is_knowable use Result instead of Option

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -404,12 +404,12 @@ fn resolve_negative_obligation<'cx, 'tcx>(
 pub fn trait_ref_is_knowable<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::TraitRef<'tcx>,
-) -> Option<Conflict> {
+) -> Result<(), Conflict> {
     debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
     if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
         // A downstream or cousin crate is allowed to implement some
         // substitution of this trait-ref.
-        return Some(Conflict::Downstream);
+        return Err(Conflict::Downstream);
     }
 
     if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
@@ -418,7 +418,7 @@ pub fn trait_ref_is_knowable<'tcx>(
         // allowed to implement a substitution of this trait ref, which
         // means impls could only come from dependencies of this crate,
         // which we already know about.
-        return None;
+        return Ok(());
     }
 
     // This is a remote non-fundamental trait, so if another crate
@@ -431,10 +431,10 @@ pub fn trait_ref_is_knowable<'tcx>(
     // we are an owner.
     if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
         debug!("trait_ref_is_knowable: orphan check passed");
-        None
+        Ok(())
     } else {
         debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
-        Some(Conflict::Upstream)
+        Err(Conflict::Upstream)
     }
 }
 

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -75,7 +75,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         &mut self,
         stack: &TraitObligationStack<'o, 'tcx>,
     ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
-        if let Some(conflict) = self.is_knowable(stack) {
+        if let Err(conflict) = self.is_knowable(stack) {
             debug!("coherence stage: not knowable");
             if self.intercrate_ambiguity_causes.is_some() {
                 debug!("evaluate_stack: intercrate_ambiguity_causes is some");

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1265,11 +1265,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         Ok(Some(candidate))
     }
 
-    fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> {
+    fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result<(), Conflict> {
         debug!("is_knowable(intercrate={:?})", self.intercrate);
 
         if !self.intercrate || stack.obligation.polarity() == ty::ImplPolarity::Negative {
-            return None;
+            return Ok(());
         }
 
         let obligation = &stack.obligation;