Auto merge of #128835 - matthiaskrgr:rollup-apzlbxy, r=matthiaskrgr

Rollup of 7 pull requests

Successful merges:

 - #128306 (Update NonNull::align_offset quarantees)
 - #128612 (Make `validate_mir` ensure the final MIR for all bodies)
 - #128648 (Add regression test)
 - #128749 (Mark `{f32,f64}::{next_up,next_down,midpoint}` inline)
 - #128795 (Update E0517 message to reflect RFC 2195.)
 - #128825 (rm `declared_features` field in resolver)
 - #128826 (Only suggest `#[allow]` for `--warn` and `--deny` lint level flags)

r? `@ghost`
`@rustbot` modify labels: rollup

1 files changed, 12 insertions, 7 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0517.md b/compiler/rustc_error_codes/src/error_codes/E0517.md
index ae802245bd1..5354a08bf31 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0517.md
+++ b/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