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Clarify error on casting larger integers to char #91939

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Feb 6, 2022
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Clarify error on casting larger integers to char #91939

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a15cb49
#91836: Clarify error on casting larger integers to char
GKFX Dec 14, 2021
badb81a
#91939: integer to char cast error, make more targeted
GKFX Jan 25, 2022
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11 changes: 8 additions & 3 deletions compiler/rustc_error_codes/src/error_codes/E0604.md
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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1 change: 1 addition & 0 deletions compiler/rustc_typeck/src/check/cast.rs
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Expand Up @@ -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")
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@camelid camelid Dec 14, 2021 •
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This should only be suggested if self.expr_ty is u32. Pseudocode:

Suggested change
.span_help(self.span, "try `char::from_u32` instead")
let err = type_error_struct!(...).span_label(...);
if self.expr_ty == fcx.tcx.types.u32 {
err.span_help(self.span, "try `char::from_u32` instead");
}
err.emit();

.emit();
}
CastError::NonScalar => {
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