Ambiguous macro call is expanded anyway

[theemathas]

I tried this code:

macro_rules! foo {
    () => {}
}

macro_rules! make_foo {
    () => {
        macro_rules! foo {
            () => {
                compile_error!("why is this expanded?");
            }
        }
    }
}

make_foo!();

foo!();

I got the following compiler output:

error: why is this expanded?
  --> src/lib.rs:9:17
   |
 9 |                 compile_error!("why is this expanded?");
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
17 | foo!();
   | ------ in this macro invocation
   |
   = note: this error originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0659]: `foo` is ambiguous
  --> src/lib.rs:17:1
   |
17 | foo!();
   | ^^^ ambiguous name
   |
   = note: ambiguous because of a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution
note: `foo` could refer to the macro defined here
  --> src/lib.rs:7:9
   |
 7 | /         macro_rules! foo {
 8 | |             () => {
 9 | |                 compile_error!("why is this expanded?");
10 | |             }
11 | |         }
   | |_________^
...
15 |   make_foo!();
   |   ----------- in this macro invocation
note: `foo` could also refer to the macro defined here
  --> src/lib.rs:1:1
   |
 1 | / macro_rules! foo {
 2 | |     () => {}
 3 | | }
   | |_^
   = note: this error originates in the macro `make_foo` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: unused macro definition: `foo`
 --> src/lib.rs:1:14
  |
1 | macro_rules! foo {
  |              ^^^
  |
  = note: `#[warn(unused_macros)]` on by default

For more information about this error, try `rustc --explain E0659`.
warning: `playground` (lib) generated 1 warning
error: could not compile `playground` (lib) due to 2 previous errors; 1 warning emitted

It seems incorrect that foo!() would be expanded, since the macro call is ambiguous, so we don't know which macro call is the right one.

@rustbot labels +A-macros

Meta

Reproducible on the playground with version 1.91.0-nightly (2025-08-09 ca77504943887037504c)

[theemathas]

Oddly enough, this code doesn't produce any errors:

macro_rules! make_foo {
    () => {
        macro_rules! foo {
            () => {
                compile_error!("This is never expanded");
            }
        }
    }
}

make_foo!();

macro_rules! foo {
    () => {}
}

foo!();