Unhelpful rustdoc [src] link for struct generated by macro

[dtolnay]

In dtolnay/syn#304 we have a macro_rules macro that adds some derive attributes to most of our syntax tree structs. Conceptually it looks like this:

macro_rules! ast_struct {
    (pub struct $name:ident $body:tt) => {
        #[derive(Clone)]
        pub struct $name $body
    }
}

ast_struct! {
    pub struct S {
        /* ... */
    }
}

In our case the macro definition and invocation are always in different files but the code above is sufficient to reproduce the issue. If you document this code with cargo doc, the [src] link at the top of struct.S.html points to the line pub struct $name $body, as far as I can tell because this is where the first non-attribute token (pub) of the final struct definition came from.

I propose that it would be more often correct to have [src] point to wherever the *name* token of the item came from, in this case the $name token which originates from the line pub struct S.

[jyn514]

Closing as duplicate of #39726