Automatically provide fallback constructor for all-public Rust structs

In my application I have many "plain-old-data" structs, i.e. structs with only public fields that are supposed to be instantiated with the StructName { field1: ..., field2: ... } syntax in Rust.

Making those structs usable from Python-land currently involves a bit of avoidable boilerplate:

struct StructName {
    #[pyo3(get, set)] pub field1: u32,
    #[pyo3(get, set)] pub field1: u32,
}

// this is the boilerplate I'm talking about
#[pymethods]
impl StructName {
    #[new]
    pub fn new(field1: u32, field2: u32) -> Self {
        Self { field1, field2 }
    }
}

This kind of class is equivalent to Python's dataclasses. Python's dataclasses include default constructors which is very convenient and sensible. I think it would be nice for PyO3 to do the same and automatically provide default constructors like above, for plain-old-data structs with all pub fields.

Start by tracing how PyO3 handles the #[pymethods] and #[new] entry points for Rust structs. Determine the supported scope for detecting structs whose fields are all public, then verify that such structs receive a constructor matching their fields without the existing boilerplate.