safely transmute<&List<Ty<'tcx>>, &List<GenericArg<'tcx>>>

compiler/rustc_middle/src/ty/context.rs

@@ -101,7 +101,6 @@ pub struct CtxtInterners<'tcx> {
     // Specifically use a speedy hash algorithm for these hash sets, since
     // they're accessed quite often.
     type_: InternedSet<'tcx, TyS<'tcx>>,
-    type_list: InternedSet<'tcx, List<Ty<'tcx>>>,
     substs: InternedSet<'tcx, InternalSubsts<'tcx>>,
     canonical_var_infos: InternedSet<'tcx, List<CanonicalVarInfo<'tcx>>>,
     region: InternedSet<'tcx, RegionKind>,
@@ -129,7 +128,6 @@ impl<'tcx> CtxtInterners<'tcx> {
         CtxtInterners {
             arena,
             type_: Default::default(),
-            type_list: Default::default(),
             substs: Default::default(),
             region: Default::default(),
             poly_existential_predicates: Default::default(),
@@ -1666,6 +1664,23 @@ macro_rules! nop_lift {
     };
 }
 
+// Can't use the macros as we have reuse the `substs` here.
+//
+// See `intern_type_list` for more info.
+impl<'a, 'tcx> Lift<'tcx> for &'a List<Ty<'a>> {
+    type Lifted = &'tcx List<Ty<'tcx>>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
+        if self.is_empty() {
+            return Some(List::empty());
+        }
+        if tcx.interners.substs.contains_pointer_to(&InternedInSet(self.as_substs())) {
+            Some(unsafe { mem::transmute(self) })
+        } else {
+            None
+        }
+    }
+}
+
 macro_rules! nop_list_lift {
     ($set:ident; $ty:ty => $lifted:ty) => {
         impl<'a, 'tcx> Lift<'tcx> for &'a List<$ty> {
@@ -1690,7 +1705,6 @@ nop_lift! {const_; Const<'a> => Const<'tcx>}
 nop_lift_old! {const_allocation; &'a Allocation => &'tcx Allocation}
 nop_lift! {predicate; Predicate<'a> => Predicate<'tcx>}
 
-nop_list_lift! {type_list; Ty<'a> => Ty<'tcx>}
 nop_list_lift! {poly_existential_predicates; ty::Binder<'a, ExistentialPredicate<'a>> => ty::Binder<'tcx, ExistentialPredicate<'tcx>>}
 nop_list_lift! {predicates; Predicate<'a> => Predicate<'tcx>}
 nop_list_lift! {canonical_var_infos; CanonicalVarInfo<'a> => CanonicalVarInfo<'tcx>}
@@ -2189,7 +2203,6 @@ macro_rules! slice_interners {
 }
 
 slice_interners!(
-    type_list: _intern_type_list(Ty<'tcx>),
     substs: _intern_substs(GenericArg<'tcx>),
     canonical_var_infos: _intern_canonical_var_infos(CanonicalVarInfo<'tcx>),
     poly_existential_predicates:
@@ -2611,7 +2624,19 @@ impl<'tcx> TyCtxt<'tcx> {
     }
 
     pub fn intern_type_list(self, ts: &[Ty<'tcx>]) -> &'tcx List<Ty<'tcx>> {
-        if ts.is_empty() { List::empty() } else { self._intern_type_list(ts) }
+        if ts.is_empty() {
+            List::empty()
+        } else {
+            // Actually intern type lists as lists of `GenericArg`s.
+            //
+            // Transmuting from `Ty<'tcx>` to `GenericArg<'tcx>` is sound
+            // as explained in ty_slice_as_generic_arg`. With this,
+            // we guarantee that even when transmuting between `List<Ty<'tcx>>`
+            // and `List<GenericArg<'tcx>>`, the uniqueness requirement for
+            // lists is upheld.
+            let substs = self._intern_substs(ty::subst::ty_slice_as_generic_args(ts));
+            substs.try_as_type_list().unwrap()
+        }
     }
 
     pub fn intern_substs(self, ts: &[GenericArg<'tcx>]) -> &'tcx List<GenericArg<'tcx>> {

compiler/rustc_middle/src/ty/impls_ty.rs

@@ -61,6 +61,28 @@ impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ty::subst::GenericArg<'t
     }
 }
 
+impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ty::subst::GenericArgKind<'tcx> {
+    fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
+        match self {
+            // WARNING: We dedup cache the `HashStable` results for `List`
+            // while ignoring types and freely transmute
+            // between `List<Ty<'tcx>>` and `List<GenericArg<'tcx>>`.
+            // See `fn intern_type_list` for more details.
+            //
+            // We therefore hash types without adding a hash for their discriminant.
+            ty::subst::GenericArgKind::Type(ty) => ty.hash_stable(hcx, hasher),
+            ty::subst::GenericArgKind::Const(ct) => {
+                mem::discriminant(self).hash_stable(hcx, hasher);
+                ct.hash_stable(hcx, hasher);
+            }
+            ty::subst::GenericArgKind::Lifetime(lt) => {
+                mem::discriminant(self).hash_stable(hcx, hasher);
+                lt.hash_stable(hcx, hasher);
+            }
+        }
+    }
+}
+
 impl<'a> HashStable<StableHashingContext<'a>> for ty::RegionKind {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
         mem::discriminant(self).hash_stable(hcx, hasher);

compiler/rustc_middle/src/ty/subst.rs

@@ -20,6 +20,7 @@ use std::marker::PhantomData;
 use std::mem;
 use std::num::NonZeroUsize;
 use std::ops::ControlFlow;
+use std::slice;
 
 /// An entity in the Rust type system, which can be one of
 /// several kinds (types, lifetimes, and consts).
@@ -40,13 +41,37 @@ const TYPE_TAG: usize = 0b00;
 const REGION_TAG: usize = 0b01;
 const CONST_TAG: usize = 0b10;
 
-#[derive(Debug, TyEncodable, TyDecodable, PartialEq, Eq, PartialOrd, Ord, HashStable)]
+#[derive(Debug, TyEncodable, TyDecodable, PartialEq, Eq, PartialOrd, Ord)]
 pub enum GenericArgKind<'tcx> {
     Lifetime(ty::Region<'tcx>),
     Type(Ty<'tcx>),
     Const(ty::Const<'tcx>),
 }
 
+/// This function goes from `&'a [Ty<'tcx>]` to `&'a [GenericArg<'tcx>]`
+///
+/// This is sound as, for types, `GenericArg` is just
+/// `NonZeroUsize::new_unchecked(ty as *const _ as usize)`.
+pub fn ty_slice_as_generic_args<'a, 'tcx>(ts: &'a [Ty<'tcx>]) -> &'a [GenericArg<'tcx>] {
+    assert_eq!(TYPE_TAG, 0);
+    // SAFETY: the whole slice is valid and immutable.
+    // `Ty` and `GenericArg` is explained above.
+    unsafe { slice::from_raw_parts(ts.as_ptr().cast(), ts.len()) }
+}
+
+impl<'tcx> List<Ty<'tcx>> {
+    /// Allows to freely switch betwen `List<Ty<'tcx>>` and `List<GenericArg<'tcx>>`.
+    ///
+    /// As lists are interned, `List<Ty<'tcx>>` and `List<GenericArg<'tcx>>` have
+    /// be interned together, see `intern_type_list` for more details.
+    #[inline]
+    pub fn as_substs(&'tcx self) -> SubstsRef<'tcx> {
+        assert_eq!(TYPE_TAG, 0);
+        // SAFETY: `List<T>` is `#[repr(C)]`. `Ty` and `GenericArg` is explained above.
+        unsafe { &*(self as *const List<Ty<'tcx>> as *const List<GenericArg<'tcx>>) }
+    }
+}
+
 impl<'tcx> GenericArgKind<'tcx> {
     #[inline]
     fn pack(self) -> GenericArg<'tcx> {
@@ -208,6 +233,17 @@ pub type InternalSubsts<'tcx> = List<GenericArg<'tcx>>;
 pub type SubstsRef<'tcx> = &'tcx InternalSubsts<'tcx>;
 
 impl<'a, 'tcx> InternalSubsts<'tcx> {
+    /// Checks whether all elements of this list are types, if so, transmute.
+    pub fn try_as_type_list(&'tcx self) -> Option<&'tcx List<Ty<'tcx>>> {
+        if self.iter().all(|arg| matches!(arg.unpack(), GenericArgKind::Type(_))) {
+            assert_eq!(TYPE_TAG, 0);
+            // SAFETY: All elements are types, see `List<Ty<'tcx>>::as_substs`.
+            Some(unsafe { &*(self as *const List<GenericArg<'tcx>> as *const List<Ty<'tcx>>) })
+        } else {
+            None
+        }
+    }
+
     /// Interpret these substitutions as the substitutions of a closure type.
     /// Closure substitutions have a particular structure controlled by the
     /// compiler that encodes information like the signature and closure kind;