Push the decision to skip fields further down

compiler/rustc_pattern_analysis/src/lib.rs

@@ -82,6 +82,10 @@ use crate::usefulness::{compute_match_usefulness, ValidityConstraint};
 pub trait Captures<'a> {}
 impl<'a, T: ?Sized> Captures<'a> for T {}
 
+/// `bool` newtype that indicates whether we should skip this field during analysis.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct SkipField(pub bool);
+
 /// Context that provides type information about constructors.
 ///
 /// Most of the crate is parameterized on a type that implements this trait.
@@ -105,13 +109,13 @@ pub trait TypeCx: Sized + fmt::Debug {
     /// The number of fields for this constructor.
     fn ctor_arity(&self, ctor: &Constructor<Self>, ty: &Self::Ty) -> usize;
 
-    /// The types of the fields for this constructor. The result must have a length of
-    /// `ctor_arity()`.
+    /// The types of the fields for this constructor. The result must contain `ctor_arity()`-many
+    /// fields that are not skipped.
     fn ctor_sub_tys<'a>(
         &'a self,
         ctor: &'a Constructor<Self>,
         ty: &'a Self::Ty,
-    ) -> impl Iterator<Item = Self::Ty> + ExactSizeIterator + Captures<'a>;
+    ) -> impl Iterator<Item = (Self::Ty, SkipField)> + ExactSizeIterator + Captures<'a>;
 
     /// The set of all the constructors for `ty`.
     ///

compiler/rustc_pattern_analysis/src/pat.rs

@@ -5,7 +5,7 @@ use std::fmt;
 use smallvec::{smallvec, SmallVec};
 
 use crate::constructor::{Constructor, Slice, SliceKind};
-use crate::TypeCx;
+use crate::{SkipField, TypeCx};
 
 use self::Constructor::*;
 
@@ -300,7 +300,11 @@ impl<Cx: TypeCx> WitnessPat<Cx> {
     /// For example, if `ctor` is a `Constructor::Variant` for `Option::Some`, we get the pattern
     /// `Some(_)`.
     pub(crate) fn wild_from_ctor(cx: &Cx, ctor: Constructor<Cx>, ty: Cx::Ty) -> Self {
-        let fields = cx.ctor_sub_tys(&ctor, &ty).map(|ty| Self::wildcard(ty)).collect();
+        let fields = cx
+            .ctor_sub_tys(&ctor, &ty)
+            .filter(|(_, SkipField(skip))| !skip)
+            .map(|(ty, _)| Self::wildcard(ty))
+            .collect();
         Self::new(ctor, fields, ty)
     }
 

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -18,7 +18,7 @@ use rustc_target::abi::{FieldIdx, Integer, VariantIdx, FIRST_VARIANT};
 use crate::constructor::{
     IntRange, MaybeInfiniteInt, OpaqueId, RangeEnd, Slice, SliceKind, VariantVisibility,
 };
-use crate::{errors, Captures, TypeCx};
+use crate::{errors, Captures, SkipField, TypeCx};
 
 use crate::constructor::Constructor::*;
 
@@ -208,12 +208,15 @@ impl<'p, 'tcx: 'p> RustcMatchCheckCtxt<'p, 'tcx> {
         &'a self,
         ctor: &'a Constructor<'p, 'tcx>,
         ty: RevealedTy<'tcx>,
-    ) -> impl Iterator<Item = RevealedTy<'tcx>> + ExactSizeIterator + Captures<'a> {
+    ) -> impl Iterator<Item = (RevealedTy<'tcx>, SkipField)> + ExactSizeIterator + Captures<'a>
+    {
         fn reveal_and_alloc<'a, 'tcx>(
             cx: &'a RustcMatchCheckCtxt<'_, 'tcx>,
             iter: impl Iterator<Item = Ty<'tcx>>,
-        ) -> &'a [RevealedTy<'tcx>] {
-            cx.dropless_arena.alloc_from_iter(iter.map(|ty| cx.reveal_opaque_ty(ty)))
+        ) -> &'a [(RevealedTy<'tcx>, SkipField)] {
+            cx.dropless_arena.alloc_from_iter(
+                iter.map(|ty| cx.reveal_opaque_ty(ty)).map(|ty| (ty, SkipField(false))),
+            )
         }
         let cx = self;
         let slice = match ctor {
@@ -229,8 +232,7 @@ impl<'p, 'tcx: 'p> RustcMatchCheckCtxt<'p, 'tcx> {
                             &adt.variant(RustcMatchCheckCtxt::variant_index_for_adt(&ctor, *adt));
                         let tys = cx
                             .list_variant_nonhidden_fields(ty, variant)
-                            .filter(|(_, _, skip)| !skip)
-                            .map(|(_, ty, _)| ty);
+                            .map(|(_, ty, skip)| (ty, SkipField(skip)));
                         cx.dropless_arena.alloc_from_iter(tys)
                     }
                 }
@@ -872,7 +874,7 @@ impl<'p, 'tcx: 'p> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
         &'a self,
         ctor: &'a crate::constructor::Constructor<Self>,
         ty: &'a Self::Ty,
-    ) -> impl Iterator<Item = Self::Ty> + ExactSizeIterator + Captures<'a> {
+    ) -> impl Iterator<Item = (Self::Ty, SkipField)> + ExactSizeIterator + Captures<'a> {
         self.ctor_sub_tys(ctor, *ty)
     }
     fn ctors_for_ty(

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -716,7 +716,7 @@ use std::fmt;
 
 use crate::constructor::{Constructor, ConstructorSet, IntRange};
 use crate::pat::{DeconstructedPat, PatId, PatOrWild, WitnessPat};
-use crate::{Captures, MatchArm, TypeCx};
+use crate::{Captures, MatchArm, SkipField, TypeCx};
 
 use self::ValidityConstraint::*;
 
@@ -833,7 +833,9 @@ impl<Cx: TypeCx> PlaceInfo<Cx> {
     ) -> impl Iterator<Item = Self> + ExactSizeIterator + Captures<'a> {
         let ctor_sub_tys = cx.ctor_sub_tys(ctor, &self.ty);
         let ctor_sub_validity = self.validity.specialize(ctor);
-        ctor_sub_tys.map(move |ty| PlaceInfo {
+        // Collect to keep the `ExactSizeIterator` bound. This is a temporary measure.
+        let tmp: Vec<_> = ctor_sub_tys.filter(|(_, SkipField(skip))| !skip).collect();
+        tmp.into_iter().map(move |(ty, _)| PlaceInfo {
             ty,
             validity: ctor_sub_validity,
             is_scrutinee: false,