Tracking Issue for `Vec::recycle`

Feature gate: #![feature(vec_recycle)]

This is a tracking issue for the Vec::recycle method, which allows reusing a Vec in a loop even if the lifetime unification would otherwise disallow it.

Public API

// in alloc::vec
#![feature(transmutability)]
#![feature(trait_alias)]

use std::mem::{Assume, MaybeUninit, TransmuteFrom};

/// Implemented if the size and alignment `T` and `Self` are both equal.
trait Recyclable<T> =
    for<'a> TransmuteFrom<&'a MaybeUninit<T>, { Assume::SAFETY }> 
    where 
        T: for<'a> TransmuteFrom<&'a MaybeUninit<Self>, { Assume::SAFETY }>;

impl<T> Vec<T> {
    #[feature(vec_recycle)]
    pub fn recycle<U>(self) -> Vec<U> where U: Recyclable<T>;
}

Steps / History

• ACP: rust-lang/libs-team#674
• Implementation: #...
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

• Make sure that we don't unintentionally create reliance on this as workaround for lack of stable TransmuteFrom, when this stabilizes.