#[pyfunction] inside #[pymethods] could emit a better error

[davidhewitt]

Consider this code:

#[pyo3::pymodule]
mod pyo3_scratch {
    use pyo3::prelude::*;

    #[pyclass]
    struct Foo {}

    #[pymethods]
    impl Foo {
        #[pyfunction]
        fn bug() {}
    }
}

The #[pyfunction] inside #[pymethods] is an innocent user error that I've seen by folks new to PyO3. At the moment this creates a load of confusing garbage errors (due to the #[pyfunction] implementation details):

Click to show error details

error: module is not supported in `trait`s or `impl`s
  --> src/lib.rs:10:9
   |
10 |         #[pyfunction]
   |         ^^^^^^^^^^^^^
   |
   = help: consider moving the module out to a nearby module scope
   = note: this error originates in the attribute macro `pyfunction` (in Nightly builds, run with -Z macro-backtrace for more info)

error: implementation is not supported in `trait`s or `impl`s
  --> src/lib.rs:10:9
   |
10 |         #[pyfunction]
   |         ^^^^^^^^^^^^^
   |
   = help: consider moving the implementation out to a nearby module scope
   = note: this error originates in the attribute macro `pyfunction` (in Nightly builds, run with -Z macro-backtrace for more info)

error: static method needs #[staticmethod] attribute
  --> src/lib.rs:11:9
   |
11 |         fn bug() {}
   |         ^^

error[E0425]: cannot find value `bug` in this scope
  --> src/lib.rs:11:12
   |
11 |         fn bug() {}
   |            ^^^
   |            |
   |            an associated function by that name is available on `Self` here
   |            not found in this scope

For more information about this error, try `rustc --explain E0425`.

---

We should be able to detect the #[pyfunction] attribute the same way declarative modules detect #[pyclass] / #[pyfunction] / #[pymodule] attributes. In this case, we should emit a helpful error like "functions inside #[pymethods] do not need to be annotated with #[pyfunction]".

I think this is a good first issue for someone wanting to improve the macro code.