paritytech / paritytech/parity-scale-codec
Invalid bound for for field type like `<T>::A`
Open
Nobody has claimed this yet.
- Dominant language
- Rust
- Stars
- 287
- Forks
- 103
- Avg merge
- 6d 13h
- Merged PRs (30d)
- 1
Description
The following test:
trait Types {
type A;
}
#[derive(Encode, Decode)]
struct Assoc<T: Types> {
a: <T>::A,
}
Fails with:
error: generic parameters on `where` clauses are reserved for future use
--> tests/foo.rs:11:5
|
11 | a: <T>::A,
| ^^^ currently unsupported
error: proc-macro derive produced unparseable tokens
--> tests/foo.rs:9:18
|
9 | #[derive(Encode, Decode)]
| ^^^^^^
error: proc-macro derive produced unparseable tokens
--> tests/foo.rs:9:10
|
9 | #[derive(Encode, Decode)]
| ^^^^^^
error[E0412]: cannot find type `A` in the crate root
--> tests/foo.rs:11:10
|
11 | a: <T>::A,
| ^ not found in the crate root
error[E0282]: type annotations needed
--> tests/foo.rs:10:14
|
10 | struct Assoc<T: Types> {
| ^ cannot infer type for type parameter `T`
This is probably due to a wrong expansion in some of the code in derive/src/trait_bound.rs
Contributor guide
No contributing guide indexed for this repository
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 associated-type example from the issue, then inspect derive/src/trait_bound.rs, where the suspected incorrect expansion is located. Confirm that the generated Encode and Decode derives produce parseable tokens and compile successfully for the Assoc<T: Types> field using ::A.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- tooling
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 42/100