Silence `unused_mut` when only `span` is mutated with `span` feature

src/entry.rs

@@ -579,6 +579,8 @@ mod test {
         );
 
         let entry: KdlEntry = " \\\n (\"m\\\"eh\")0xDEADbeef\t\\\n".parse()?;
+        #[cfg_attr(not(feature = "span"), allow(unused_mut))]
+        {
             let mut ty: KdlIdentifier = "\"m\\\"eh\"".parse()?;
             #[cfg(feature = "span")]
             {
@@ -600,6 +602,7 @@ mod test {
                     span: SourceSpan::from(0..26),
                 }
             );
+        }
 
         let entry: KdlEntry = " \\\n \"foo\"=(\"m\\\"eh\")0xDEADbeef\t\\\n".parse()?;
         assert_eq!(

src/v2_parser.rs

@@ -759,6 +759,7 @@ fn node_children(input: &mut Input<'_>) -> PResult<KdlDocument> {
             .map(|_| KdlDocument::new())
             .or_else(|mut e: ErrMode<KdlParseError>| {
                 e = match e {
+                    #[cfg_attr(not(feature = "span"), allow(unused_mut))]
                     ErrMode::Cut(mut pe) => {
                         #[cfg(feature = "span")]
                         {