Non-doc comments are whitespace, you can get them from the input string in between the Span of any two consecutive tokens.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=d7c2055b2f3762958151c838c76e2b7f

// [dependencies]
// proc-macro2 = { version = "1", features = ["span-locations"] }
// quote = "1"
// syn = { version = "2", features = ["full"] }

use quote::quote;
use syn::ItemImpl;

fn main() {
    let input = r#"
        impl Thing {
            // non-doc comment
            fn f(&self) {}
            // also comment
            fn g(&self) {}
        }
    "#;

    let impl_block: ItemImpl = syn::parse_str(input).unwrap();
    let mut cur = impl_block.brace_token.span.open().byte_range().end;
    for method in impl_block.items {
        let mut tokens = quote!(#method).into_iter();
        let first = tokens.next().unwrap().span();
        let last = tokens.last().map_or(first, |last| last.span());
        let comment = &input[cur..first.byte_range().start];
        cur = last.byte_range().end;
        println!("comment: {:?}", comment.trim());
    }
}

Uh oh!

Non-doc comments #946

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