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

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Clarify error on casting larger integers to char

Closes #91836 with changes to E0604.md and a `span_help`.
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Matthias Krüger 2022-02-06 04:13:29 +01:00 • committed by GitHub
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11
compiler/rustc_error_codes/src/error_codes/E0604.md
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@ -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