Added SmallVec::push_mut andSmallVec::insert_mut

[mematthias]

The push_mut feature (rust-lang/rust#135974) has already been merged and is expected to be included in the Rust 1.95 stable release. For this reason, we should also add push_mut and insert_mut here.

[emilio]

Would be nice to split the formatting into a separate (preliminary?) PR.

[emilio]

Maybe just &mut *ptr? Or why would as_mut().unwrap_unchecked() be preferred?

[mematthias]

That is just my writing style; it makes it easier to search.
I simplified it anyway

[emilio]

Miri seems to be complaining.

[balt-dev]

miri seems to be getting confused on the provenance?

[balt-dev]

as the guy who did push_mut in std, i feel like the change just made kind of defeats the point? the whole point of push_mut is that you're not rederiving the pointer
i say the old version is fine, we just need some way to tell miri it's okay

[mematthias]

@balt-dev While I understand the concerns raised, I am currently unable to resolve the Miri report. That said, this should not block us from shipping the feature. At the moment, users neither benefit from nor are harmed by it. But by making it available now, we ensure that once the optimization is in place, existing users can immediately take advantage of it without any disruption to their experience.

One possible (but dirty) solution would be to use set_len directly (inline) within the push_mut function:

#[inline]
pub fn push(&mut self, value: T) {
    _ = self.push_mut(value);
}

#[inline]
#[must_use]
pub fn push_mut(&mut self, value: T) -> &mut T {
    let len = self.len();
    if len == self.capacity() {
        self.reserve(1);
    }
    // SAFETY: both the input and output are within the allocation
    let ptr = unsafe { self.as_mut_ptr().add(len) };
    // SAFETY: we allocated enough space in case it wasn't enough, so the address is valid for
    // writes.
    unsafe { ptr.write(value) };

    // unsafe { self.set_len(len + 1) };
    {
        let new_len = len + 1;
        debug_assert!(new_len <= self.capacity());
        let on_heap = self.len.on_heap();
        self.len = TaggedLen::new(new_len, on_heap);
    }

    unsafe { &mut *ptr }
}