refactor path pattern checking to get info for peeling

See the doc comment on `ResolvedPat` for more information. This and the
next couple commits split resolution apart from checking for path,
struct, and tuple struct patterns, in order to find the pattern's type
before peeling the scrutinee. This helps us avoid peeling the scrutinee
when the pattern could match it.

The reason this handles errors from resolution after peeling is for
struct and tuple struct patterns: we check their subpatterns even when
they fail to resolve, to potentially catch more resolution errors. By
doing this after peeling, we're able to use the updated `PatInfo`. I
don't know if there's currently any observable difference from using the
outdated `PatInfo`, but it could potentially be a source of subtle
diagnostic bugs in the future, so I'm opting to peel first.

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 +258,44 @@ 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, and to finish checking the pattern once we know its adjusted type.
+#[derive(Clone, Copy, Debug)]
+struct ResolvedPat<'tcx> {
+    /// The type of the pattern, to be checked against the type of the scrutinee after peeling. This
+    /// is also used to avoid peeling the scrutinee's constructors (see the `Cow` example above).
+    ty: Ty<'tcx>,
+    kind: ResolvedPatKind<'tcx>,
+}
+
+#[derive(Clone, Copy, Debug)]
+enum ResolvedPatKind<'tcx> {
+    Path { res: Res, pat_res: Res, segments: &'tcx [hir::PathSegment<'tcx>] },
+}
+
+impl<'tcx> ResolvedPat<'tcx> {
+    fn adjust_mode(&self) -> AdjustMode {
+        let ResolvedPatKind::Path { res, .. } = self.kind;
+        if matches!(res, Res::Def(DefKind::Const | DefKind::AssocConst, _)) {
+            // These constants can be of a reference type, e.g. `const X: &u8 = &0;`.
+            // Peeling the reference types too early will cause type checking failures.
+            // Although it would be possible to *also* peel the types of the constants too.
+            AdjustMode::Pass
+        } else {
+            // The remaining possible resolutions for path, struct, and tuple struct patterns are
+            // ADT constructors. As such, we may peel references freely, but we must not peel the
+            // ADT itself from the scrutinee if it's a smart pointer.
+            AdjustMode::Peel { kind: PeelKind::Overloaded }
+        }
+    }
+}
+
 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 +374,17 @@ 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 {
+        // For patterns containing paths, we need the path's resolution to determine whether to
+        // implicitly dereference the scrutinee before matching.
+        let opt_res_pat_or_err = 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))
+                Some(self.resolve_pat_path(*hir_id, *span, qpath))
             }
             _ => None,
         };
-        let adjust_mode = self.calc_adjust_mode(pat, path_res.map(|(res, ..)| res));
+        let opt_res_pat = opt_res_pat_or_err.map(Result::ok).flatten();
+
+        let adjust_mode = self.calc_adjust_mode(pat, opt_res_pat.as_ref());
         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 +399,13 @@ 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 = match opt_res_pat_or_err.unwrap() {
+                    Ok(ref res_pat) => {
+                        self.check_pat_path(pat.hir_id, pat.span, res_pat, expected, &ti)
+                    }
+                    Err(guar) => Ty::new_error(self.tcx, guar),
+                };
                 self.write_ty(*hir_id, ty);
                 ty
             }
@@ -490,8 +529,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     /// How should the binding mode and expected type be adjusted?
     ///
-    /// When the pattern is a path pattern, `opt_path_res` must be `Some(res)`.
-    fn calc_adjust_mode(&self, pat: &'tcx Pat<'tcx>, opt_path_res: Option<Res>) -> AdjustMode {
+    /// When the pattern contains a path, `opt_resolved_pat` must be `Some(resolved_pat)`.
+    fn calc_adjust_mode(
+        &self,
+        pat: &'tcx Pat<'tcx>,
+        opt_resolved_pat: Option<&ResolvedPat<'tcx>>,
+    ) -> AdjustMode {
         // When we perform destructuring assignment, we disable default match bindings, which are
         // unintuitive in this context.
         if !pat.default_binding_modes {
@@ -512,17 +555,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             | PatKind::Deref(_) => AdjustMode::Peel { kind: PeelKind::RefOnly },
             // A never pattern behaves somewhat like a literal or unit variant.
             PatKind::Never => AdjustMode::Peel { kind: PeelKind::Overloaded },
-            PatKind::Expr(PatExpr { kind: PatExprKind::Path(_), .. }) => match opt_path_res.unwrap() {
-                // These constants can be of a reference type, e.g. `const X: &u8 = &0;`.
-                // Peeling the reference types too early will cause type checking failures.
-                // Although it would be possible to *also* peel the types of the constants too.
-                Res::Def(DefKind::Const | DefKind::AssocConst, _) => AdjustMode::Pass,
-                // In the `ValueNS`, we have `SelfCtor(..) | Ctor(_, Const), _)` remaining which
-                // could successfully compile. The former being `Self` requires a unit struct.
-                // In either case, and unlike constants, the pattern itself cannot be
-                // a reference type wherefore peeling doesn't give up any expressiveness.
-                _ => AdjustMode::Peel { kind: PeelKind::Overloaded },
-            },
+            // For patterns with paths, how we peel the scrutinee depends on the path's resolution.
+            PatKind::Expr(PatExpr { kind: PatExprKind::Path(_), .. }) => {
+                opt_resolved_pat.map_or(AdjustMode::Peel { kind: PeelKind::Overloaded }, ResolvedPat::adjust_mode)
+            }
 
             // String and byte-string literals result in types `&str` and `&[u8]` respectively.
             // All other literals result in non-reference types.
@@ -1241,31 +1277,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    fn check_pat_path(
+    fn resolve_pat_path(
         &self,
         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>,
-        ti: &TopInfo<'tcx>,
-    ) -> Ty<'tcx> {
+        qpath: &'tcx hir::QPath<'_>,
+    ) -> Result<ResolvedPat<'tcx>, ErrorGuaranteed> {
         let tcx = self.tcx;
 
-        // We have already resolved the path.
-        let (res, opt_ty, segments) = path_resolution;
+        let (res, opt_ty, segments) =
+            self.resolve_ty_and_res_fully_qualified_call(qpath, path_id, span);
         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);
+                return 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);
+                return Err(e);
             }
             Res::SelfCtor(def_id) => {
                 if let ty::Adt(adt_def, _) = *tcx.type_of(def_id).skip_binder().kind()
@@ -1283,7 +1315,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         E0533,
                         "unit struct",
                     );
-                    return Ty::new_error(tcx, e);
+                    return Err(e);
                 }
             }
             Res::Def(
@@ -1296,15 +1328,26 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => bug!("unexpected pattern resolution: {:?}", res),
         }
 
-        // Type-check the path.
+        // Find the type of the path pattern, for later checking.
         let (pat_ty, pat_res) =
             self.instantiate_value_path(segments, opt_ty, res, span, span, path_id);
+        Ok(ResolvedPat { ty: pat_ty, kind: ResolvedPatKind::Path { res, pat_res, segments } })
+    }
+
+    fn check_pat_path(
+        &self,
+        pat_id_for_diag: HirId,
+        span: Span,
+        resolved: &ResolvedPat<'tcx>,
+        expected: Ty<'tcx>,
+        ti: &TopInfo<'tcx>,
+    ) -> Ty<'tcx> {
         if let Err(err) =
-            self.demand_suptype_with_origin(&self.pattern_cause(ti, span), expected, pat_ty)
+            self.demand_suptype_with_origin(&self.pattern_cause(ti, span), expected, resolved.ty)
         {
-            self.emit_bad_pat_path(err, pat_id_for_diag, span, res, pat_res, pat_ty, segments);
+            self.emit_bad_pat_path(err, pat_id_for_diag, span, resolved);
         }
-        pat_ty
+        resolved.ty
     }
 
     fn maybe_suggest_range_literal(
@@ -1347,11 +1390,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         mut e: Diag<'_>,
         hir_id: HirId,
         pat_span: Span,
-        res: Res,
-        pat_res: Res,
-        pat_ty: Ty<'tcx>,
-        segments: &'tcx [hir::PathSegment<'tcx>],
+        resolved_pat: &ResolvedPat<'tcx>,
     ) {
+        let ResolvedPatKind::Path { res, pat_res, segments } = resolved_pat.kind;
+
         if let Some(span) = self.tcx.hir_res_span(pat_res) {
             e.span_label(span, format!("{} defined here", res.descr()));
             if let [hir::PathSegment { ident, .. }] = &*segments {
@@ -1374,7 +1416,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         );
                     }
                     _ => {
-                        let (type_def_id, item_def_id) = match pat_ty.kind() {
+                        let (type_def_id, item_def_id) = match resolved_pat.ty.kind() {
                             ty::Adt(def, _) => match res {
                                 Res::Def(DefKind::Const, def_id) => (Some(def.did()), Some(def_id)),
                                 _ => (None, None),