Arbitrary self types v2: no deshadowing on traits

This is a hacky temporary "fix" for problems encountered where our
deshadowing algorithm is blocking legitimate code. This will need to be
reworked, but pushing to see if we get through CI.

The solution here involves only applying the deshadowing algorithm in
cases where both methods are inherent rather than in traits. That's
probably what we want to do? - but it needs discussion, and the code
needs a bit of restructuring.

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -88,8 +88,9 @@ pub(crate) struct ProbeContext<'a, 'tcx> {
 
     /// Number of times we've hopped along the chain of `Receiver::Target`.
     /// Used to spot cases where an "outer" method in a smart pointer might
-    /// "shadow" a pre-existing method in the pointee.
-    receiver_trait_derefs: usize,
+    /// "shadow" a pre-existing method in the pointee. Only applies for inherent
+    /// methods.
+    receiver_trait_derefs: Option<usize>,
 }
 
 impl<'a, 'tcx> Deref for ProbeContext<'a, 'tcx> {
@@ -104,7 +105,7 @@ pub(crate) struct Candidate<'tcx> {
     pub(crate) item: ty::AssocItem,
     pub(crate) kind: CandidateKind<'tcx>,
     pub(crate) import_ids: SmallVec<[LocalDefId; 1]>,
-    receiver_trait_derefs: usize,
+    receiver_trait_derefs: Option<usize>,
 }
 
 #[derive(Debug, Clone)]
@@ -183,7 +184,7 @@ struct PickDiagHints<'a, 'tcx> {
 #[derive(Debug)]
 struct PickConstraintsForShadowed {
     autoderefs: usize,
-    receiver_trait_derefs: usize,
+    receiver_trait_derefs: Option<usize>,
     def_id: DefId,
 }
 
@@ -192,8 +193,11 @@ impl PickConstraintsForShadowed {
         autoderefs == self.autoderefs
     }
 
-    fn may_shadow_based_on_receiver_trait_derefs(&self, receiver_trait_derefs: usize) -> bool {
-        receiver_trait_derefs != self.receiver_trait_derefs
+    fn may_shadow_based_on_receiver_trait_derefs(
+        &self,
+        receiver_trait_derefs: Option<usize>,
+    ) -> bool {
+        receiver_trait_derefs != self.receiver_trait_derefs && receiver_trait_derefs.is_some()
     }
 
     fn may_shadow_based_on_defid(&self, def_id: DefId) -> bool {
@@ -223,7 +227,8 @@ pub(crate) struct Pick<'tcx> {
 
     /// Number of jumps along the `Receiver::Target` chain we followed
     /// to identify this method. Used only for deshadowing errors.
-    pub receiver_trait_derefs: usize,
+    /// Only applies for inherent impls.
+    pub receiver_trait_derefs: Option<usize>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -708,7 +713,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             static_candidates: RefCell::new(Vec::new()),
             scope_expr_id,
             is_suggestion,
-            receiver_trait_derefs: 0usize,
+            receiver_trait_derefs: None,
         }
     }
 
@@ -765,9 +770,11 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
     fn assemble_inherent_candidates(&mut self) {
         for step in self.steps.iter() {
-            self.receiver_trait_derefs = step.autoderefs;
+            self.receiver_trait_derefs = Some(step.autoderefs);
             self.assemble_probe(&step.self_ty);
         }
+        // FIXME pass on stack again probably
+        self.receiver_trait_derefs = None;
     }
 
     #[instrument(level = "debug", skip(self))]
@@ -2039,11 +2046,11 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             item: probes[0].0.item,
             kind: TraitPick,
             import_ids: probes[0].0.import_ids.clone(),
-            autoderefs: probes[0].0.receiver_trait_derefs,
+            autoderefs: 0,
             autoref_or_ptr_adjustment: None,
             self_ty,
             unstable_candidates: vec![],
-            receiver_trait_derefs: 0,
+            receiver_trait_derefs: probes[0].0.receiver_trait_derefs,
         })
     }