Description
There seems to be a bug with implementing the From trait when the type argument is an associated type defined in an external crate.
To reproduce this bug you need to have 2 crates.
the first crate, let's call it aaa, is a library crate and contains the following code in its src/lib.rs file:
pub trait DummyTrait {
type DummyAssociatedType;
}
pub struct DummyStruct;
pub struct DummyAssociatedTypeValue;
impl DummyTrait for DummyStruct {
type DummyAssociatedType = DummyAssociatedTypeValue;
}the second crate, let's call it bbb, is also a library crate. it depends on the aaa crate, and it contains the following code in its src/lib.rs file:
pub struct DummyStructInCrateB;
impl From<<aaa::DummyStruct as aaa::DummyTrait>::DummyAssociatedType> for DummyStructInCrateB {
fn from(value: <aaa::DummyStruct as aaa::DummyTrait>::DummyAssociatedType) -> Self {
todo!()
}
}I expected both crates to compile properly.
Instead, i get the following error when trying to compile crate bbb:
error[E0119]: conflicting implementations of trait `From<DummyStructInCrateB>` for type `DummyStructInCrateB`
--> src/lib.rs:3:1
|
3 | impl From<<aaa::DummyStruct as aaa::DummyTrait>::DummyAssociatedType> for DummyStructInCrateB {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: conflicting implementation in crate `core`:
- impl<T> From<T> for T;
For some reason, the compiler thinks that my implementation conflicts with the blanket implementation, as if it thinks that <aaa::DummyStruct as aaa::DummyTrait>::DummyAssociatedType is somehow the same type as DummyStructInCrateB, which is obviously wrong.
Meta
rustc --version --verbose:
rustc 1.85.1 (4eb161250 2025-03-15)
binary: rustc
commit-hash: 4eb161250e340c8f48f66e2b929ef4a5bed7c181
commit-date: 2025-03-15
host: x86_64-unknown-linux-gnu
release: 1.85.1
LLVM version: 19.1.7
the bug also reproduces on the following nightly version:
rustc 1.87.0-nightly (78948ac25 2025-03-20)
binary: rustc
commit-hash: 78948ac259253ce89effca1e8bb64d16f4684aa4
commit-date: 2025-03-20
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.1