`Self` not allowed in type of const generics even when it's concrete

[theemathas]

I tried this code:

trait MyTrait {
    fn foo<const N: i32>();
}

impl MyTrait for i32 {
    fn foo<const N: Self>() {}
}

I expected this code to compile, but I got the following error instead:

error[E0770]: the type of const parameters must not depend on other generic parameters
 --> src/lib.rs:6:21
  |
6 |     fn foo<const N: Self>() {}
  |                     ^^^^ the type must not depend on the parameter `Self`

error[E0053]: associated function `foo` has an incompatible generic parameter for trait `MyTrait`
 --> src/lib.rs:6:12
  |
1 | trait MyTrait {
  |       -------
2 |     fn foo<const N: i32>();
  |            ------------ expected const parameter of type `i32`
...
5 | impl MyTrait for i32 {
  | --------------------
6 |     fn foo<const N: Self>() {}
  |            ^^^^^^^^^^^^^ found const parameter of type `{type error}`

Some errors have detailed explanations: E0053, E0770.
For more information about an error, try `rustc --explain E0053`.

The Self type here just means i32, so it doesn't actually depend on any generic parameters.

In contrast, the following code compiles:

trait MyTrait {
    fn foo() -> i32;
}

impl MyTrait for i32 {
    fn foo() -> Self { 1 }
}

Meta

Reproducible on the playground with version 1.93.0-nightly (2025-11-21 27b076af7e3e7a363975)

[BoxyUwU]

This is inconsistent with how we allow uses of the Self alias inside of const arguments so long as it is fully concrete. E.g.

trait Trait { type Assoc; }
impl Trait for () {
    type Assoc = [(); size_of::<Self>()];
}

We should definitely be supporting this

[BoxyUwU]

Though I'm not sure what the interactions there are with generic_const_parameter_types and wanting to forbid forward declared parameters from appearing in the type of const parameters 🤔 That seems kinda tricky to support