text_signature and raw identifiers don't interact well

[roblabla]

🐛 Bug Reports

The text_signature attributes automatically sets the first line of the python documentation to the function signature, which is supposed to be fnname(arg1, arg2). However, this seems to interact badly with rust's raw identifiers. For instance, a methods defined like this:

#[pymethods]
impl PythonStuff {
    #[text_signature = "()"]
    fn r#match(&self) {}
}

Will have a signature of r#match() instead of the correct match(). This breaks several tools, like sphinx documentation generation.

wrongly documents match's signature as r#match() instead of simply match(). Python script showcasing the issue:

>>> import pyo3_bug
>>> help(pyo3_bug.PythonStuff.match)
Help on method_descriptor:

match(...)
    r#match()
    --

🌍 Environment

• Your operating system and version: Ubuntu 20
• Your python version: Python 3.8.5
• How did you install python (e.g. apt or pyenv)? Did you use a virtualenv?: apt, yes
• Your Rust version (rustc --version): 1.47.0
• Your PyO3 version: 0.12
• Have you tried using latest PyO3 master (replace version = "0.x.y" with git = "https://github.com/PyO3/pyo3")?: yes, same issue.

💥 Reproducing

Here's a quick script that will quickly reproduce the bug. It creates a simple pyo3 module with the r#match method and tries to access its help text.

cargo new --lib pyo3_bug
cd pyo3_bug
echo 'pyo3 = { version = "0.12", features = ["extension-module"] }' >> Cargo.toml
echo '[lib]' >> Cargo.toml
echo 'crate-type = ["rlib", "cdylib"]' >> Cargo.toml
cat > src/lib.rs << HERE
use pyo3::prelude::*;
#[pymodule]
fn pyo3_bug(_py: Python, m: &PyModule) -> PyResult<()> {
    m.add_class::<PythonStuff>()?;
    Ok(())
}
#[pyclass]
struct PythonStuff {}
#[pymethods]
impl PythonStuff {
    #[text_signature = "()"]
    fn r#match(&self) {}
}
HERE
maturin develop
cat << HERE | python3
import pyo3_bug
help(pyo3_bug.PythonStuff.match)
HERE

This script should eventually show:

Help on method_descriptor:

match(...)
    r#match()
    --