Non-unique argument names should not be allowed in a trait method

[greenhat]

I tried this code (see the A::f arguments have the same names - a):

  trait A {
      fn f(a: u32, a: u32) -> u32;
  }

  struct B;

  impl A for B {
      fn f(a: u32, b: u32) -> u32 {
          a + b
      }
  }

  fn main() {
      println!("{}", B::f(1, 2));
  }

I expected to see this happen: the compiler throws an error. the compiler shows a warning (if enabled). EDIT: The hard error is too much. The lint fits better. See the comments below.

Instead, this happened: the compiler accepted this code.

Meta

rustc --version --verbose:

rustc 1.67.0-nightly (6284998a2 2022-11-12)
binary: rustc
commit-hash: 6284998a2677d7e3e8420db783f3aa4fd80d7423
commit-date: 2022-11-12
host: x86_64-unknown-linux-gnu
release: 1.67.0-nightly
LLVM version: 15.0.4

[greenhat]

I'd like to take a stab at it.

[compiler-errors]

This is not something that can be turned into a hard-error straight away, since it would break backwards compatibility. This should probably be made a lint first, then discussion can be had whether or not to promote it to a hard error.

[Urgau]

This has been valid code (no compiler error and no warnings) since Rust 1.0: https://rust.godbolt.org/z/7daPYqce7

And as @compiler-errors said should really be a lint first, whenever it should be enable by default or made into a error is a question for later.

[est31]

It used to be allowed to have no parameter name at all, so fn(u32, u32) -> u32; until that was deprecated for editions 2018+, see #41686 .

[greenhat]

Got it. I'll start with a lint.

[petrochenkov]

Duplicate of #33995.
I'm not even sure it's something that needs to be fixed.

[greenhat]

Duplicate of #33995. I'm not even sure it's something that needs to be fixed.

I stumbled upon it at bytecodealliance/wasmtime#5259, and I would appreciate rustc's help to avoid such cases.