Call pinned method from Trait on generic parameter

[Jonathas-Conceicao]

I've found a problem regarding Pin that I just can't solve and the current Nightly compiler gives a misleading suggestion.

This playground example serves well as a minimalist example. I can't manege to call quack on the Animal inside the Transporter even with the restriction to Animal: Quack.

And the Nightly compiler suggests:

   = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following trait defines an item `quack`, perhaps you need to restrict type parameter `Animal` with it:
   |
17 | impl<Animal: Quack + Quack> Quack for Transporter<Animal> {
   |      ^^^^^^^^^^^^^^^

Which clearly doesn't really solve anything.

If a take off the Pin from the Traits and it's impl and just use a &self I can get the expected Quack! from inside the transporter.

The compiler suggestion surely doesn't seams right to me, but what about the Pin? Am I missing something or is there a bug too?

[memoryruins]

The suggestions are strange with other self types as well, e.g. using Box and Arc instead of Pin.

playground 1.39.0-nightly 2019-08-23 9eae1fc0ea9b00341b8f

mod First {
    trait Foo { fn m(self: Box<Self>); }
    fn foo<T: Foo>(a: T) {
        a.m();
    }
}
mod Second {
    trait Bar { fn m(self: std::sync::Arc<Self>); }
    fn bar(b: impl Bar) {
        b.m();
    }
}

help: the following traits define an item `m`, perhaps you need to restrict type parameter `T` with one of them:
  |
3 |     fn foo<T: First::Foo + Foo>(a: T) {
  |            ^^^^^^^^^^^^^^^
3 |     fn foo<T: Second::Bar + Foo>(a: T) {
  |            ^^^^^^^^^^^^^^^^

The above suggests a trait from another module with a different name

help: the following traits define an item `m`, perhaps you need to restrict type parameter `impl Bar` with one of them:
   |
9  |     fn bar(b: impl Bar: First::Foo + {
   |               ^^^^^^^^^^^^^^^^^^^^^^
9  |     fn bar(b: impl Bar: Second::Bar + {
   |               ^^^^^^^^^^^^^^^^^^^^^^^

and the next suggestion isn't syntactically correct.

[Aaron1011]

@Jonathas-Conceicao: as @memoryruins mentioned, the diagnostic suggestions are wrong in this case.

You're trying to go from a Pin<&MyType> to a Pin<&MyField>, where &MyField is a reference to a field of MyType. This is known as a pinned projection, and has additional requirements that you must meet in order for it to be sound.

<shameless-plug>You might want to take a look at the pin-project crate (disclaiminer: I'm a contributor), which makes it easy to write safe pin projections.</shameless-plug>

[taiki-e]

The diagnostic issue seems to dup of #57994.

[Jonathas-Conceicao]

@Aaron1011, thank you for the explanation and the suggestion, that does indeed solve the error.

Regarding the suggestion, it really does seams to be a duplicate, will be closing this in favor of #57994.