Stable regression in tcod-0.8.0 resolving AsRef<Path>

[brson]

Originally reported by crater.

tcod-0.8.0 builds on 1.4 but not 1.5. The errors (two of them) are:

brian@ip-10-235-20-183:~/dev/tcod-0.8.0⟫ cargo build
   Compiling tcod-sys v3.0.2
   Compiling tcod v0.8.0 (file:///mnt/dev/tcod-0.8.0)
src/console.rs:479:39: 479:62 error: mismatched types:
 expected `&std::path::Path`,
    found `&core::convert::AsRef<std::path::Path>`
(expected struct `std::path::Path`,
    found trait core::convert::AsRef) [E0308]
src/console.rs:479                 Root::set_custom_font(self.font_path.as_ref(),
                                                         ^~~~~~~~~~~~~~~~~~~~~~~
src/console.rs:479:39: 479:62 help: run `rustc --explain E0308` to see a detailed explanation
src/console.rs:486:60: 486:70 error: no method named `as_bytes` found for type `&core::convert::AsRef<str>` in the current scope
src/console.rs:486             let c_title = CString::new(self.title.as_ref().as_bytes()).unwrap();
                                                                              ^~~~~~~~~~
error: aborting due to 2 previous errors
Could not compile `tcod`.

[brson]

You can get the source with curl -sSfL https://crates.io/api/v1/crates/tcod/0.8.0/download > tcod-0.8.0.tar.gz. Note that you'll have to edit Cargo.toml to remove the path line in the tcod-sys dep.

[brson]

I've updated the OP to indicate that this is actually a regression from 1.4->1.5 stable.

[nikomatsakis]

likely related to adding an AsRef impl?

cc @alexcrichton

[aturon]

Note that self.font_path has type Box<AsRef<Path> + 'a>, and the error we're getting here is connected to some intermediate trait object.

I don't think this is related to new AsRef impls or anything like that. Possibly an issue with method dispatch choking on the multiple layers at play here?

[brson]

We can try to bisect.

[aturon]

@brson OK -- FWIW, the current behavior looks buggy, so we can can also just try to reduce the test and make a fix directly.

[nikomatsakis]

Probably a good starting point would be trying to create a minimized test case.

[bltavares]

Would this be a minimized test case?

use std::path::Path;

struct Root<'a> {
    font_path: Box<AsRef<Path> + 'a>
}

fn main() {
    let path = Path::new("/");
    let root = Root {
        font_path: Box::new(&path)
    };

    let path_ref : &Path = root.font_path.as_ref();
}

I get the following with rustc 1.7.0-nightly (d5e229057 2016-01-04)

30744.rs:13:28: 13:51 error: mismatched types:
 expected `&std::path::Path`,
    found `&core::convert::AsRef<std::path::Path>`
(expected struct `std::path::Path`,
    found trait core::convert::AsRef) [E0308]
30744.rs:13     let path_ref : &Path = root.font_path.as_ref();
                                       ^~~~~~~~~~~~~~~~~~~~~~~
30744.rs:13:28: 13:51 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error

[pnkfelix]

triage: P-high

someone should indeed try to figure out what the problem is; whether the test case in the previous comment is a representative (i.e. try it against Rust 1.4 or so), et cetera.

[bltavares]

I was able to confirm that the code fails to compile on rustc 1.5.0 (3d7cd77e4 2015-12-04) but compiles on rustc 1.4.0 (8ab8581f6 2015-10-27).

On January 7, 2016 7:07:41 PM GMT-02:00, Felix S Klock II notifications@github.com wrote:

triage: P-high

someone should indeed try to figure out what the problem is; whether
the test case in the previous comment is a representative (i.e. try it
against Rust 1.4 or so), et cetera.

---

Reply to this email directly or view it on GitHub:
#30744 (comment)

Sent from my Android device with K-9 Mail. Please excuse my brevity.

[brson]

@bltavares Thanks for the investigation!

[aturon]

I'm going to look into this.

[arielb1]

I am quite sure this impl is guilty:

#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
impl<T: ?Sized> AsRef<T> for Box<T> {
    fn as_ref(&self) -> &T {
        &**self
    }
}

[arielb1]

I am quite sure we have seen this issue before - and our consensus is that methods like as_ref should not be used in non-generic code, because autoderef may start picking the wrong method. This is however an annoying gotcha in Rust. Better add it to the FAQ.

[aturon]

@arielb1 Good catch; I'm sure you're correct, and I agree with the analysis. Thanks!

[brson]

Thanks everyone for investigating.