Fix ScalarInt to char conversion

to avoid panic for invalid Unicode scalar values

compiler/rustc_middle/src/ty/consts/int.rs

@@ -294,12 +294,22 @@ impl From<char> for ScalarInt {
     }
 }
 
+/// Error returned when a conversion from ScalarInt to char fails.
+#[derive(Debug)]
+pub struct CharTryFromScalarInt;
+
 impl TryFrom<ScalarInt> for char {
-    type Error = Size;
+    type Error = CharTryFromScalarInt;
+
     #[inline]
-    fn try_from(int: ScalarInt) -> Result<Self, Size> {
-        int.to_bits(Size::from_bytes(std::mem::size_of::<char>()))
-            .map(|u| char::from_u32(u.try_into().unwrap()).unwrap())
+    fn try_from(int: ScalarInt) -> Result<Self, Self::Error> {
+        let Ok(bits) = int.to_bits(Size::from_bytes(std::mem::size_of::<char>())) else  {
+            return Err(CharTryFromScalarInt);
+        };
+        match char::from_u32(bits.try_into().unwrap()) {
+            Some(c) => Ok(c),
+            None => Err(CharTryFromScalarInt),
+        }
     }
 }