Rust: "cannot infer the value of const parameter" when a default is provided

Question

I'm providing a default value for a const generic type but the Rust compiler tells me it "cannot infer the value of const parameter". It seems to ignore the default. Am I using this feature wrong? Is this how it is supposed to work? Then why use defaults at all? I use nightly 1.60.

const DEFAULT_N: usize = 73;

struct Foo<const N: usize = DEFAULT_N>;

impl<const N: usize> Foo<N> {
    fn new() -> Self {
        println!("N is: {}", N);
        Self
    }
}

fn main() {
    Foo::new();
}

I never used this feature, but should't you specify the default value also in the impl declaration ? — Peterrabbit
– Peterrabbit, Commented Feb 23, 2022 at 9:51
@Peterrabbit No, specifying a default in an impl block is an error. (error: defaults for const parameters are only allowed in struct, enum, type, or trait definitions) — cdhowie
– cdhowie, Commented Feb 23, 2022 at 9:58

Peterrabbit · Accepted Answer · 2022-02-23 10:28:23Z

4

I got it working like this

const DEFAULT_N: usize = 73;

struct Foo<const N: usize = DEFAULT_N>;

impl<const N: usize> Foo<N> {
    fn new() -> Self {
        println!("N is: {}", N);
        Self
    }
}

fn main() {
    let f:Foo = Foo::new(); // just added the asked type annotation here
}

answered Feb 23, 2022 at 10:28

Peterrabbit

2,2751 gold badge5 silver badges19 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

cdhowie Over a year ago

Interesting, so it looks like currently defaults only apply when naming the type itself and not using a member of it. The N on the right is inferred from the N on the left, and the N on the left is inferred to be the default. This seems like it could be a bug and might be worth opening a github issue to see what the devs think.

Jonathan Woollett-light Over a year ago

Has anyone opened an issue on github in regards to this? If not, I can.

Overlook Motel Over a year ago

@JonathanWoollett-light Sounds like no-one did. Did you?

Collectives™ on Stack Overflow

Rust: "cannot infer the value of const parameter" when a default is provided

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related