error: could not evaluate constant pattern

[MatsPalmgren]

In the Firefox style system we have this enum type and in the impl I'm adding a const, like so:
pub const INITIAL_VALUE: Self = Self::None;
That compiles fine. Then I'd like to add this function:

    pub fn is_initial(&self) -> bool {
        matches!(*self, Self::INITIAL_VALUE)
    }

which results in:

error: could not evaluate constant pattern
   --> servo/components/style/values/generics/grid.rs:770:25
    |
770 |         matches!(*self, Self::INITIAL_VALUE)
    |                         ^^^^^^^^^^^^^^^^^^^

which makes no sense to me given that matches!(*self, Self::None) compiles just fine.

(This is in a local mozilla-central build, and I think the compiler it uses is: rustc 1.39.0-nightly (6e19f3f 2019-09-06))

BTW, why is the type required in the const declaration? Why can't I just say:
pub const INITIAL_VALUE = Self::None;
It seems like the compiler should be able to deduce the type I wanted there...

[rodrimati1992]

Ì believe this is a minimized version of the error:

#![feature(matches_macro)]

enum Maybe<T>{
    Just(T),
    Nothing,
}

impl<T> Maybe<T>{
    pub const INITIAL_VALUE: Self = Self::Nothing;

    pub fn is_initial(&self) -> bool {
        matches!(*self, Self::INITIAL_VALUE)
    }
}

error: could not evaluate constant pattern
  --> src/lib.rs:12:25
   |
12 |         matches!(*self, Self::INITIAL_VALUE)
   |                         ^^^^^^^^^^^^^^^^^^^

[jonas-schievink]

Also happens on stable when omitting the matches! macro:

enum Maybe<T>{
    Just(T),
    Nothing,
}

impl<T> Maybe<T>{
    pub const INITIAL_VALUE: Self = Self::Nothing;

    pub fn is_initial(&self) -> bool {
        if let Self::INITIAL_VALUE = *self {
            true
        } else {
            false
        }
    }
}

error: could not evaluate constant pattern
  --> src/lib.rs:10:16
   |
10 |         if let Self::INITIAL_VALUE = *self {
   |                ^^^^^^^^^^^^^^^^^^^

[Spoonbender]

triage: still reproduces, error message improved:

error: constant pattern depends on a generic parameter
  --> src/lib.rs:10:16
   |
10 |         if let Self::INITIAL_VALUE = *self {
   |                ^^^^^^^^^^^^^^^^^^^

warning: irrefutable `if let` pattern
  --> src/lib.rs:10:12
   |
10 |         if let Self::INITIAL_VALUE = *self {
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(irrefutable_let_patterns)]` on by default
   = note: this pattern will always match, so the `if let` is useless
   = help: consider replacing the `if let` with a `let`