Clarify error on casting larger integers to char

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

@@ -328,16 +328,28 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                 err.emit();
             }
             CastError::CastToChar => {
-                type_error_struct!(
+                let mut err = type_error_struct!(
                     fcx.tcx.sess,
                     self.span,
                     self.expr_ty,
                     E0604,
                     "only `u8` can be cast as `char`, not `{}`",
                     self.expr_ty
-                )
-                .span_label(self.span, "invalid cast")
-                .emit();
+                );
+                err.span_label(self.span, "invalid cast");
+                if self.expr_ty.is_numeric() {
+                    err.span_help(
+                        self.span,
+                        if self.expr_ty == fcx.tcx.types.i8 {
+                            "try casting from `u8` instead"
+                        } else if self.expr_ty == fcx.tcx.types.u32 {
+                            "try `char::from_u32` instead"
+                        } else {
+                            "try `char::from_u32` instead (via a `u32`)"
+                        },
+                    );
+                }
+                err.emit();
             }
             CastError::NonScalar => {
                 let mut err = type_error_struct!(

src/test/ui/consts/const-eval/const-eval-overflow-4b.stderr

@@ -17,6 +17,12 @@ error[E0604]: only `u8` can be cast as `char`, not `i8`
    |
 LL |     : [u32; 5i8 as char as usize]
    |             ^^^^^^^^^^^ invalid cast
+   |
+help: try casting from `u8` instead
+  --> $DIR/const-eval-overflow-4b.rs:22:13
+   |
+LL |     : [u32; 5i8 as char as usize]
+   |             ^^^^^^^^^^^
 
 error: aborting due to 3 previous errors
 

src/test/ui/error-codes/E0604.stderr

@@ -3,6 +3,12 @@ error[E0604]: only `u8` can be cast as `char`, not `u32`
    |
 LL |     1u32 as char;
    |     ^^^^^^^^^^^^ invalid cast
+   |
+help: try `char::from_u32` instead
+  --> $DIR/E0604.rs:2:5
+   |
+LL |     1u32 as char;
+   |     ^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/error-festival.stderr

@@ -58,6 +58,12 @@ error[E0604]: only `u8` can be cast as `char`, not `u32`
    |
 LL |     0u32 as char;
    |     ^^^^^^^^^^^^ invalid cast
+   |
+help: try `char::from_u32` instead
+  --> $DIR/error-festival.rs:25:5
+   |
+LL |     0u32 as char;
+   |     ^^^^^^^^^^^^
 
 error[E0605]: non-primitive cast: `u8` as `Vec<u8>`
   --> $DIR/error-festival.rs:29:5

src/test/ui/mismatched_types/cast-rfc0401.stderr

@@ -99,6 +99,12 @@ error[E0604]: only `u8` can be cast as `char`, not `u32`
    |
 LL |     let _ = 0x61u32 as char;
    |             ^^^^^^^^^^^^^^^ invalid cast
+   |
+help: try `char::from_u32` instead
+  --> $DIR/cast-rfc0401.rs:41:13
+   |
+LL |     let _ = 0x61u32 as char;
+   |             ^^^^^^^^^^^^^^^
 
 error[E0606]: casting `bool` as `f32` is invalid
   --> $DIR/cast-rfc0401.rs:43:13