Does not fail on multiple methods definition

[rom1v]

This obviously fails to compile:

struct X;
impl X { 
   fn f(&self) {}
}
impl X {
   fn f(&self) {}
}

error[E0592]: duplicate definitions with name `f`
 --> a.rs:3:4
  |
3 |    fn f(&self) {}
  |    ^^^^^^^^^^^^^^ duplicate definitions for `f`
...
6 |    fn f(&self) {}
  |    -------------- other definition for `f`

But this does not:

struct X;
impl X {
   fn f(&self) {}
}
macro_rules! do_impl {
    ($name: ident) => {
        impl $name {
            fn f(&self) {}
        }
    }
}
do_impl!(X);

unless you use it:

fn main() {
    X.f()
}

error[E0034]: multiple applicable items in scope
  --> a.rs:14:7
   |
14 |     X.f()
   |       ^ multiple `f` found
   |
note: candidate #1 is defined in an impl for the type `X`
  --> a.rs:3:4
   |
3  |    fn f(&self) {}
   |    ^^^^^^^^^^^
note: candidate #2 is defined in an impl for the type `X`
  --> a.rs:8:13
   |
8  |             fn f(&self) {}
   |             ^^^^^^^^^^^
...
12 | do_impl!(X);
   | ------------ in this macro invocation

I think it should fail even when we don't use the duplicate function.

[Centril]

cc @petrochenkov

This compiles all the way back to 1.0.0 (tested with godbolt).

[petrochenkov]

Ugh, fn check_for_common_items_in_impls compares Idents (which are not equal if they come from different macros).

I would actually check the intermediate versions of Rust between 1.0 and 1.33, IIRC this error should be impossible in some version range due to how identifiers worked back then.

Slightly more minimized reproduction:

struct X;

impl X {
   fn f() {}
}

macro_rules! do_impl { () => {
    impl X {
       fn f() {}
    }
}}
do_impl!();

fn main() {}

[petrochenkov]

Fixed in #58948

[petrochenkov]

The regression was introduced in Rust 1.29, the error was reported before that.
(This case became an error in Rust 1.18 after a long deprecation lint period.)