Coherence rejects otherwise valid impl
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Description
I tried this code:
// in crate a
pub trait Table {}
pub trait IsTable {
type Table: Table;
}
pub struct Wrapper<'a, T>(&'a T);
impl<'a, T> IsTable for Wrapper<'a, T> where T: Table {
type Table = T;
}
pub struct Only<T>(T);
impl<T> Table for Only<T> where T: Table {}
pub trait TableNotEqual<T: Table>: Table {}
// in crate b
struct TableA;
struct TableB;
impl Table for TableA {}
impl Table for TableB {}
impl TableNotEqual<<Wrapper<'static, TableA> as IsTable>::Table>
for Only<<Wrapper<'static, TableB> as IsTable>::Table>
where
for<'a> Wrapper<'a, TableA>: IsTable<Table = TableA>,
for<'a> Wrapper<'a, TableB>: IsTable<Table = TableB>,
{
}
I expected to see this happen: Code compiles without error
Instead, this happened: Compilation fails with the following error:
error[E0117]: only traits defined in the current crate can be implemented for types defined outside of the crate
--> src/lib.rs:9:1
|
9 | impl TableNotEqual<<Wrapper<'static, TableA> as IsTable>::Table>
| ^ ----------------------------------------------------------- `<crate_a::Wrapper<'_, TableA> as crate_a::IsTable>::Table` is not defined in the current crate
| _|
| |
10 | | for Only<<Wrapper<'static, TableB> as IsTable>::Table>
| | -------------------------------------------------- `crate_a::Only` is not defined in the current crate
11 | | where
12 | | for<'a> Wrapper<'a, TableA>: IsTable<Table = TableA>,
13 | | for<'a> Wrapper<'a, TableB>: IsTable<Table = TableB>,
| |_________________________________________________________^
|
= note: impl doesn't have any local type before any uncovered type parameters
= note: for more information see https://doc.rust-lang.org/reference/items/implementations.html#orphan-rules
= note: define and implement a trait or new type instead
The impl is restricted in such a way that the Table types used in the definition are local types. The equivalent impl impl TableNotEqual<TableA> for Only<TableB> {} compiles without error.
Meta
rustc --version --verbose:
rustc 1.92.0-nightly (b925a865e 2025-10-09)
binary: rustc
commit-hash: b925a865e2c9a0aefe5a2877863cb4df796f2eaf
commit-date: 2025-10-09
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.2
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Reproduce the two-crate example from src/lib.rs with rustc 1.92.0-nightly, then compare the failing associated-type impl with the equivalent impl using TableA and TableB. The issue is done when the restricted impl is accepted without weakening the reported orphan-rule checks, with the reproduction covered by an appropriate compiler test.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100