use enum to represent ObligationCause::dummy without allocating

Author: lcnr

src/librustc_middle/traits/mod.rs

@@ -89,14 +89,12 @@ pub enum Reveal {
 /// only live for a short period of time.
 #[derive(Clone, PartialEq, Eq, Hash)]
 pub struct ObligationCause<'tcx> {
-    data: Rc<ObligationCauseData<'tcx>>,
+    /// `None` for `ObligationCause::dummy`, `Some` otherwise.
+    data: Option<Rc<ObligationCauseData<'tcx>>>,
 }
 
-// A dummy obligation. As the parralel compiler does not share `Obligation`s between
-// threads, we use a `thread_local` here so we can keep using an `Rc` inside of `ObligationCause`.
-thread_local! {
-    static DUMMY_OBLIGATION_CAUSE: ObligationCause<'static> = ObligationCause::new(DUMMY_SP, hir::CRATE_HIR_ID, MiscObligation);
-}
+const DUMMY_OBLIGATION_CAUSE_DATA: ObligationCauseData<'static> =
+    ObligationCauseData { span: DUMMY_SP, body_id: hir::CRATE_HIR_ID, code: MiscObligation };
 
 // Correctly format `ObligationCause::dummy`.
 impl<'tcx> fmt::Debug for ObligationCause<'tcx> {
@@ -108,8 +106,9 @@ impl<'tcx> fmt::Debug for ObligationCause<'tcx> {
 impl Deref for ObligationCause<'tcx> {
     type Target = ObligationCauseData<'tcx>;
 
+    #[inline(always)]
     fn deref(&self) -> &Self::Target {
-        &self.data
+        self.data.as_deref().unwrap_or(&DUMMY_OBLIGATION_CAUSE_DATA)
     }
 }
 
@@ -135,7 +134,7 @@ impl<'tcx> ObligationCause<'tcx> {
         body_id: hir::HirId,
         code: ObligationCauseCode<'tcx>,
     ) -> ObligationCause<'tcx> {
-        ObligationCause { data: Rc::new(ObligationCauseData { span, body_id, code }) }
+        ObligationCause { data: Some(Rc::new(ObligationCauseData { span, body_id, code })) }
     }
 
     pub fn misc(span: Span, body_id: hir::HirId) -> ObligationCause<'tcx> {
@@ -148,11 +147,11 @@ impl<'tcx> ObligationCause<'tcx> {
 
     #[inline(always)]
     pub fn dummy() -> ObligationCause<'tcx> {
-        DUMMY_OBLIGATION_CAUSE.with(Clone::clone)
+        ObligationCause { data: None }
     }
 
     pub fn make_mut(&mut self) -> &mut ObligationCauseData<'tcx> {
-        Rc::make_mut(&mut self.data)
+        Rc::make_mut(self.data.get_or_insert_with(|| Rc::new(DUMMY_OBLIGATION_CAUSE_DATA)))
     }
 
     pub fn span(&self, tcx: TyCtxt<'tcx>) -> Span {