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impl const Default for Box<[T]> and Box<str>

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This commit is contained in:
Josh Stone 2022-04-11 11:43:21 -07:00
parent 625e4dd13a
commit a2902ebe57
2 changed files with 18 additions and 4 deletions
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16
library/alloc/src/boxed.rs
View File

@ -1192,17 +1192,25 @@ impl<T: Default> Default for Box<T> {
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Default for Box<[T]> {
#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
impl<T> const Default for Box<[T]> {
fn default() -> Self {
Box::<[T; 0]>::new([])
let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
Box(ptr, Global)
}
}
#[cfg(not(no_global_oom_handling))]
#[stable(feature = "default_box_extra", since = "1.17.0")]
impl Default for Box<str> {
#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
impl const Default for Box<str> {
fn default() -> Self {
unsafe { from_boxed_utf8_unchecked(Default::default()) }
// SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
let ptr: Unique<str> = unsafe {
let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
Unique::new_unchecked(bytes.as_ptr() as *mut str)
};
Box(ptr, Global)
}
}

6
library/alloc/tests/const_fns.rs
View File

@ -6,6 +6,9 @@ pub const MY_VEC2: Vec<usize> = Default::default();
pub const MY_STRING: String = String::new();
pub const MY_STRING2: String = Default::default();
pub const MY_BOXED_SLICE: Box<[usize]> = Default::default();
pub const MY_BOXED_STR: Box<str> = Default::default();
use std::collections::{BTreeMap, BTreeSet};
pub const MY_BTREEMAP: BTreeMap<usize, usize> = BTreeMap::new();
@ -23,6 +26,9 @@ fn test_const() {
assert_eq!(MY_VEC, MY_VEC2);
assert_eq!(MY_STRING, MY_STRING2);
assert_eq!(MY_VEC, *MY_BOXED_SLICE);
assert_eq!(MY_STRING, *MY_BOXED_STR);
assert_eq!(MAP_LEN, 0);
assert_eq!(SET_LEN, 0);
assert!(MAP_IS_EMPTY && SET_IS_EMPTY);