Diagnostic for const generic enum on stable suggests requirements for nightly-only feature (PartialEq, Eq)

[joshtriplett]

Given the following code (playground link):

enum E {
    V1,
    V2,
}

struct S<const X: E>;

Rust gives a diagnostic suggesting deriving PartialEq, Eq:

error: `E` is forbidden as the type of a const generic parameter
 --> src/lib.rs:6:19
  |
6 | struct S<const X: E>;
  |                   ^
  |
  = note: the only supported types are integers, `bool` and `char`

error[E0741]: `E` must be annotated with `#[derive(PartialEq, Eq)]` to be used as the type of a const parameter
 --> src/lib.rs:6:19
  |
6 | struct S<const X: E>;
  |                   ^ `E` doesn't derive both `PartialEq` and `Eq`

For more information about this error, try `rustc --explain E0741`.

However, adding #[derive(PartialEq, Eq)] leads the compiler to still give an error about not supporting this.

A non-nightly compiler should not give the second error at all; it should only give the first, saying that the only supported types are integers, bool and char.

[lcnr]

assigning this to project-const-generics, getting worked on in https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/error.20msg.20referencing.20stable

[in42]

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[lcnr]

To fix this issue, you can do the following steps:

• grep for "forbidden as the type of a const generic parameter"
• look at that function for a bit
• change it so that the second error isn't emitted on stable (probably actually change the control flow to start by branching on tcx.features().adt_const_params

In case you are interested but need more help to fix this, feel free to reach out to me on zulip

[OliverMD]

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