Use proper span when emitting 'self' identifier (#210)

Normally, the span of a field has the same hygiene context as
`Span::call_site`. However, it's possible for a struct definition
to be 'constructed' via a `macro_rules` macro such that the field
has a different hygiene context. This will cause the expanded code
to be unable to resolve any references to `self`, resulting in a
compilation error.

This pull request uses `quote!` instead of `quote_spanned!` when
emitting a 'self' identifier. `quote_spanned!` is still used for
everything else in the emitted method, meaning that error messages
will still point to the proper field.

I've included a test case which triggers this issue on
Rust 1.43.1. It's current difficult to hit this issue
other than in this artificial case, but that will change
once rust-lang/rust#72622 is re-landed.

derive/src/encode.rs

@@ -70,16 +70,19 @@ fn encode_single_field(
 		}
 	};
 
+	// This may have different hygiene than the field span
+	let i_self = quote! { self };
+
 	quote_spanned! { field.span() =>
-			fn encode_to<EncOut: _parity_scale_codec::Output>(&self, dest: &mut EncOut) {
+			fn encode_to<EncOut: _parity_scale_codec::Output>(&#i_self, dest: &mut EncOut) {
 				_parity_scale_codec::Encode::encode_to(&#final_field_variable, dest)
 			}
 
-			fn encode(&self) -> _parity_scale_codec::alloc::vec::Vec<u8> {
+			fn encode(&#i_self) -> _parity_scale_codec::alloc::vec::Vec<u8> {
 				_parity_scale_codec::Encode::encode(&#final_field_variable)
 			}
 
-			fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&self, f: F) -> R {
+			fn using_encoded<R, F: FnOnce(&[u8]) -> R>(&#i_self, f: F) -> R {
 				_parity_scale_codec::Encode::using_encoded(&#final_field_variable, f)
 			}
 	}

tests/mod.rs

@@ -526,3 +526,18 @@ fn crafted_input_for_vec_t() {
 	);
 }
 
+#[test]
+fn weird_derive() {
+    // Tests that compilation succeeds when the macro invocation
+    // hygiene context is different from the field hygiene context.
+    macro_rules! make_struct {
+        (#[$attr:meta]) => (
+            #[$attr]
+            pub struct MyStruct {
+                field: u8
+            }
+        )
+    }
+
+    make_struct!(#[derive(Encode, Decode)]);
+}