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Rollup merge of #90162 - WaffleLapkin:const_array_slice_from_ref_mut, r=oli-obk

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Mark `{array, slice}::{from_ref, from_mut}` as const fn

This PR marks the following APIs as `const`:
```rust
// core::array
pub const fn from_ref<T>(s: &T) -> &[T; 1];
pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1];

// core::slice
pub const fn from_ref<T>(s: &T) -> &[T];
pub const fn from_mut<T>(s: &mut T) -> &mut [T];
```

Note that `from_ref` methods require `const_raw_ptr_deref` feature (which seems totally fine, since it's being stabilized, see #89551), `from_mut` methods require `const_mut_refs` (which seems fine too since this PR marks `from_mut` functions as const unstable).

r? ````@oli-obk````
This commit is contained in:
Matthias Krüger
2021-10-24 15:48:44 +02:00
committed by GitHub
parent b837605012 5f390cfb72
commit c16ee19dd4
6 changed files with 25 additions and 4 deletions
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6
library/core/src/array/mod.rs
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@@ -85,14 +85,16 @@ where
/// Converts a reference to `T` into a reference to an array of length 1 (without copying).
#[stable(feature = "array_from_ref", since = "1.53.0")]
pub fn from_ref<T>(s: &T) -> &[T; 1] {
#[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
pub const fn from_ref<T>(s: &T) -> &[T; 1] {
// SAFETY: Converting `&T` to `&[T; 1]` is sound.
unsafe { &*(s as *const T).cast::<[T; 1]>() }
}
/// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
#[stable(feature = "array_from_ref", since = "1.53.0")]
pub fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
#[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
// SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
}