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Feature gate: #![feature(vec_push_within_capacity)]

This is a tracking issue for Vec::push_within_capacity

It enables pushing into a Vec if there is any capacity left and otherwise returns the value. I.e. it avoids implicit resizing. This can be useful when one wants to explicitly control it (e.g. via try_reserve) or uses a Vec in an unsafe manner where moving the elements would be a problem.

Public API

// alloc::Vec
impl Vec<T, A> {
    pub fn push_within_capacity(&mut self, value: T) -> Result<&mut T, T>;
}

Steps / History

• Implementation: add Vec::push_within_capacity - fallible, does not allocate #89123, Merge Vec::push{,_mut}_within_capacity #148534
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• naming (e.g. push_in_capacity)
• may become redundant due to larger-scale work in RFC: Add more support for fallible allocations in Vec rfcs#3271