Incorrect error suggestion when using a type that does not recursively derive PartialEq, Eq

[CDirkx]

The following code

#![feature(const_generics)]
#![feature(structural_match)]

struct Inner;

#[derive(PartialEq, Eq)]
struct Outer {
    inner: Inner
}

struct S<const O: Outer>;
const C : S<{ Outer { inner: Inner } }> = S;

results in the error

error[E0741]: `Outer` must be annotated with `#[derive(PartialEq, Eq)]` to be used as the type of a const parameter
  --> src/lib.rs:11:19
   |
   | struct S<const O: Outer>;
   |                   ^^^^^ `Outer` doesn't derive both `PartialEq` and `Eq`

Which is not correct: Outer does derive PartialEq and Eq.

The true error is that Inner does not derive the traits (or implements StructuralEq and StructuralPartialEq), however it is probably not a good idea to leak this type in the error message, as it is an implementation detail?

[estebank]

The suggestions now guide you in the right direction:

error[[E0741]](https://doc.rust-lang.org/nightly/error_codes/E0741.html): `Outer` must implement `ConstParamTy` to be used as the type of a const generic parameter
  --> src/lib.rs:12:19
   |
12 | struct S<const O: Outer>;
   |                   ^^^^^
   |
help: add `#[derive(ConstParamTy)]` to the struct
   |
8  + #[derive(ConstParamTy)]
9  | struct Outer {
   |

error[[E0204]](https://doc.rust-lang.org/nightly/error_codes/E0204.html): the trait `ConstParamTy` cannot be implemented for this type
 --> src/lib.rs:7:25
  |
7 | #[derive(PartialEq, Eq, ConstParamTy)]
  |                         ^^^^^^^^^^^^
8 | struct Outer {
9 |     inner: Inner
  |     ------------ this field does not implement `ConstParamTy`
  |
  = note: this error originates in the derive macro `ConstParamTy` (in Nightly builds, run with -Z macro-backtrace for more info)

error[[E0277]](https://doc.rust-lang.org/nightly/error_codes/E0277.html): the type `Inner` does not `#[derive(PartialEq)]`
 --> src/lib.rs:5:10
  |
5 | #[derive(ConstParamTy)]
  |          ^^^^^^^^^^^^ the trait `StructuralPartialEq` is not implemented for `Inner`
  |
note: required by a bound in `ConstParamTy`
 --> /rustc/32303b219d4dffa447aa606bc11c7a648f44a862/library/core/src/marker.rs:993:1
  = note: this error originates in the derive macro `ConstParamTy` (in Nightly builds, run with -Z macro-backtrace for more info)

error[[E0277]](https://doc.rust-lang.org/nightly/error_codes/E0277.html): the type `Inner` does not `#[derive(Eq)]`
 --> src/lib.rs:5:10
  |
5 | #[derive(ConstParamTy)]
  |          ^^^^^^^^^^^^ the trait `StructuralEq` is not implemented for `Inner`
  |
note: required by a bound in `ConstParamTy`
 --> /rustc/32303b219d4dffa447aa606bc11c7a648f44a862/library/core/src/marker.rs:993:1
  = note: this error originates in the derive macro `ConstParamTy` (in Nightly builds, run with -Z macro-backtrace for more info)