How do you get the type of an impl block in a Rust procedural macro?

Question

I'm trying to write a Rust procedural macro that can be applied to an impl block like this;

struct SomeStruct { }

#[my_macro]
impl SomeStruct { }

I'm using syn and quote to parse and format TokenStreams in the macro. It looks something like this:

#[proc_macro_attribute]
pub fn my_macro(meta: TokenStream, code: TokenStream) -> TokenStream {
    let input = parse_macro_input!(code as ItemImpl);

    // ...

    TokenStream::from(quote!(#input))
}

Is there a way to access the type name of the impl block using syn? I don't see any field in ItemImpl that gives me that information.

Answer 1

The documentation lists 9 fields on an ItemImpl:

1. attrs: Vec<Attribute>
2. defaultness: Option<Default>
3. unsafety: Option<Unsafe>
4. impl_token: Impl
5. generics: Generics
6. trait_: Option<(Option<Bang>, Path, For)>
7. self_ty: Box<Type>
8. brace_token: Brace
9. items: Vec<ImplItem>

Only one of those has the word "type" in it: self_ty.

use syn; // 0.15.23

fn example(input: syn::ItemImpl) {
    println!("{:#?}", input.self_ty);
}

fn main() {
    let code = syn::parse_str(
        r###"
        impl Foo {}
        "###,
    )
    .unwrap();

    example(code);
}

Path(
    TypePath {
        qself: None,
        path: Path {
            leading_colon: None,
            segments: [
                PathSegment {
                    ident: Ident(
                        Foo
                    ),
                    arguments: None
                }
            ]
        }
    }
)