Higher ranked types in trait bounds result in confusing diagnostics
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- Dominant language
- Rust
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Description
Code
trait Trait {}
impl Trait for for<'a> fn(&'a u32) {}
fn f() where for<'a> fn(&'a u32): Trait {}
fn main() {
f();
}
Current output
error: implementation of `Trait` is not general enough
--> ./t.rs:9:5
|
9 | f();
| ^^^ implementation of `Trait` is not general enough
|
= note: `for<'a> fn(&'a u32)` must implement `Trait`, for any lifetime `'0`...
= note: ...but `Trait` is actually implemented for the type `for<'a> fn(&'a u32)`
Desired output
error: implementation of `Trait` is not general enough
--> ./t.rs:9:5
|
9 | f();
| ^^^ implementation of `Trait` is not general enough
|
= note: `fn(&'0 u32)` must implement `Trait`, for any lifetime `'0`...
= note: ...but `Trait` is actually implemented for the type `for<'a> fn(&'a u32)`
= note: `where for<'a> fn(&'a u32): Trait` is parsed as `where for<'a> (fn(&'a u32): Trait)`, not `where (for<'a> fn(&'a u32)): Trait`
Rationale and extra context
for<'a> fn(&'a u32) in the note makes it seem like it is right (as it's the same as the type for which the trait is implemented), when we actually want to use the '0. Also it's a good idea to mention how the for binders in where clauses are parsed, since this is not that intuitive (the user could have literally pasted the type from the trait impl).
Other cases
No response
Rust Version
rustc 1.79.0-nightly (ef8b9dcf2 2024-04-24)
binary: rustc
commit-hash: ef8b9dcf23700f2e2265317611460d3a65c19eff
commit-date: 2024-04-24
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.4
Anything else?
No response
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
Compile the provided Rust example with the stated rustc nightly and compare the current and desired diagnostics. Trace the compiler path that produces the “implementation of Trait is not general enough” message. Done means the diagnostic uses the instantiated '0 lifetime and explains how the for binder in the where clause is parsed.
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
- 38/100