Very confusing error on attempt to pass `path::Path` by value

[kmcallister]

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

[brson]

I've hit this too and was completely baffled about what the compiler was saying.

[Byron]

Same here, this was my code:

pub fn assure_config_dir_exists(dir: &str) -> Result<Path, ArgumentError> {
    Ok(Path::new(dir))
}

This works. Something that I didn't expect is that Path::new(...) returns a borrow.

pub fn assure_config_dir_exists(dir: &str) -> Result<&Path, ArgumentError> {
    Ok(Path::new(dir))
}

[marcusklaas]

Can confirm that this behaviour is still present in rustc 1.2.0-nightly (9cb7b3ffb 2015-06-01) (built 2015-06-01).

[fcourchesne]

Confirmed in rustc 1.0 and 1.2.0-nightly (613e57b 2015-06-01) (built 2015-06-02).

[luke-gru]

I just hit this and was confused as well. rustc 1.2.0-nightly (c6b148337 2015-06-12)

[aturon]

Nominating. This seems like an important error to try to improve.

triage: I-nominated

[arielb1]

The issue here is how we report errors on trait-matching in the presence of nested obligations - we report the predicate that failed to shallowly match and not the original obligation. We essentially have:

struct Path([u8]);

unsafe impl Sized for Path where foreach_field!(_: T in Struct, T: Sized) {}
// expanded:
unsafe impl Sized for Path where [u8]: Sized {}

and we require that Path: Sized - which then requires [u8]: Sized, which doesn't hold.

[nikomatsakis]

All right, I'll give it P-high. I've been thinking of tackling this issue that @arielb1 is describing as part of another change. This is kind of a dup but I'm going to keep it separate.

triage: P-high

[liigo]

Does #26617 fix this issue?