E0121 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0121

The type placeholder `_` was used within a type on an item’s signature.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    fn foo() -> _ { 5 } // error

    static BAR: _ = "test"; // error
    }

In those cases, you need to provide the type explicitly:

    #![allow(unused)]
    fn main() {
    fn foo() -> i32 { 5 } // ok!

    static BAR: &str = "test"; // ok!
    }

The type placeholder `_` can be used outside item’s signature as follows:

    #![allow(unused)]
    fn main() {
    let x = "a4a".split('4')
        .collect::<Vec<_>>(); // No need to precise the Vec's generic type.
    }