Make source references for generated builder methods link to original field/parameter location instead of macro callsite

[Eisverygoodletter]

Overview

Make generated builder methods have source links that don't just go to the macro callsite.

Details

This feature request is similar to #57 and follows from #341.

#341 made it so that methods attached to some struct MyStruct, i.e. something along the lines of MyStruct::do_thing(..) -> MyStructDoThingBuilder<...> link to the original definition of MyStruct::do_thing instead of the macro callsite. This has not been implemented for methods on the builder, which is what this feature request covers. For example, consider

use bon::Builder;
#[derive(Builder)]
struct MyStruct {
    property: i32,
}

The generated docs for MyStructBuilder::property(self, value: i32) -> MyStructBuilder<SetProperty<S>> link to the callsite line

#[derive(Builder)]

instead of the line where property came from

    property: i32,

. This issue is also present with params, with

#[bon]
impl MyStruct {
    #[builder]
    pub fn many_params(
        a: i32,
        b: i32,
        c: i32
    ) -> i32 {
        a + b + c
    }
}

Generating a a(self, value: i32) -> ThingManyParamsBuilder<SetA<S>> which links to callsite

#[bon]

instead of the line where param a was defined

         a: i32,

Examples

An example can be seen at https://docs.rs/remoteit-api/latest/remoteit_api/struct.R3ClientBuilder.html#method.credentials where clicking the "Source" links to https://docs.rs/remoteit-api/latest/src/remoteit_api/lib.rs.html#83 instead of https://docs.rs/remoteit-api/latest/src/remoteit_api/lib.rs.html#85.

Additional note

I'm not sure how easy or consistent implementing this will be. Based off implementing #341 it might be as easy as just keeping a span around, but I'm not sure given that we will need to traverse each parameter name and struct field to get the spans.

[Veetaha]

Makes sense. Let me know if you'd like to work on this, and I'll include it as part of the same patch as #341, otherwise I can release #341 separately

[Eisverygoodletter]

I'll probably leave it for someone else or do it way later. This one is (to me) a lot more complex and I don't have the time for it for now.

[Veetaha]

Created a small draft #345, which works for setters and links to original fields/arguments

The main nuance is that the prefix of the generated method i.e. visibility and const keywords need to also use the member's span, so I had to respan them with set_span() where appropriate.

        // Respan the tokens to make sure the function's signature begins with
        // the member's span. This is important to make rustdoc's source links
        // point to the original member's location.
        let fn_modifiers = quote!(#vis #const_).into_iter().map(|mut token| {
            token.set_span(span);
            token
        });

I'll finish it today or over the weekends and make a patch

[Veetaha]

This was released in v3.8.0. I decided to do a minor bump because of the amount of new potential warnings that may be emitted from the generated code which I also observed when doing #345