Invalid suggestion for ambiguous associated type

[junbl]

I tried this code:
Playground link

trait Parent {
    type Assoc;
}
trait Child: Parent {
    // send bound is not necessary for the error, but that's why this associated
    // type exists at all
    type Assoc: Send; 
    fn get_associated() -> <Self as Child>::Assoc;
}


pub struct Implementor;
pub struct AssociatedNamedFields {
    field: i32,
}


impl Parent for Implementor {
    type Assoc = AssociatedNamedFields;
}

impl Child for Implementor {
    type Assoc = <Self as Parent>::Assoc;
    fn get_associated() -> <Self as Child>::Assoc {
        // this line produces the issue:
        Self::Assoc { field: 1 }
        
        // this is what rustc suggests, which is incorrect:
        // <Self as Child>::Assoc { field: 1 }
    }
}

I expected to see this happen:

rustc wouldn't suggest unstable features - either wouldn't suggest anything or suggest a different fix.

Not sure what the best fix in this case would be - maybe renaming one of the associated types? Or ideally not using it at all.

Instead, this happened:

rustc (and rust-analyzer) suggest the following replacement:

        <Self as Child>::Assoc { field: 1 }

which is not supported syntax:

error[E0658]: usage of qualified paths in this context is experimental
  --> src/main.rs:31:9
   |
31 |         <Self as Child>::Assoc { field: 1 }
   |         ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #86935 <https://github.com/rust-lang/rust/issues/86935> for more information

Side note - interestingly, this only happens for structs with named fields - for tuple or unit structs, rustc just says:

error[E0599]: no associated item named `Assoc` found for struct `Implementor` in the current scope
  --> src/main.rs:33:15
   |
12 | pub struct Implementor;
   | ---------------------- associated item `Assoc` not found for this struct
...
33 |         Self::Assoc
   |               ^^^^^ associated item not found in `Implementor`

which is a confusing message by itself.

Meta

rustc --version --verbose:

rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: x86_64-unknown-linux-gnu
release: 1.81.0
LLVM version: 18.1.7

I do see the same issue on nightly and beta. I also checked the last several versions and all of them had the same issue.

Backtrace

➜  dev/rust/assoc (main) ✗ RUST_BACKTRACE=1 cargo build  
   Compiling assoc v0.1.0 (/home/erhodes/dev/rust/assoc)
error[E0221]: ambiguous associated type `Assoc` in bounds of `Self`
  --> src/main.rs:30:28
   |
2  |     type Assoc;
   |     ---------- ambiguous `Assoc` from `Parent`
...
7  |     type Assoc: Send;
   |     ---------------- ambiguous `Assoc` from `Child`
...
30 |     fn get_associated() -> Self::Assoc {
   |                            ^^^^^^^^^^^ ambiguous associated type `Assoc`
   |
help: use fully-qualified syntax to disambiguate
   |
30 |     fn get_associated() -> <Self as Child>::Assoc {
   |                            ~~~~~~~~~~~~~~~~~
help: use fully-qualified syntax to disambiguate
   |
30 |     fn get_associated() -> <Self as Parent>::Assoc {
   |                            ~~~~~~~~~~~~~~~~~~

error[E0221]: ambiguous associated type `Assoc` in bounds of `Self`
  --> src/main.rs:31:9
   |
2  |     type Assoc;
   |     ---------- ambiguous `Assoc` from `Parent`
...
7  |     type Assoc: Send;
   |     ---------------- ambiguous `Assoc` from `Child`
...
31 |         Self::Assoc { field: 1 }
   |         ^^^^^^^^^^^ ambiguous associated type `Assoc`
   |
help: use fully-qualified syntax to disambiguate
   |
31 |         <Self as Child>::Assoc { field: 1 }
   |         ~~~~~~~~~~~~~~~~~
help: use fully-qualified syntax to disambiguate
   |
31 |         <Self as Parent>::Assoc { field: 1 }
   |         ~~~~~~~~~~~~~~~~~~

For more information about this error, try `rustc --explain E0221`.
error: could not compile `assoc` (bin "assoc") due to 2 previous errors