This error means that an incorrect number of generic arguments were provided:
struct Foo<T> { x: T } struct Bar { x: Foo } // error: wrong number of type arguments: // expected 1, found 0 struct Baz<S, T> { x: Foo<S, T> } // error: wrong number of type arguments: // expected 1, found 2 fn foo<T, U>(x: T, y: U) {} fn main() { let x: bool = true; foo::<bool>(x); // error: wrong number of type arguments: // expected 2, found 1 foo::<bool, i32, i32>(x, 2, 4); // error: wrong number of type arguments: // expected 2, found 3 } fn f() {} fn main() { f::<'static>(); // error: wrong number of lifetime arguments: // expected 0, found 1 }