Add helper fn for checking if opened input is theme preview file

src/assets.rs

@@ -191,11 +191,7 @@ impl HighlightingAssets {
         input: &mut OpenedInput,
         mapping: &SyntaxMapping,
     ) -> &SyntaxReference {
-        let syntax = if match input.kind {
+        let syntax = if input.kind.is_theme_preview_file() {
-            OpenedInputKind::ThemePreviewFile => true,
-            _ => false,
-        } {
-            // FIXME: replace the above match statement with matches macro when min Rust >= 1.42.0
             self.syntax_set.find_syntax_by_name("Rust")
         } else if let Some(language) = language {
             self.syntax_set.find_syntax_by_token(language)

src/input.rs

@@ -39,6 +39,15 @@ pub(crate) enum OpenedInputKind {
     CustomReader,
 }
 
+impl OpenedInputKind {
+    pub(crate) fn is_theme_preview_file(&self) -> bool {
+        match self {
+            OpenedInputKind::ThemePreviewFile => true,
+            _ => false,
+        }
+    }
+}
+
 pub(crate) struct OpenedInput<'a> {
     pub(crate) kind: OpenedInputKind,
     pub(crate) metadata: InputMetadata,