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Rollup merge of #100872 - JanBeh:PR_vec_default_alloc_doc, r=fee1-dead

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Add guarantee that Vec::default() does not alloc

Currently `Vec::new()` is guaranteed to not allocate until elements are pushed onto the `Vec`, but such a guarantee is missing for `Vec`'s implementation of `Default::default`.

This adds such a guarantee for `Vec::default()` to the API reference.

See also [this discussion on URLO](https://users.rust-lang.org/t/guarantee-that-vec-default-does-not-allocate/79903).
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Dylan DPC 2022-08-22 20:34:16 +05:30 • committed by GitHub
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library/alloc/src/vec/mod.rs
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@ -2928,6 +2928,8 @@ unsafe impl<#[may_dangle] T, A: Allocator> Drop for Vec<T, A> {
#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")] #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
impl<T> const Default for Vec<T> { impl<T> const Default for Vec<T> {
/// Creates an empty `Vec<T>`. /// Creates an empty `Vec<T>`.
///
/// The vector will not allocate until elements are pushed onto it.
fn default() -> Vec<T> { fn default() -> Vec<T> {
Vec::new() Vec::new()
} }