Allow optimizing u32::from::<char>.

Author: reitermarkus

library/core/src/char/convert.rs

@@ -48,7 +48,12 @@ impl From<char> for u32 {
     /// ```
     #[inline]
     fn from(c: char) -> Self {
-        c as u32
+        let c = c as Self;
+
+        // SAFETY: `char::MAX` is the highest valid Unicode code point.
+        unsafe { core::hint::assert_unchecked(c <= char::MAX as Self) };
+
+        c
     }
 }
 
@@ -69,7 +74,12 @@ impl From<char> for u64 {
     fn from(c: char) -> Self {
         // The char is casted to the value of the code point, then zero-extended to 64 bit.
         // See [https://doc.rust-lang.org/reference/expressions/operator-expr.html#semantics]
-        c as u64
+        let c = c as Self;
+
+        // SAFETY: `char::MAX` is the highest valid Unicode code point.
+        unsafe { core::hint::assert_unchecked(c <= char::MAX as Self) };
+
+        c
     }
 }
 
@@ -90,7 +100,12 @@ impl From<char> for u128 {
     fn from(c: char) -> Self {
         // The char is casted to the value of the code point, then zero-extended to 128 bit.
         // See [https://doc.rust-lang.org/reference/expressions/operator-expr.html#semantics]
-        c as u128
+        let c = c as Self;
+
+        // SAFETY: `char::MAX` is the highest valid Unicode code point.
+        unsafe { core::hint::assert_unchecked(c <= char::MAX as Self) };
+
+        c
     }
 }