subscriber: fix dead code on some feature combos

tracing-subscriber/src/filter/directive.rs

@@ -45,7 +45,8 @@ enum ParseErrorKind {
 // === impl DirectiveSet ===
 
 impl<T> DirectiveSet<T> {
-    #[cfg(feature = "std")]
+    // this is only used by `env-filter`.
+    #[cfg(all(feature = "std", feature = "env-filter"))]
     pub(crate) fn is_empty(&self) -> bool {
         self.directives.is_empty()
     }
@@ -388,7 +389,7 @@ impl FromStr for StaticDirective {
 // === impl ParseError ===
 
 impl ParseError {
-    #[cfg(feature = "std")]
+    #[cfg(all(feature = "std", feature = "env-filter"))]
     pub(crate) fn new() -> Self {
         ParseError {
             kind: ParseErrorKind::Other(None),

tracing-subscriber/src/fmt/mod.rs

@@ -1287,6 +1287,8 @@ mod test {
             Self { buf }
         }
 
+        // this is currently only used by the JSON formatter tests. if we need it elsewhere in the future, feel free to remove the `#[cfg]` attribute!
+        #[cfg(feature = "json")]
         pub(crate) fn buf(&self) -> MutexGuard<'_, Vec<u8>> {
             self.buf.lock().unwrap()
         }