implicit Deref coercion not happening

[rbtcollins]

use std::path::{Path,PathBuf};

#[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
pub(crate) struct PathBasedToolchain(PathBuf);

impl TryFrom<&Path> for PathBasedToolchain {
    type Error = anyhow::Error;

    fn try_from(value: &Path) -> std::result::Result<Self, Self::Error> {
                Ok(PathBasedToolchain(value.into()))
    }
}


fn main() -> anyhow::Result<()> {
    let path = PathBuf::new();
    let t =  PathBasedToolchain::try_from(&path);
    let u =  PathBasedToolchain::try_from(&path as &Path);
    
    Ok(())
}

(playground link)[https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=53696b2684233ffd81680129f6c076d2]

I expected to see this happen: compile with no error (because of https://doc.rust-lang.org/book/ch15-02-deref.html#implicit-deref-coercions-with-functions-and-methods and https://doc.rust-lang.org/stable/std/path/struct.PathBuf.html#impl-Deref-for-PathBuf ).

Instead, this happened:

Compiling playground v0.0.1 (/playground)
error[[E0277]](https://doc.rust-lang.org/nightly/error_codes/E0277.html): the trait bound `PathBasedToolchain: From<&PathBuf>` is not satisfied
  --> src/lib.rs:17:43
   |
17 |     let t =  PathBasedToolchain::try_from(&path);
   |              ---------------------------- ^^^^^ the trait `From<&PathBuf>` is not implemented for `PathBasedToolchain`
   |              |
   |              required by a bound introduced by this call
   |
   = help: the trait `TryFrom<&Path>` is implemented for `PathBasedToolchain`
   = note: required for `&PathBuf` to implement `Into<PathBasedToolchain>`
   = note: required for `PathBasedToolchain` to implement `TryFrom<&PathBuf>`

error[[E0277]](https://doc.rust-lang.org/nightly/error_codes/E0277.html): the trait bound `PathBasedToolchain: From<&PathBuf>` is not satisfied
  --> src/lib.rs:17:14
   |
17 |     let t =  PathBasedToolchain::try_from(&path);
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<&PathBuf>` is not implemented for `PathBasedToolchain`
   |
   = help: the trait `TryFrom<&Path>` is implemented for `PathBasedToolchain`
   = note: required for `&PathBuf` to implement `Into<PathBasedToolchain>`
   = note: required for `PathBasedToolchain` to implement `TryFrom<&PathBuf>`

Meta

rustc --version --verbose:

Nightly channel

Build using the Nightly version: 1.70.0-nightly

(2023-03-31 5e1d3299a290026b8578)

This doesn't cause a backtrace from the compiler.

Backtrace

<backtrace>

[SkiFire13]

AFAIK this is expected, the compiler doesn't do coercions when matching traits (third paragraph), i.e. here the compiler is looking for an implementation of TryFrom<typeof(&path)> for PathBasedToolchain (the syntax typeof(&path) doesn't exist but I hope the meaning is clear), but that doesn't exist, and the implementation of TryFrom<&Path> does not constitute an implementation of TryFrom<&PathBuf> too.

[rbtcollins]

Thank you. It might be nice to link to the nomicon from the docs on implicit coercions, because while the nomicon is clear, I didn't find it after non-trivial googling.

[chenyukang]

But why there are two almost the same diagnostics show up? (only the span is different)
I think the second one could be removed.

[Spartan2909]

I've opened rust-lang/book#3670 to try and clarify this behaviour.

[Aditya-PS-05]

@rustbot claim

[oli-obk]

Unassigning due to inactivity

[p172913]

@rustbot claim