internal: document that ascription is preferred to a turbo fish

crates/vfs/src/loader.rs

@@ -161,7 +161,7 @@ impl Directories {
     ///   - This path is longer than any element in `self.exclude` that is a prefix
     ///     of `path`. In case of equality, exclusion wins.
     fn includes_path(&self, path: &AbsPath) -> bool {
-        let mut include = None::<&AbsPathBuf>;
+        let mut include: Option<&AbsPathBuf> = None;
         for incl in &self.include {
             if path.starts_with(incl) {
                 include = Some(match include {