Add test: builtin_mappings_are_lazily_evaluated

src/syntax_mapping.rs

@@ -128,6 +128,9 @@ impl<'a> SyntaxMapping<'a> {
 
 #[cfg(test)]
 mod tests {
+    use once_cell::sync::Lazy;
+    use rusty_fork::rusty_fork_test;
+
     use super::*;
     #[test]
     fn builtin_mappings_work() {
@@ -148,6 +151,31 @@ mod tests {
         let _mappings = map.builtin_mappings().collect::<Vec<_>>();
     }
 
+    // lazy initialisation test needs to be run on a separate instance because
+    // it will race with other tests
+    // see: https://github.com/rust-lang/rust/issues/47506
+    rusty_fork_test! {
+        #[test]
+        fn builtin_mappings_are_lazily_evaluated() {
+            let map = SyntaxMapping::new();
+
+            assert!(BUILTIN_MAPPINGS
+                .iter()
+                .all(|(matcher, _)| Lazy::get(matcher).is_none()));
+
+            // calling `builtin_mappings` should not trigger evaluation
+            let mappings_iter = map.builtin_mappings();
+            assert!(BUILTIN_MAPPINGS
+                .iter()
+                .all(|(matcher, _)| Lazy::get(matcher).is_none()));
+
+            let _mappings: Vec<_> = mappings_iter.collect();
+            assert!(BUILTIN_MAPPINGS
+                .iter()
+                .all(|(matcher, _)| Lazy::get(matcher).is_some()));
+        }
+    }
+
     #[test]
     fn builtin_mappings_matcher_only_compile_once() {
         let map = SyntaxMapping::new();