CODEWITHSUNDEEP
rust
phip1611
phip1611

Reputation: 6170

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

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();
}

Upvotes: 6

Views: 1578

Answers (1)

Peterrabbit
Peterrabbit

Reputation: 2247

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
}

Upvotes: 4

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