Resolve unused field of ReplaceSelf syntax tree visitor

    warning: field `0` is never read
      --> src/receiver.rs:81:24
       |
    81 | pub struct ReplaceSelf(pub Span);
       |            ----------- ^^^^^^^^
       |            |
       |            field in this struct
       |
       = note: `#[warn(dead_code)]` on by default
    help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
       |
    81 | pub struct ReplaceSelf(());
       |                        ~~

src/expand.rs

@@ -335,7 +335,7 @@ fn transform_sig(
 //         ___ret
 //     })
 fn transform_block(context: Context, sig: &mut Signature, block: &mut Block) {
-    let mut self_span = None;
+    let mut replace_self = false;
     let decls = sig
         .inputs
         .iter()
@@ -346,8 +346,8 @@ fn transform_block(context: Context, sig: &mut Signature, block: &mut Block) {
                 mutability,
                 ..
             }) => {
+                replace_self = true;
                 let ident = Ident::new("__self", self_token.span);
-                self_span = Some(self_token.span);
                 quote!(let #mutability #ident = #self_token;)
             }
             FnArg::Typed(arg) => {
@@ -389,9 +389,8 @@ fn transform_block(context: Context, sig: &mut Signature, block: &mut Block) {
         })
         .collect::<Vec<_>>();
 
-    if let Some(span) = self_span {
-        let mut replace_self = ReplaceSelf(span);
-        replace_self.visit_block_mut(block);
+    if replace_self {
+        ReplaceSelf.visit_block_mut(block);
     }
 
     let stmts = &block.stmts;

src/receiver.rs

@@ -1,4 +1,4 @@
-use proc_macro2::{Group, Span, TokenStream, TokenTree};
+use proc_macro2::{Group, TokenStream, TokenTree};
 use syn::visit_mut::{self, VisitMut};
 use syn::{
     Block, ExprPath, Ident, Item, Macro, Pat, PatIdent, Path, Receiver, Signature, Token, TypePath,
@@ -78,7 +78,7 @@ impl VisitMut for HasSelf {
     }
 }
 
-pub struct ReplaceSelf(pub Span);
+pub struct ReplaceSelf;
 
 impl ReplaceSelf {
     #[cfg_attr(not(self_span_hack), allow(clippy::unused_self))]