E0423 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0423

An identifier was used like a function name or a value was expected and the identifier exists but it belongs to a different namespace.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    struct Foo { a: bool };

    let f = Foo();
    // error: expected function, tuple struct or tuple variant, found `Foo`
    // `Foo` is a struct name, but this expression uses it like a function name
    }

Please verify you didn’t misspell the name of what you actually wanted to use here. Example:

    #![allow(unused)]
    fn main() {
    fn Foo() -> u32 { 0 }

    let f = Foo(); // ok!
    }

It is common to forget the trailing `!` on macro invocations, which would also yield this error:

    #![allow(unused)]
    fn main() {
    println("");
    // error: expected function, tuple struct or tuple variant,
    // found macro `println`
    // did you mean `println!(...)`? (notice the trailing `!`)
    }

Another case where this error is emitted is when a value is expected, but something else is found:

    #![allow(unused)]
    fn main() {
    pub mod a {
        pub const I: i32 = 1;
    }

    fn h1() -> i32 {
        a.I
        //~^ ERROR expected value, found module `a`
        // did you mean `a::I`?
    }
    }