Move get_associated_type from clippy to rustc_lint

compiler/rustc_lint/src/context.rs

@@ -1245,6 +1245,23 @@ impl<'tcx> LateContext<'tcx> {
 
         AbsolutePathPrinter { tcx: self.tcx }.print_def_path(def_id, &[]).unwrap()
     }
+
+    /// Returns the associated type `name` for `self_ty` as an implementation of `trait_id`.
+    /// Do not invoke without first verifying that the type implements the trait.
+    pub fn get_associated_type(
+        &self,
+        self_ty: Ty<'tcx>,
+        trait_id: DefId,
+        name: &str,
+    ) -> Option<Ty<'tcx>> {
+        let tcx = self.tcx;
+        tcx.associated_items(trait_id)
+            .find_by_name_and_kind(tcx, Ident::from_str(name), ty::AssocKind::Type, trait_id)
+            .and_then(|assoc| {
+                let proj = tcx.mk_projection(assoc.def_id, tcx.mk_substs_trait(self_ty, []));
+                tcx.try_normalize_erasing_regions(self.param_env, proj).ok()
+            })
+    }
 }
 
 impl<'tcx> abi::HasDataLayout for LateContext<'tcx> {