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

library/core/src/char/methods.rs

@@ -1202,6 +1202,20 @@ 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 {
+        // 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

@@ -482,6 +482,20 @@ 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 {
+        // 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

@@ -2633,6 +2633,21 @@ 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] {
+        // 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)`,