Calling a function within a module derived from concat_idents fails to resolve

[jdm]

mod barfoo {
  fn foo() {}
}

fn main() {
  concat_idents!(bar, foo)::foo();
}

<anon>:5:37: 5:63 error: unresolved name `barfoo`.
<anon>:5          mod barfoo { fn foo() {} }; concat_idents!(bar, foo)::foo()
                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of concat_idents!
<anon>:5:37: 5:63 note: expansion site
<anon>:5:61: 5:66 error: unresolved name `foo`.
<anon>:5          mod barfoo { fn foo() {} }; concat_idents!(bar, foo)::foo()
                                                                      ^~~~~
error: aborting due to 2 previous errors
application terminated with error code 101

[huonw]

This is a dupe of #5941 and (implicitly) #12249. That is, as written, it's parsing as:

mod barfoo {
  fn foo() {}
}

fn main() {
  concat_idents!(bar, foo);

  ::foo();
}

Fixing this by writing (concat_idents!(bar, foo)::foo()) to force it to be a single expression triggers:

8167.rs:6:28: 6:30 error: expected one of `,`, `)` but found `::`
8167.rs:6   (concat_idents!(bar, foo)::foo());
                                     ^~

which is the similarity to #12249: can't use a macro in purely ident position.