Unexpected error when resolving bounds involving associated types
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A-impl-trait
A-trait-system
C-bug
fixed-by-next-solver
T-compiler
- Dominant language
- Rust
- Stars
- 119k
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Description
I am trying to compile the following code
struct MyBlock;
trait Block {
type Header;
}
impl Block for MyBlock {
type Header = ();
}
trait Header {}
impl Header for () {}
trait FullBlock: Block<Header: Header> {}
impl<T> FullBlock for T where T: Block<Header: Header> {}
trait Primitives {
type Block;
}
trait FullPrimitives: Primitives<Block: FullBlock<Header = Self::Header>> {
type Header;
}
impl<T> FullPrimitives for T where T: Primitives<Block: FullBlock> {
type Header = <<T as Primitives>::Block as Block>::Header;
}
fn test<P: FullPrimitives<Block = MyBlock>>() {}
The compilation is failing with:
error[E0283]: type annotations needed: cannot satisfy `MyBlock: FullBlock`
--> src/main.rs:30:12
|
30 | fn test<P: FullPrimitives<Block = MyBlock>>() {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: cannot satisfy `MyBlock: FullBlock`
note: required by a bound in `FullPrimitives`
--> src/main.rs:22:41
|
22 | trait FullPrimitives: Primitives<Block: FullBlock<Header = Self::Header>> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `FullPrimitives`
For more information about this error, try `rustc --explain E0283`.
error: could not compile `my_project` (bin "my") due to 1 previous error
However, if I relax the FullBlock trait requirements it compiles fine
-trait FullBlock: Block<Header: Header> {}
-impl<T> FullBlock for T where T: Block<Header: Header> {}
+trait FullBlock: Block {}
+impl<T> FullBlock for T where T: Block {}
Meta
rustc --version --verbose:
rustc 1.84.0-nightly (c1db4dc24 2024-10-25)
binary: rustc
commit-hash: c1db4dc24267a707409c9bf2e67cf3c7323975c8
commit-date: 2024-10-25
host: aarch64-apple-darwin
release: 1.84.0-nightly
LLVM version: 19.1.1
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 example from src/main.rs with the reported nightly rustc, then compare it with the relaxed FullBlock bounds. Trace how the compiler handles the associated-type and nested trait bounds, and consider the original example fixed when it compiles without the erroneous E0283 diagnostic and has regression coverage.
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
- 42/100