rust-lang#91836: Clarify error on casting larger integers to char

Author: GKFX

compiler/rustc_error_codes/src/error_codes/E0604.md

@@ -6,11 +6,16 @@ Erroneous code example:
 0u32 as char; // error: only `u8` can be cast as `char`, not `u32`
 ```
 
-As the error message indicates, only `u8` can be cast into `char`. Example:
+`char` is a Unicode Scalar Value, an integer value from 0 to 0xD7FF and
+0xE000 to 0x10FFFF. (The gap is for surrogate pairs.) Only `u8` always fits in
+those ranges so only `u8` may be cast to `char`.
+
+To allow larger values, use `char::from_u32`, which checks the value is valid.
 
 ```
-let c = 86u8 as char; // ok!
-assert_eq!(c, 'V');
+assert_eq!(86u8 as char, 'V'); // ok!
+assert_eq!(char::from_u32(0x3B1), Some('α')); // ok!
+assert_eq!(char::from_u32(0xD800), None); // not a USV.
 ```
 
 For more information about casts, take a look at the Type cast section in

compiler/rustc_typeck/src/check/cast.rs

@@ -337,6 +337,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     self.expr_ty
                 )
                 .span_label(self.span, "invalid cast")
+                .span_help(self.span, "try `char::from_u32` instead")
                 .emit();
             }
             CastError::NonScalar => {