refactor path pattern checking to get more info for peeling

See the doc comment on `ResolvedPat` for more information. This is to
get the pattern's type's ADT def for (tuple) struct patterns, in order
to know when to stop adding implicit deref patterns. Since we don't know
the type we're matching against until we've peeled the scrutinee,
`ResolvedPat` provides a way to finish checking the pattern once that
type is known. The refactor's unfortunate, but this was the least
disruptive way I could think of doing it. All the state that needs to be
kept between resolution and type-checking is captured in a closure so it
doesn't need to be manually passed around in `check_pat`.

This first part only handles path patterns. Their resolutions were
already needed early for adjust mode calculation, so I think it makes
sense to handle that with `ResolvedPat` too.

Author: dianne

compiler/rustc_hir_typeck/src/pat.rs

@@ -33,7 +33,7 @@ use ty::VariantDef;
 use super::report_unexpected_variant_res;
 use crate::expectation::Expectation;
 use crate::gather_locals::DeclOrigin;
-use crate::{FnCtxt, LoweredTy, errors};
+use crate::{FnCtxt, errors};
 
 const CANNOT_IMPLICITLY_DEREF_POINTER_TRAIT_OBJ: &str = "\
 This error indicates that a pointer to a trait type cannot be implicitly dereferenced by a \
@@ -258,6 +258,24 @@ enum InheritedRefMatchRule {
     },
 }
 
+/// When checking patterns containing paths, we need to know the path's resolution to determine
+/// whether to apply match ergonomics and implicitly dereference the scrutinee. For instance, when
+/// the `deref_patterns` feature is enabled and we're matching against a scrutinee of type
+/// `Cow<'a, Option<u8>>`, we insert an implicit dereference to allow the pattern `Some(_)` to type,
+/// but we must not dereference it when checking the pattern `Cow::Borrowed(_)`.
+///
+/// `ResolvedPat` contains the information from resolution needed to determine match ergonomics
+/// adjustments, plus a callback to finish checking the pattern once we know its adjusted type.
+// TODO: add an `adt_def: Option<DefId>` field to handle the `Cow` case above.
+struct ResolvedPat<F: ?Sized> {
+    /// For path patterns, this must be `Some(res)`, where `res` is the resolution of the pattern's
+    /// `QPath`. Otherwise, this is `None`.
+    path_res: Option<Res>,
+    /// Given the expected type of the scrutinee and the [`PatInfo`] after applying match ergonomics
+    /// adjustments, finish checking the pattern.
+    check: F,
+}
+
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Experimental pattern feature: after matching against a shared reference, do we limit the
     /// default binding mode in subpatterns to be `ref` when it would otherwise be `ref mut`?
@@ -336,13 +354,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn check_pat(&self, pat: &'tcx Pat<'tcx>, expected: Ty<'tcx>, pat_info: PatInfo<'tcx>) {
         let PatInfo { binding_mode, max_ref_mutbl, top_info: ti, current_depth, .. } = pat_info;
 
-        let path_res = match pat.kind {
-            PatKind::Expr(PatExpr { kind: PatExprKind::Path(qpath), hir_id, span }) => {
-                Some(self.resolve_ty_and_res_fully_qualified_call(qpath, *hir_id, *span))
-            }
-            _ => None,
-        };
-        let adjust_mode = self.calc_adjust_mode(pat, path_res.map(|(res, ..)| res));
+        // For patterns containing paths, we need the path's resolution to determine whether to
+        // implicitly dereference the scrutinee before matching.
+        let resolved_pat: Option<&ResolvedPat<dyn Fn(Ty<'tcx>, PatInfo<'tcx>) -> Ty<'tcx>>> =
+            match pat.kind {
+                PatKind::Expr(PatExpr { kind: PatExprKind::Path(qpath), hir_id, span }) => {
+                    Some { 0: &self.check_pat_path(*hir_id, pat.hir_id, *span, qpath, &ti) }
+                }
+                _ => None,
+            };
+        let adjust_mode = self.calc_adjust_mode(pat, resolved_pat.and_then(|r| r.path_res));
         let (expected, binding_mode, max_ref_mutbl) =
             self.calc_default_binding_mode(pat, expected, binding_mode, adjust_mode, max_ref_mutbl);
         let pat_info = PatInfo {
@@ -357,16 +378,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             PatKind::Wild | PatKind::Err(_) => expected,
             // We allow any type here; we ensure that the type is uninhabited during match checking.
             PatKind::Never => expected,
-            PatKind::Expr(PatExpr { kind: PatExprKind::Path(qpath), hir_id, span }) => {
-                let ty = self.check_pat_path(
-                    *hir_id,
-                    pat.hir_id,
-                    *span,
-                    qpath,
-                    path_res.unwrap(),
-                    expected,
-                    &pat_info.top_info,
-                );
+            PatKind::Expr(PatExpr { kind: PatExprKind::Path(_), hir_id, .. }) => {
+                let ty = (resolved_pat.unwrap().check)(expected, pat_info);
                 self.write_ty(*hir_id, ty);
                 ty
             }
@@ -1251,33 +1264,31 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         path_id: HirId,
         pat_id_for_diag: HirId,
         span: Span,
-        qpath: &hir::QPath<'_>,
-        path_resolution: (Res, Option<LoweredTy<'tcx>>, &'tcx [hir::PathSegment<'tcx>]),
-        expected: Ty<'tcx>,
+        qpath: &'tcx hir::QPath<'tcx>,
         ti: &TopInfo<'tcx>,
-    ) -> Ty<'tcx> {
+    ) -> ResolvedPat<impl Fn(Ty<'tcx>, PatInfo<'tcx>) -> Ty<'tcx>> {
         let tcx = self.tcx;
 
-        // We have already resolved the path.
-        let (res, opt_ty, segments) = path_resolution;
-        match res {
+        let (res, opt_ty, segments) =
+            self.resolve_ty_and_res_fully_qualified_call(qpath, path_id, span);
+        let res_ok = match res {
             Res::Err => {
                 let e =
                     self.dcx().span_delayed_bug(qpath.span(), "`Res::Err` but no error emitted");
                 self.set_tainted_by_errors(e);
-                return Ty::new_error(tcx, e);
+                Err(e)
             }
             Res::Def(DefKind::AssocFn | DefKind::Ctor(_, CtorKind::Fn) | DefKind::Variant, _) => {
                 let expected = "unit struct, unit variant or constant";
                 let e = report_unexpected_variant_res(tcx, res, None, qpath, span, E0533, expected);
-                return Ty::new_error(tcx, e);
+                Err(e)
             }
             Res::SelfCtor(def_id) => {
                 if let ty::Adt(adt_def, _) = *tcx.type_of(def_id).skip_binder().kind()
                     && adt_def.is_struct()
                     && let Some((CtorKind::Const, _)) = adt_def.non_enum_variant().ctor
                 {
-                    // Ok, we allow unit struct ctors in patterns only.
+                    Ok(()) // Ok, we allow unit struct ctors in patterns only.
                 } else {
                     let e = report_unexpected_variant_res(
                         tcx,
@@ -1288,7 +1299,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         E0533,
                         "unit struct",
                     );
-                    return Ty::new_error(tcx, e);
+                    Err(e)
                 }
             }
             Res::Def(
@@ -1297,19 +1308,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 | DefKind::AssocConst
                 | DefKind::ConstParam,
                 _,
-            ) => {} // OK
+            ) => Ok(()), // OK
             _ => bug!("unexpected pattern resolution: {:?}", res),
-        }
+        };
 
         // Type-check the path.
-        let (pat_ty, pat_res) =
-            self.instantiate_value_path(segments, opt_ty, res, span, span, path_id);
-        if let Err(err) =
-            self.demand_suptype_with_origin(&self.pattern_cause(ti, span), expected, pat_ty)
-        {
-            self.emit_bad_pat_path(err, pat_id_for_diag, span, res, pat_res, pat_ty, segments);
-        }
-        pat_ty
+        let pat_ty_and_res =
+            res_ok.map(|_| self.instantiate_value_path(segments, opt_ty, res, span, span, path_id));
+
+        let check = move |expected, _pat_info| -> Ty<'tcx> {
+            let (pat_ty, pat_res) = match pat_ty_and_res {
+                Ok(pat_ty_and_res) => pat_ty_and_res,
+                Err(guar) => return Ty::new_error(tcx, guar),
+            };
+
+            if let Err(err) =
+                self.demand_suptype_with_origin(&self.pattern_cause(ti, span), expected, pat_ty)
+            {
+                self.emit_bad_pat_path(err, pat_id_for_diag, span, res, pat_res, pat_ty, segments);
+            }
+            pat_ty
+        };
+        ResolvedPat { path_res: Some(res), check }
     }
 
     fn maybe_suggest_range_literal(