Let ctor_sub_tys return any Iterator they want

Since we always clone and allocate the types somewhere else ourselves,
no need to ask for `Cx` to do the allocation.

compiler/rustc_pattern_analysis/src/lib.rs

@@ -101,7 +101,11 @@ pub trait TypeCx: Sized + fmt::Debug {
 
     /// The types of the fields for this constructor. The result must have a length of
     /// `ctor_arity()`.
-    fn ctor_sub_tys(&self, ctor: &Constructor<Self>, ty: &Self::Ty) -> &[Self::Ty];
+    fn ctor_sub_tys<'a>(
+        &'a self,
+        ctor: &'a Constructor<Self>,
+        ty: &'a Self::Ty,
+    ) -> impl Iterator<Item = Self::Ty> + ExactSizeIterator + Captures<'a>;
 
     /// The set of all the constructors for `ty`.
     ///

compiler/rustc_pattern_analysis/src/pat.rs

@@ -241,8 +241,7 @@ 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(pcx: &PlaceCtxt<'_, Cx>, ctor: Constructor<Cx>) -> Self {
-        let field_tys = pcx.ctor_sub_tys(&ctor);
-        let fields = field_tys.iter().cloned().map(|ty| Self::wildcard(ty)).collect();
+        let fields = pcx.ctor_sub_tys(&ctor).map(|ty| Self::wildcard(ty)).collect();
         Self::new(ctor, fields, pcx.ty.clone())
     }
 

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -3,7 +3,6 @@ use std::fmt;
 use std::iter::once;
 
 use rustc_arena::{DroplessArena, TypedArena};
-use rustc_data_structures::captures::Captures;
 use rustc_hir::def_id::DefId;
 use rustc_hir::HirId;
 use rustc_index::{Idx, IndexVec};
@@ -20,7 +19,7 @@ use rustc_target::abi::{FieldIdx, Integer, VariantIdx, FIRST_VARIANT};
 use crate::constructor::{
     IntRange, MaybeInfiniteInt, OpaqueId, RangeEnd, Slice, SliceKind, VariantVisibility,
 };
-use crate::{errors, TypeCx};
+use crate::{errors, Captures, TypeCx};
 
 use crate::constructor::Constructor::*;
 
@@ -210,19 +209,19 @@ impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
     /// Returns the types of the fields for a given constructor. The result must have a length of
     /// `ctor.arity()`.
     #[instrument(level = "trace", skip(self))]
-    pub(crate) fn ctor_sub_tys(
-        &self,
-        ctor: &Constructor<'p, 'tcx>,
+    pub(crate) fn ctor_sub_tys<'a>(
+        &'a self,
+        ctor: &'a Constructor<'p, 'tcx>,
         ty: RevealedTy<'tcx>,
-    ) -> &[RevealedTy<'tcx>] {
+    ) -> impl Iterator<Item = RevealedTy<'tcx>> + 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)))
         }
         let cx = self;
-        match ctor {
+        let slice = match ctor {
             Struct | Variant(_) | UnionField => match ty.kind() {
                 ty::Tuple(fs) => reveal_and_alloc(cx, fs.iter()),
                 ty::Adt(adt, args) => {
@@ -263,7 +262,8 @@ impl<'p, 'tcx> RustcMatchCheckCtxt<'p, 'tcx> {
             Or => {
                 bug!("called `Fields::wildcards` on an `Or` ctor")
             }
-        }
+        };
+        slice.iter().copied()
     }
 
     /// The number of fields for this constructor.
@@ -964,11 +964,11 @@ impl<'p, 'tcx> TypeCx for RustcMatchCheckCtxt<'p, 'tcx> {
     fn ctor_arity(&self, ctor: &crate::constructor::Constructor<Self>, ty: &Self::Ty) -> usize {
         self.ctor_arity(ctor, *ty)
     }
-    fn ctor_sub_tys(
-        &self,
-        ctor: &crate::constructor::Constructor<Self>,
-        ty: &Self::Ty,
-    ) -> &[Self::Ty] {
+    fn ctor_sub_tys<'a>(
+        &'a self,
+        ctor: &'a crate::constructor::Constructor<Self>,
+        ty: &'a Self::Ty,
+    ) -> impl Iterator<Item = Self::Ty> + ExactSizeIterator + Captures<'a> {
         self.ctor_sub_tys(ctor, *ty)
     }
     fn ctors_for_ty(

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -750,7 +750,10 @@ impl<'a, Cx: TypeCx> PlaceCtxt<'a, Cx> {
     pub(crate) fn ctor_arity(&self, ctor: &Constructor<Cx>) -> usize {
         self.mcx.tycx.ctor_arity(ctor, self.ty)
     }
-    pub(crate) fn ctor_sub_tys(&self, ctor: &Constructor<Cx>) -> &[Cx::Ty] {
+    pub(crate) fn ctor_sub_tys(
+        &'a self,
+        ctor: &'a Constructor<Cx>,
+    ) -> impl Iterator<Item = Cx::Ty> + ExactSizeIterator + Captures<'a> {
         self.mcx.tycx.ctor_sub_tys(ctor, self.ty)
     }
     pub(crate) fn ctors_for_ty(&self) -> Result<ConstructorSet<Cx>, Cx::Error> {
@@ -1058,8 +1061,7 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
     ) -> Matrix<'p, Cx> {
         let ctor_sub_tys = pcx.ctor_sub_tys(ctor);
         let arity = ctor_sub_tys.len();
-        let specialized_place_ty =
-            ctor_sub_tys.iter().chain(self.place_ty[1..].iter()).cloned().collect();
+        let specialized_place_ty = ctor_sub_tys.chain(self.place_ty[1..].iter().cloned()).collect();
         let ctor_sub_validity = self.place_validity[0].specialize(ctor);
         let specialized_place_validity = std::iter::repeat(ctor_sub_validity)
             .take(arity)