Add test: builtin_mappings_are_lazily_evaluated

Cargo.lock

@@ -155,6 +155,7 @@ dependencies = [
  "predicates",
  "regex",
  "run_script",
+ "rusty-fork",
  "semver",
  "serde",
  "serde_with",
@@ -1039,6 +1040,12 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "quick-error"
+version = "1.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a1d01941d82fa2ab50be1e79e6714289dd7cde78eba4c074bc5a4374f650dfe0"
+
 [[package]]
 name = "quick-xml"
 version = "0.30.0"
@@ -1171,6 +1178,18 @@ dependencies = [
  "windows-sys 0.48.0",
 ]
 
+[[package]]
+name = "rusty-fork"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb3dcc6e454c328bb824492db107ab7c0ae8fcffe4ad210136ef014458c1bc4f"
+dependencies = [
+ "fnv",
+ "quick-error",
+ "tempfile",
+ "wait-timeout",
+]
+
 [[package]]
 name = "ryu"
 version = "1.0.11"

Cargo.toml

@@ -67,6 +67,7 @@ bytesize = { version = "1.3.0" }
 encoding_rs = "0.8.33"
 os_str_bytes = { version = "~6.6", optional = true }
 run_script = { version = "^0.10.1", optional = true}
+rusty-fork = "0.3.0"
 
 [dependencies.git2]
 version = "0.18"

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();