Make sure Session.plugin_attributes is only used on one thread

src/librustc/session/mod.rs

@@ -91,7 +91,7 @@ pub struct Session {
     /// in order to avoid redundantly verbose output (Issue #24690, #44953).
     pub one_time_diagnostics: RefCell<FxHashSet<(DiagnosticMessageId, Option<Span>, String)>>,
     pub plugin_llvm_passes: OneThread<RefCell<Vec<String>>>,
-    pub plugin_attributes: RefCell<Vec<(String, AttributeType)>>,
+    pub plugin_attributes: OneThread<RefCell<Vec<(String, AttributeType)>>>,
     pub crate_types: RefCell<Vec<config::CrateType>>,
     pub dependency_formats: RefCell<dependency_format::Dependencies>,
     /// The crate_disambiguator is constructed out of all the `-C metadata`
@@ -1095,7 +1095,7 @@ pub fn build_session_(
         buffered_lints: OneThread::new(RefCell::new(Some(lint::LintBuffer::new()))),
         one_time_diagnostics: RefCell::new(FxHashSet()),
         plugin_llvm_passes: OneThread::new(RefCell::new(Vec::new())),
-        plugin_attributes: RefCell::new(Vec::new()),
+        plugin_attributes: OneThread::new(RefCell::new(Vec::new())),
         crate_types: RefCell::new(Vec::new()),
         dependency_formats: RefCell::new(FxHashMap()),
         crate_disambiguator: Once::new(),