refactor struct pattern checking to get info for peeling

See the previous commit for details. This doesn't yet extract the struct
pat's type's ADT def before peeling, but it should now be possible.

Author: dianne

compiler/rustc_hir_typeck/src/pat.rs

@@ -361,6 +361,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 PatKind::Expr(PatExpr { kind: PatExprKind::Path(qpath), hir_id, span }) => {
                     Some { 0: &self.check_pat_path(*hir_id, pat.hir_id, *span, qpath, &ti) }
                 }
+                PatKind::Struct(ref qpath, fields, has_rest_pat) => {
+                    Some { 0: &self.check_pat_struct(pat, qpath, fields, has_rest_pat) }
+                }
                 _ => None,
             };
         let adjust_mode = self.calc_adjust_mode(pat, resolved_pat.and_then(|r| r.path_res));
@@ -393,9 +396,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             PatKind::TupleStruct(ref qpath, subpats, ddpos) => {
                 self.check_pat_tuple_struct(pat, qpath, subpats, ddpos, expected, pat_info)
             }
-            PatKind::Struct(ref qpath, fields, has_rest_pat) => {
-                self.check_pat_struct(pat, qpath, fields, has_rest_pat, expected, pat_info)
-            }
+            PatKind::Struct(..) => (resolved_pat.unwrap().check)(expected, pat_info),
             PatKind::Guard(pat, cond) => {
                 self.check_pat(pat, expected, pat_info);
                 self.check_expr_has_type_or_error(cond, self.tcx.types.bool, |_| {});
@@ -1234,29 +1235,34 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         qpath: &hir::QPath<'tcx>,
         fields: &'tcx [hir::PatField<'tcx>],
         has_rest_pat: bool,
-        expected: Ty<'tcx>,
-        pat_info: PatInfo<'tcx>,
-    ) -> Ty<'tcx> {
+    ) -> ResolvedPat<impl Fn(Ty<'tcx>, PatInfo<'tcx>) -> Ty<'tcx>> {
         // Resolve the path and check the definition for errors.
-        let (variant, pat_ty) = match self.check_struct_path(qpath, pat.hir_id) {
-            Ok(data) => data,
-            Err(guar) => {
-                let err = Ty::new_error(self.tcx, guar);
-                for field in fields {
-                    self.check_pat(field.pat, err, pat_info);
+        let variant_and_pat_ty = self.check_struct_path(qpath, pat.hir_id);
+
+        let check = move |expected: Ty<'tcx>, pat_info: PatInfo<'tcx>| -> Ty<'tcx> {
+            let (variant, pat_ty) = match variant_and_pat_ty {
+                Ok(data) => data,
+                Err(guar) => {
+                    let err = Ty::new_error(self.tcx, guar);
+                    for field in fields {
+                        self.check_pat(field.pat, err, pat_info);
+                    }
+                    return err;
                 }
-                return err;
+            };
+
+            // Type-check the path.
+            let _ = self.demand_eqtype_pat(pat.span, expected, pat_ty, &pat_info.top_info);
+
+            // Type-check subpatterns.
+            match self.check_struct_pat_fields(pat_ty, pat, variant, fields, has_rest_pat, pat_info)
+            {
+                Ok(()) => pat_ty,
+                Err(guar) => Ty::new_error(self.tcx, guar),
             }
         };
 
-        // Type-check the path.
-        let _ = self.demand_eqtype_pat(pat.span, expected, pat_ty, &pat_info.top_info);
-
-        // Type-check subpatterns.
-        match self.check_struct_pat_fields(pat_ty, pat, variant, fields, has_rest_pat, pat_info) {
-            Ok(()) => pat_ty,
-            Err(guar) => Ty::new_error(self.tcx, guar),
-        }
+        ResolvedPat { path_res: None, check }
     }
 
     fn check_pat_path(