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.