early return in trait detection for non-trait item

Author: bvanjoi

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -4111,6 +4111,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return false;
         }
         let trait_def_id = self.tcx.parent(item_def_id);
+        if !self.tcx.is_trait(trait_def_id) {
+            return false;
+        }
         let krate = self.tcx.crate_name(trait_def_id.krate);
         let name = self.tcx.item_name(trait_def_id);
         let candidates: Vec<_> = traits

tests/crashes/135863.rs

@@ -0,0 +1,12 @@
+// issue#135863
+
+struct A;
+
+impl A {
+    fn len(self: &&A) {}
+}
+
+fn main() {
+    A.len();
+    //~^ ERROR: no method named `len` found for struct `A` in the current scope
+}

tests/ui/suggestions/double-reference-ty-in-self-ty.rs

@@ -0,0 +1,19 @@
+error[E0599]: no method named `len` found for struct `A` in the current scope
+  --> $DIR/double-reference-ty-in-self-ty.rs:10:7
+   |
+LL | struct A;
+   | -------- method `len` not found for this struct
+...
+LL |     fn len(self: &&A) {}
+   |        --- the method is available for `&A` here
+...
+LL |     A.len();
+   |       ^^^ method not found in `A`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+   = note: the following trait defines an item `len`, perhaps you need to implement it:
+           candidate #1: `ExactSizeIterator`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0599`.