fn foo(arg: Foo) {} //Foo is a trait
fn bar (arg: [i32]) {}
fn quz (arg: str) {}

All those functions return the following error: the trait core::marker::Sized is not implemented for the type...
This is confusing for a beginner, because it makes you think you need to implement a trait to make it work. The message could suggest using &Foo/&[i32]/&str instead.