Write a python script to generate type assertions for Servo_* functions

[Manishearth]

This is the first step towards fixing #12992

https://github.com/emilio/servo/blob/c7e1a575a5225ccfb00a38fb50e052d986c0a78d/components/style/gecko_bindings/bindings.rs#L802 contains a bunch of Servo_* function declarations. These functions are defined in ports/geckolib/glue.rs.

We need a way of testing that the functions have the same signatures. This can be done by writing code that will not compile if they don't.

The main step here will be to write a python script in gecko_bindings that reads bindings.rs (assume it is in the same directory), and writes a type_assertions.rs file in the same directory. This file just contains code like the following:

fn assert_types() {
    [Servo_Foo, bindings::Servo_Foo];
    [Servo_Bar, bindings::Servo_Bar];
}

for each function in bindings.rs with a Servo_* signature. Be sure to include the generated file in your pull request.

This file can be include!()ed into a test in tests/unit/stylo, where all of the imports have been set up correctly. (This part isn't part of this bug, but feel free to work on it and ask me questions if you have any).

I'll hook it up to the main bindgen script later.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[highfive]

cc @wafflespeanut

[Rafagd]

Is this what you want? #13603

[wafflespeanut]

In the future, please claim the relevant issue before working on it, so that we can reduce duplicate efforts.

[Rafagd]

I'll do it.