Add assert_unsafe_precondition!()s to as_ascii_unchecked() methods

Author: sam skeoch

library/core/src/char/methods.rs

@@ -1212,6 +1212,12 @@ impl char {
     #[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) }
     }

library/core/src/num/mod.rs

@@ -492,6 +492,12 @@ impl u8 {
     #[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) }
     }

library/core/src/str/mod.rs

@@ -2643,6 +2643,12 @@ impl str {
     #[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() }