Add link to RFC 2915 for data-ful enums with reprs

and clarify which attribute is being referred to in the next paragraph.

Author: zachs18

compiler/rustc_error_codes/src/error_codes/E0517.md

@@ -31,8 +31,11 @@ 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. 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 2915][rfc2915].
+
+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.
+
+[rfc2915]: https://github.com/rust-lang/rfcs/blob/master/text/2195-really-tagged-unions.md