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Update E0517 message to reflect RFC 2195. #128795

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Aug 8, 2024
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Update E0517 message to reflect RFC 2195. #128795

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19 changes: 12 additions & 7 deletions compiler/rustc_error_codes/src/error_codes/E0517.md
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Original file line number Diff line number Diff line change
Expand Up @@ -25,14 +25,17 @@ impl Foo {
These attributes do not work on typedefs, since typedefs are just aliases.

Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
discriminant size for enums with no data fields on any of the variants, e.g.
`enum Color {Red, Blue, Green}`, effectively setting the size of the enum to
the size of the provided type. Such an enum can be cast to a value of the same
type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
with a constrained set of allowed values.
discriminant size for enums. For enums with no data fields on any of the
variants, e.g. `enum Color {Red, Blue, Green}`, this effectively sets the size
of the enum to the size of the provided type. Such an enum can be cast to a
value of the same type as well. In short, `#[repr(u8)]` makes a field-less enum
behave like an integer with a constrained set of allowed values.
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Only field-less enums can be cast to numerical primitives, so this attribute
will not apply to structs.
For a description of how `#[repr(C)]` and representations like `#[repr(u8)]`
affect the layout of enums with data fields, see [RFC 2195][rfc2195].

Only field-less enums can be cast to numerical primitives. Representations like
`#[repr(u8)]` will not apply to structs.

`#[repr(packed)]` reduces padding to make the struct size smaller. The
representation of enums isn't strictly defined in Rust, and this attribute
Expand All @@ -42,3 +45,5 @@ won't work on enums.
types (i.e., `u8`, `i32`, etc) a representation that permits vectorization via
SIMD. This doesn't make much sense for enums since they don't consist of a
single list of data.

[rfc2195]: https://github.com/rust-lang/rfcs/blob/master/text/2195-really-tagged-unions.md
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