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Auto merge of #75157 - rodrimati1992:patch-1, r=oli-obk

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Constified str::from_utf8_unchecked

This would be useful for const code to use an array to construct a string using guaranteed utf8 inputs, and then create a `&str` from it.
This commit is contained in:
bors 2020-08-14 14:08:05 +00:00
commit 55b9adfafa
1 changed files with 9 additions and 13 deletions
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22
library/core/src/str/mod.rs
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@ -414,12 +414,13 @@ pub fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
/// ``` /// ```
#[inline] #[inline]
#[stable(feature = "rust1", since = "1.0.0")] #[stable(feature = "rust1", since = "1.0.0")]
pub unsafe fn from_utf8_unchecked(v: &[u8]) -> &str { #[rustc_const_unstable(feature = "const_str_from_utf8_unchecked", issue = "75196")]
// SAFETY: the caller must guarantee that the bytes `v` #[allow(unused_attributes)]
// are valid UTF-8, thus the cast to `*const str` is safe. #[allow_internal_unstable(const_fn_transmute)]
// Also, the pointer dereference is safe because that pointer pub const unsafe fn from_utf8_unchecked(v: &[u8]) -> &str {
// comes from a reference which is guaranteed to be valid for reads. // SAFETY: the caller must guarantee that the bytes `v` are valid UTF-8.
unsafe { &*(v as *const [u8] as *const str) } // Also relies on `&str` and `&[u8]` having the same layout.
unsafe { mem::transmute(v) }
} }
/// Converts a slice of bytes to a string slice without checking /// Converts a slice of bytes to a string slice without checking
@ -2357,15 +2358,10 @@ impl str {
#[rustc_const_stable(feature = "str_as_bytes", since = "1.32.0")] #[rustc_const_stable(feature = "str_as_bytes", since = "1.32.0")]
#[inline(always)] #[inline(always)]
#[allow(unused_attributes)] #[allow(unused_attributes)]
#[allow_internal_unstable(const_fn_union)] #[allow_internal_unstable(const_fn_transmute)]
pub const fn as_bytes(&self) -> &[u8] { pub const fn as_bytes(&self) -> &[u8] {
#[repr(C)]
union Slices<'a> {
str: &'a str,
slice: &'a [u8],
}
// SAFETY: const sound because we transmute two types with the same layout // SAFETY: const sound because we transmute two types with the same layout
unsafe { Slices { str: self }.slice } unsafe { mem::transmute(self) }
} }
/// Converts a mutable string slice to a mutable byte slice. /// Converts a mutable string slice to a mutable byte slice.