn leak<'b>(self) -> &'b mut T
where T: 'b,
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Do we need this extra 'b lifetime? If 'a is 'static already, won't returning &'a mut T already get you the static lifetime?

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Not 100% sure!
If 'a is 'static and we want a &'static mut T, then sure, that would work.
But what if we actually need a shorter lifetime? I guess it's still fine because &'a T is covariant in 'a, https://doc.rust-lang.org/reference/subtyping.html#variance, so any &'a T is a subtype of &'static T for all lifetimes 'a?

'a: 'b,
{
let value = core::mem::ManuallyDrop::new(self);
let ptr = value.as_ptr();
// Safety: We leak self, so the reference stays valid as long as T and 'a are valid
unsafe { &mut *ptr }
}
}

impl<'a, T> Drop for SingleBox<'a, T> {
Expand Down Expand Up @@ -162,5 +177,9 @@ pub mod test {
// look ma no stack
let bx3 = SINGLE_DEMO.alloc_const_val().unwrap();
println!("{:?}", bx3.deref());

// we can get a &'static mut:
let bx3_ref: &'static mut _ = bx3.leak();
println!("{:?}", bx3_ref);
}
}