Paths to extension trait functions in diagnostics can be confusing

[vporton]

The following code compiles with a warning (on take function):

use std::ops::Deref;

pub trait MutexGuard<T>: Deref<Target = T> /*+ DerefMut<Target = T>*/ {
    fn set(&mut self, value: T);
    fn take(&self) -> Option<T>;
}

impl<T> MutexGuard<T> for tokio::sync::MutexGuard<'_, T> {
    fn set(&mut self, value: T) {
        *self.deref_mut() = value;
    }

    fn take(&self) -> Option<T> {
        tokio::sync::MutexGuard::take(self)
    }
}

warning: function cannot return without recursing
  --> src/lib.rs:13:5
   |
13 |     fn take(&self) -> Option<T> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
14 |         tokio::sync::MutexGuard::take(self)
   |         ----------------------------------- recursive call site
   |
   = help: a `loop` may express intention better if this is on purpose
   = note: `#[warn(unconditional_recursion)]` on by default

I specified a "full path" (tokio::sync::MutexGuard::take) to take function and it should use that path rather than using a trait.

$ rustc --version
rustc 1.78.0 (9b00956e5 2024-04-29)

[workingjubilee]

@vporton Is there a take function on tokio::sync::MutexGuard?

Not seeing one here? https://docs.rs/tokio/latest/tokio/sync/struct.MutexGuard.html

[workingjubilee]

To be clear, @vporton, what you said would in fact be true if there was in fact a function to call. The detail is that by specifying the path to the type, you still perform lookup on the type, which will prefer to look up an inherent impl, and then will look for a trait impl, in that order, as demonstrated by this Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a6ddeec5f1848915158f205f5b1ffff0

Is there a diagnostic you would prefer? We obviously cannot emit a diagnostic every time someone gives the path to a type and it performs lookup that reaches a trait implementation, as very often that will be intentional. I suppose we could diagnose this specific recursion case?