Orphan rule checking too strict for associated types

[axos88]

I tried this code:

// trait Crypto is foreign

pub struct MyAccountKey;
pub struct MyCrypto;

impl Crypto for MyCrypto {
  type AccountKey = MyAccountKey
}

// impl<'a> From<&'a <MyAccountKey as Crypto>::AccountKey> for ForeignFoo {
//     fn from(_: &'a <MyAccountKey as Crypto>::AccountKey) -> Self {
//         todo!()
//     }
// }

impl<'a> From<&'a AcmeEspAccountKey> for ForeignFoo {
    fn from(_: &'a AcmeEspAccountKey) -> Self {
        todo!()
    }
}

I expected to see this happen: both commented code and uncommented code should compile fine

Instead, this happened: orphan rule is triggered:

error[E0117]: only traits defined in the current crate can be implemented for types defined outside of the crate
--> src/my_crypto.rs:95:1
|
95 | impl<'a> From<&'a ::AccountKey> for ForeignFoo {
| ^^^^^^^^^---------------------------------------------------^^^^^-------------
| | | |
| | | ForeignFoo is not defined in the current crate
| | <MyAccountKey as Crypto>::AccountKey is not defined in the current crate
| impl doesn't use only types from inside the current crate
|
= note: define and implement a trait or new type instead

Meta

rustc --version --verbose:

rustc 1.66.0-nightly (57f097ea2 2022-10-01)
binary: rustc
commit-hash: 57f097ea25f2c05f424fc9b9dc50dbd6d399845c
commit-date: 2022-10-01
host: x86_64-unknown-linux-gnu
release: 1.66.0-nightly
LLVM version: 15.0.2

Backtrace

N/A

[lcnr]

your snippet fails the orphan check. the orphan check does not look into associated types as it has to run before we are able to do so.

It is therefore not able to tell that <MyAccountKey as Crypto>::AccountKey would be a local type and this is expected behavior. Opened #102734 to potentially improve the error message here