Unused self in trait impl generates warning

[aakoshh]

Aim

On my machine tests::execution_success::test_trait_impl_base_type failed with the following error:

command=`"/Users/aakoshh/Work/aztec/noir/target/debug/nargo" "--program-dir" "/Users/aakoshh/Work/aztec/noir/test_programs/execution_success/trait_impl_base_type" "execute" "--force"`
code=1
stdout=""
stderr=```
warning: unused variable self
   ┌─ /Users/aakoshh/Work/aztec/noir/test_programs/execution_success/trait_impl_base_type/src/main.nr:38:17
   │
38 │     fn to_field(self) -> Field {
   │                 ---- unused variable 

Expected Behavior

The test should compile without warning.

Bug

The method is an implementation of Fieldable::to_field. We can't use _self to silence this, and Rust does not warn about it.

To Reproduce

cargo test -p nargo_cli test_trait_impl_base_type

Workaround

Yes

Workaround Description

Add let _ = self; to the method body.

Another one suggested in the comments below is to use _self: Self (or _: Self).

Additional Context

No response

Project Impact

None

Blocker Context

No response

Nargo Version

No response

NoirJS Version

No response

Proving Backend Tooling & Version

No response

Would you like to submit a PR for this Issue?

Yes

Support Needs

No response

[jfecher]

The method is an implementation of Fieldable::to_field. We can't use _self to silence this, and Rust does not warn about it.

Why can't we use _self: Self?

[aakoshh]

You're right, if we specify the type as : Self we can use _self, I haven't tried that 👍

I meant that if I just try to use _self , the compiler says:

`Fieldable::to_field` expects 1 parameter, but this method has 0`.

NB the Rust compiler doesn't seem to like _self: Self:

method `to_field` has a `self` declaration in the trait, but not in the impl
expected `self` in impl`

[jfecher]

NB the Rust compiler doesn't seem to like _self: Self:

Yep, this is a difference between Rust and Noir. Noir is a bit more permissive with methods, you don't need a self parameter to call something as a method. Just an impl and the first type must match.