Rollup merge of #72683 - RalfJung:char-debug-check, r=Mark-Simulacrum

from_u32_unchecked: check validity when debug assertions are enabled

src/libcore/char/convert.rs

@@ -99,7 +99,7 @@ pub fn from_u32(i: u32) -> Option<char> {
 #[inline]
 #[stable(feature = "char_from_unchecked", since = "1.5.0")]
 pub unsafe fn from_u32_unchecked(i: u32) -> char {
-    transmute(i)
+    if cfg!(debug_assertions) { char::from_u32(i).unwrap() } else { transmute(i) }
 }
 
 #[stable(feature = "char_convert", since = "1.13.0")]
@@ -218,7 +218,7 @@ impl TryFrom<u32> for char {
             Err(CharTryFromError(()))
         } else {
             // SAFETY: checked that it's a legal unicode value
-            Ok(unsafe { from_u32_unchecked(i) })
+            Ok(unsafe { transmute(i) })
         }
     }
 }