Rollup merge of #91939 - GKFX:feature-91866, r=cjgillot

Clarify error on casting larger integers to char

Closes #91836 with changes to E0604.md and a `span_help`.

1 files changed, 8 insertions, 3 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0604.md b/compiler/rustc_error_codes/src/error_codes/E0604.md
index adbf76509ed..806f0001c60 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0604.md
+++ b/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