Misleading error when attempting to use `macro_rules` as an attribute

[kpreid]

Code

Code provided by Discord user baltdev on the Rust Programming Language Discord server (message link).

macro_rules! sample { () => {} }

#[sample]
pub struct S {}

Current output

error: cannot find attribute `sample` in this scope
 --> src/lib.rs:3:3
  |
3 | #[sample]
  |   ^^^^^^ consider moving the definition of `sample` before this call
  |
note: a macro with the same name exists, but it appears later at here
 --> src/lib.rs:1:14
  |
1 | macro_rules! sample { () => {} }
  |              ^^^^^^

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

Desired output

error: `sample` is not an attribute macro
help: `macro_rules!` macros cannot be used as attribute macros

Rationale and extra context

The error message is about a missing definition that is not missing, and incorrectly suggests that reordering the code might help. It should instead explain the fundamental problem with what is being attempted.

This issue is moot if rust-lang/rfcs#3697 is implemented.

Other cases

Rust Version

1.82.0

Anything else?

No response

[joshtriplett]

@kpreid It's not moot when RFC 3697 gets implemented. The error message is still misleading and should be improved. It'll just be able to say that the macro doesn't have any attr rules, rather than that this isn't possible.