constant as trait param results in incorrect dead code warning

[lcnr]

I tried this code:

#![feature(const_generics)]

const L: usize = 3;

fn main() {
    let p = Printer {};
    p.print();
}

trait Print<const N: usize> {
    fn print(&self) -> usize {
        3
    }
}

struct Printer {}
impl Print<L> for Printer {}

playground

I expected to see this happen: no dead_code warning is emitted.

Instead, this happened:

warning: the feature `const_generics` is incomplete and may cause the compiler to crash
 --> src/main.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default

warning: constant item is never used: `L`
 --> src/main.rs:3:1
  |
3 | const L: usize = 3;
  | ^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.70s
     Running `target/debug/playground`

[varkor]

I'm not sure this needs to block min_const_generics, but it affects it, so marking as blocking for now.

[lcnr]

@rustbot claim

[lcnr]

This is actually not an issue related to const generics 😅 It does not look at trait impls even if they are used as long as they don't overwrite the relevant method.

enum Foo {
    Bar,
}

fn main() {
    let p = [0; 0];
    p.print();
}

trait Print {
    fn print(&self) -> usize {
        3
    }
}

impl Print for [u32; Foo::Bar as usize] {
    // fn print(&self) -> usize {
    //    3
    // }
}