Cannot coerce dyn AsRef

[clarfonthey]

Really weird example:

use std::path::Path;
fn main() {
    let boxed_path: Box<Path> = Path::new("test").to_path_buf().into_boxed_path();
    let as_ref_works: &Path = boxed_path.as_ref();
    //let lots_of_question_marks: Box<dyn AsRef<Path>> = boxed_path;
}

Uncommenting the last line gives:

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/main.rs:5:56
  |
5 |     let lots_of_question_marks: Box<dyn AsRef<Path>> = boxed_path;
  |                                                        ^^^^^^^^^^ borrow the `Path` instead
  |
  = help: within `std::path::Path`, the trait `std::marker::Sized` is not implemented for `[u8]`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: required because it appears within the type `std::path::Path`
  = note: required for the cast to the object type `dyn std::convert::AsRef<std::path::Path>`

error: aborting due to previous error

I genuinely don't know what's going on here, but this seems to be a bug in both coercion and diagnostics.

This is on the latest nightly.

[clarfonthey]

Larger code example to show context:

let temp_dir: Box<dyn AsRef<Path>> = match matches.value_of_os("cache-dir") {
    Some(c) => Path::new(c).to_path_buf().into_boxed_path(),
    None => Box::new(TempDir::new("temp")?)
};

This is code that parses a clap argument and either a) uses the argument verbatim or b) loads a TempDir and uses that instead. In this case, it makes sense to use dyn AsRef<Path> because I want the TempDir to be destroyed at the end of execution, while also allowing a simple path to be used instead. I could store both a &Path and a TempDir but I figured I'd try my hand at this code to make things a bit easier.

Turns out that it isn't much easier, if the compiler breaks.

[kryptan]

The issue here is that you cannot put unsized objects behind dyn Trait. For example the following fails to compile with the same error:

let _: &dyn std::fmt::Debug = "a";

It is not clear to me what is the reason for this.

[clarfonthey]

I believe that it's a strict limitation on the size of fat pointers. Unsized types require extra information to encode what their size is, and ditto for polymorphism. To get around it, you have to take a reference to the reference to the unsized type, which would be a thin pointer, and that can have the polymorphic info attached.

That in particular makes a lot of sense from an implementation perspective, but it's very, very confusing from a user perspective. I'm not 100% sure what to do about it.

Perhaps discussion of this may be good to add to rfcs#2580.

[QuineDot]

See also #88141