Confusing error message when calling f(AsRef<...>, &str) with (str, &str)

[DevJac]

The following code produces a confusing error message:

use std::convert::AsRef;
use std::path::Path;

fn main() {
    foo("bar", &"baz");
    //         ^^^^^^ doesn't have a size known at compile-time???
    // Error message will clearly show that the second argument is the problem,
    // but the first argument is the problem.
}

fn foo(_bar: &dyn AsRef<Path>, _baz: &str) {}

Rust Playground link

The confusing error when compiling the above code will be:

error[E0277]: the size for values of type `str` cannot be known at compilation time
 --> src/main.rs:5:16
  |
5 |     foo("bar", &"baz");
  |                ^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `str`
  = 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 for the cast to the object type `dyn std::convert::AsRef<std::path::Path>`

I first observed this error message behavior with Rust 1.39.

[jonas-schievink]

Can you explain what exactly you find confusing and why?

[DevJac]

The error message says that the second argument in the function call is the problem, but this is not the case. It is the first argument in the function call which is the problem.

Furthermore, the error points exactly at the &str value and says it "doesn't have a size known at compile-time", which is confusing because references do have a known size.

I believe the problem is only that the error points to the wrong argument. Everything the error says make sense if applied to the first argument, but the error is pointing at the second argument.

[jonas-schievink]

Thanks for explaining, that makes sense. Reclassifying as a bug since the span is wrong.