Trait functions cannot be `const`

[jhpratt]

When implementing a trait, clippy currently suggests making one of the functions const. That is currently not possible. At least for now, it seems like this should be an exception.

[phansch]

That sounds like a bug, thanks! Could you provide an example that triggers the lint?

[jhpratt]

@phansch Here's a simple example that actually has the error twice, once for the declared function, and one for the derived Eq trait.

#![warn(clippy::missing_const_for_fn)]

#[derive(PartialEq, Eq)]
struct Point(isize, isize);

impl std::ops::Add for Point {
    type Output = Self;

    fn add(self, other: Self) -> Self {
        Point(self.0 + other.0, self.1 + other.1)
    }
}

fn main() {}

Output when running cargo clippy on the above example:

warning: this could be a const_fn
 --> src/main.rs:3:21
  |
3 | #[derive(PartialEq, Eq)]
  |                     ^^
  |
note: lint level defined here
 --> src/main.rs:1:9
  |
1 | #![warn(clippy::missing_const_for_fn)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn

warning: this could be a const_fn
  --> src/main.rs:9:5
   |
9  | /     fn add(self, other: Self) -> Self {
10 | |         Point(self.0 + other.0, self.1 + other.1)
11 | |     }
   | |_____^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_const_for_fn