Error code E0281

Note: this error code is no longer emitted by the compiler.

You tried to supply a type which doesn't implement some trait in a location which expected that trait. This error typically occurs when working with Fn-based types. Erroneous code example:

fn foo<F: Fn(usize)>(x: F) { }

fn main() {
    // type mismatch: ... implements the trait `core::ops::Fn<(String,)>`,
    // but the trait `core::ops::Fn<(usize,)>` is required
    // [E0281]
    foo(|y: String| { });

The issue in this case is that foo is defined as accepting a Fn with one argument of type String, but the closure we attempted to pass to it requires one arguments of type usize.