Rollup merge of rust-lang#128795 - zachs18:e0517-update-for-rfc-2195, r=jieyouxu

Update E0517 message to reflect RFC 2195.

E0517 occurs when a `#[repr(..)]` attribute is placed on an unsupported item. Currently, the explanation of the error implies that `#[repr(u*/i*)]` cannot be placed on fieldful enums, which is no longer the case since [RFC 2195](rust-lang/rfcs#2195) was [stabilized](rust-lang#60553), which allows placing `#[repr(u*/i*)]` and/or `#[repr(C)]` on fieldful enums to produce a defined layout.

This PR doesn't (currently) add a description of the semantics of placing `#[repr(u*/i*)]` on a fieldful enum to the error explanation, it just removes the claims/implications that it is not allowed.

Author: matthiaskrgr

compiler/rustc_error_codes/src/error_codes/E0517.md

@@ -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.
 
-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
@@ -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