This macro is [parse_quote!] + [quote_spanned!][quote::quote_spanned].

Please refer to each of their documentation.

Example

use quote::{quote, quote_spanned};
use syn::spanned::Spanned;
use syn::{parse_quote_spanned, ReturnType, Signature};

// Changes `fn()` to `fn() -> Pin<Box<dyn Future<Output = ()>>>`,
// and `fn() -> T` to `fn() -> Pin<Box<dyn Future<Output = T>>>`,
// without introducing any call_site() spans.
fn make_ret_pinned_future(sig: &mut Signature) {
    let ret = match &sig.output {
        ReturnType::Default => quote_spanned!(sig.paren_token.span=> ()),
        ReturnType::Type(_, ret) => quote!(#ret),
    };
    sig.output = parse_quote_spanned! {ret.span()=>
        -> ::std::pin::Pin<::std::boxed::Box<dyn ::std::future::Future<Output = #ret>>>
    };
}