Use Span::mixed_site to avoid let unit value warnings

[jieyouxu]

Using Span::mixed_site changes the syntax context of the generated responder outcome expr to identify as macro-expanded, helping the clippy::let_unit_value lint and a potential unit_bindings rustc lint to avoid linting on generated code.

Closes #2568.
Avoids a potential rustc unit_bindings lint (if it is merged at rust-lang/rust#112380).

[SergioBenitez]

Is this correct? Doesn't this change hygiene?

[jieyouxu]

Is this correct? Doesn't this change hygiene?

It does indeed change hygiene AFAIUI, in that it becomes mixed hygienic. That is,

Procedural macro producing tokens with Span::mixed_site() spans should be equivalent to a macro_rules macro producing the same tokens.

My understanding (might be wrong here) is that originally the proc_macro was unhygienic w.r.t. local variables (which might be why ___responder has the three underscores?) and defaults to Span::call_site hygiene. Using Span::mixed_site().located_at(ret_span) would change the macro from being unhygienic w.r.t. local variables to being mixed hygienic w.r.t. local variables but retain the line/column information of ret_span.

(If my understanding is correct, there probably is a couple more places that probably should be Span::mixed_site instead of directly forwarding the user code's span which has Span::call_site hygiene by default, since Span::call_site hygiene would expose local bindings introduced within the proc-macros to user code I think?)

[SergioBenitez]

I've merged this. Let's see how it goes!