Rollup merge of #137432 - djscythe:char_u8_str_as_ascii_unchecked, r=scottmcm

Add as_ascii_unchecked() methods to char, u8, and str

This PR adds the `as_ascii_unchecked()` method to `char`, `u8`, and `str`, allowing users to convert these types to `ascii::Char`s (see #110998) in an `unsafe` context without first checking for validity. This method was already available for `[u8]`, so this PR makes the API more consistent across other types.

library/core/src/char/methods.rs

@@ -4,6 +4,7 @@ use super::*;
 use crate::panic::const_panic;
 use crate::slice;
 use crate::str::from_utf8_unchecked_mut;
+use crate::ub_checks::assert_unsafe_precondition;
 use crate::unicode::printable::is_printable;
 use crate::unicode::{self, conversions};
 
@@ -1202,6 +1203,26 @@ impl char {
         }
     }
 
+    /// Converts this char into an [ASCII character](`ascii::Char`), without
+    /// checking whether it is valid.
+    ///
+    /// # Safety
+    ///
+    /// This char must be within the ASCII range, or else this is UB.
+    #[must_use]
+    #[unstable(feature = "ascii_char", issue = "110998")]
+    #[inline]
+    pub const unsafe fn as_ascii_unchecked(&self) -> ascii::Char {
+        assert_unsafe_precondition!(
+            check_library_ub,
+            "as_ascii_unchecked requires that the char is valid ASCII",
+            (it: &char = self) => it.is_ascii()
+        );
+
+        // SAFETY: the caller promised that this char is ASCII.
+        unsafe { ascii::Char::from_u8_unchecked(*self as u8) }
+    }
+
     /// Makes a copy of the value in its ASCII upper case equivalent.
     ///
     /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',

library/core/src/num/mod.rs

@@ -492,6 +492,26 @@ impl u8 {
         ascii::Char::from_u8(*self)
     }
 
+    /// Converts this byte to an [ASCII character](ascii::Char), without
+    /// checking whether or not it's valid.
+    ///
+    /// # Safety
+    ///
+    /// This byte must be valid ASCII, or else this is UB.
+    #[must_use]
+    #[unstable(feature = "ascii_char", issue = "110998")]
+    #[inline]
+    pub const unsafe fn as_ascii_unchecked(&self) -> ascii::Char {
+        assert_unsafe_precondition!(
+            check_library_ub,
+            "as_ascii_unchecked requires that the byte is valid ASCII",
+            (it: &u8 = self) => it.is_ascii()
+        );
+
+        // SAFETY: the caller promised that this byte is ASCII.
+        unsafe { ascii::Char::from_u8_unchecked(*self) }
+    }
+
     /// Makes a copy of the value in its ASCII upper case equivalent.
     ///
     /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',

library/core/src/str/mod.rs

@@ -17,6 +17,7 @@ use self::pattern::{DoubleEndedSearcher, Pattern, ReverseSearcher, Searcher};
 use crate::char::{self, EscapeDebugExtArgs};
 use crate::ops::Range;
 use crate::slice::{self, SliceIndex};
+use crate::ub_checks::assert_unsafe_precondition;
 use crate::{ascii, mem};
 
 pub mod pattern;
@@ -2634,6 +2635,27 @@ impl str {
         self.as_bytes().as_ascii()
     }
 
+    /// Converts this string slice into a slice of [ASCII characters](ascii::Char),
+    /// without checking whether they are valid.
+    ///
+    /// # Safety
+    ///
+    /// Every character in this string must be ASCII, or else this is UB.
+    #[unstable(feature = "ascii_char", issue = "110998")]
+    #[must_use]
+    #[inline]
+    pub const unsafe fn as_ascii_unchecked(&self) -> &[ascii::Char] {
+        assert_unsafe_precondition!(
+            check_library_ub,
+            "as_ascii_unchecked requires that the string is valid ASCII",
+            (it: &str = self) => it.is_ascii()
+        );
+
+        // SAFETY: the caller promised that every byte of this string slice
+        // is ASCII.
+        unsafe { self.as_bytes().as_ascii_unchecked() }
+    }
+
     /// Checks that two strings are an ASCII case-insensitive match.
     ///
     /// Same as `to_ascii_lowercase(a) == to_ascii_lowercase(b)`,