use std::path::Path;

fn f(p: Path) { }

produces

foo.rs:3:6: 3:7 error: the trait `core::marker::Sized` is not implemented for the type `[u8]` [E0277]
foo.rs:3 fn f(p: Path) { }
              ^
foo.rs:3:6: 3:7 note: `[u8]` does not have a constant size known at compile-time
foo.rs:3 fn f(p: Path) { }
              ^

We should give an explicit hint that Path should be passed by reference. Also the error does not mention Path and the type u8 does not appear in the program.

UPDATE: The message now includes a full stack trace of types, but it would be better to start out with the type the user knows about (Path, in this case) and only discuss [u8] last. -- @nikomatsakis