Add DefiningAnchor to FnCtxt in preparation for removing it from InferCtxt

Author: oli-obk

compiler/rustc_hir_typeck/src/inherited.rs

@@ -28,6 +28,13 @@ use std::ops::Deref;
 pub struct Inherited<'tcx> {
     pub(super) infcx: InferCtxt<'tcx>,
 
+    /// The `DefId` of the item in whose context we are performing inference or typeck.
+    /// It is used to check whether an opaque type use is a defining use.
+    ///
+    /// Its default value is `DefiningAnchor::Error`, this way it is easier to catch errors that
+    /// might come up during inference or typeck.
+    pub(super) defining_use_anchor: DefiningAnchor,
+
     pub(super) typeck_results: RefCell<ty::TypeckResults<'tcx>>,
 
     pub(super) locals: RefCell<HirIdMap<super::LocalTy<'tcx>>>,
@@ -118,7 +125,9 @@ impl<'tcx> Inherited<'tcx> {
         let tcx = infcx.tcx;
         let body_id = tcx.hir().maybe_body_owned_by(def_id);
 
+        let defining_use_anchor = DefiningAnchor::Bind(typeck_results.borrow().hir_owner.def_id);
         Inherited {
+            defining_use_anchor,
             typeck_results,
             infcx,
             fulfillment_cx: RefCell::new(<dyn TraitEngine<'_>>::new(tcx)),