Introduce an arena type which may be used to allocate a list of types with destructors

src/librustc/arena.rs

@@ -0,0 +1,84 @@
+use arena::{TypedArena, DroplessArena};
+use std::mem;
+
+#[macro_export]
+macro_rules! arena_types {
+    ($macro:path, $args:tt, $tcx:lifetime) => (
+        $macro!($args, [
+            [] vtable_method: Option<(
+                rustc::hir::def_id::DefId,
+                rustc::ty::subst::SubstsRef<$tcx>
+            )>,
+            [decode] specialization_graph: rustc::traits::specialization_graph::Graph,
+        ], $tcx);
+    )
+}
+
+macro_rules! declare_arena {
+    ([], [$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) => {
+        #[derive(Default)]
+        pub struct Arena<$tcx> {
+            dropless: DroplessArena,
+            $($name: TypedArena<$ty>,)*
+        }
+    }
+}
+
+macro_rules! impl_arena_allocatable {
+    ([], [$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) => {
+        $(
+            impl ArenaAllocatable for $ty {}
+            impl<$tcx> ArenaField<$tcx> for $ty {
+                #[inline]
+                fn arena<'a>(arena: &'a Arena<$tcx>) -> &'a TypedArena<Self> {
+                    &arena.$name
+                }
+            }
+        )*
+    }
+}
+
+arena_types!(declare_arena, [], 'tcx);
+
+arena_types!(impl_arena_allocatable, [], 'tcx);
+
+pub trait ArenaAllocatable {}
+
+impl<T: Copy> ArenaAllocatable for T {}
+
+pub trait ArenaField<'tcx>: Sized {
+    /// Returns a specific arena to allocate from.
+    fn arena<'a>(arena: &'a Arena<'tcx>) -> &'a TypedArena<Self>;
+}
+
+impl<'tcx, T> ArenaField<'tcx> for T {
+    #[inline]
+    default fn arena<'a>(_: &'a Arena<'tcx>) -> &'a TypedArena<Self> {
+        panic!()
+    }
+}
+
+impl<'tcx> Arena<'tcx> {
+    #[inline]
+    pub fn alloc<T: ArenaAllocatable>(&self, value: T) -> &mut T {
+        if mem::needs_drop::<T>() {
+            <T as ArenaField<'tcx>>::arena(self).alloc(value)
+        } else {
+            self.dropless.alloc(value)
+        }
+    }
+
+    pub fn alloc_from_iter<
+        T: ArenaAllocatable,
+        I: IntoIterator<Item = T>
+    >(
+        &self,
+        iter: I
+    ) -> &mut [T] {
+        if mem::needs_drop::<T>() {
+            <T as ArenaField<'tcx>>::arena(self).alloc_from_iter(iter)
+        } else {
+            self.dropless.alloc_from_iter(iter)
+        }
+    }
+}

src/librustc/lib.rs

@@ -40,6 +40,7 @@
 #![cfg_attr(windows, feature(libc))]
 #![feature(never_type)]
 #![feature(exhaustive_patterns)]
+#![feature(overlapping_marker_traits)]
 #![feature(extern_types)]
 #![feature(nll)]
 #![feature(non_exhaustive)]
@@ -103,6 +104,8 @@ pub mod diagnostics;
 #[macro_use]
 pub mod query;
 
+#[macro_use]
+pub mod arena;
 pub mod cfg;
 pub mod dep_graph;
 pub mod hir;

src/librustc/query/mod.rs

@@ -516,7 +516,7 @@ rustc_queries! {
 
     Other {
         query vtable_methods(key: ty::PolyTraitRef<'tcx>)
-                            -> Lrc<Vec<Option<(DefId, SubstsRef<'tcx>)>>> {
+                            -> &'tcx [Option<(DefId, SubstsRef<'tcx>)>] {
             no_force
             desc { |tcx| "finding all methods for trait {}", tcx.def_path_str(key.def_id()) }
         }
@@ -539,8 +539,7 @@ rustc_queries! {
         query trait_impls_of(key: DefId) -> Lrc<ty::trait_def::TraitImpls> {
             desc { |tcx| "trait impls of `{}`", tcx.def_path_str(key) }
         }
-        query specialization_graph_of(_: DefId)
-            -> Lrc<specialization_graph::Graph> {}
+        query specialization_graph_of(_: DefId) -> &'tcx specialization_graph::Graph {}
         query is_object_safe(key: DefId) -> bool {
             desc { |tcx| "determine object safety of trait `{}`", tcx.def_path_str(key) }
         }

src/librustc/traits/mod.rs

@@ -26,7 +26,6 @@ use crate::infer::{InferCtxt, SuppressRegionErrors};
 use crate::infer::outlives::env::OutlivesEnvironment;
 use crate::middle::region;
 use crate::mir::interpret::ErrorHandled;
-use rustc_data_structures::sync::Lrc;
 use rustc_macros::HashStable;
 use syntax::ast;
 use syntax_pos::{Span, DUMMY_SP};
@@ -984,11 +983,11 @@ fn substitute_normalize_and_test_predicates<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx
 fn vtable_methods<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     trait_ref: ty::PolyTraitRef<'tcx>)
-    -> Lrc<Vec<Option<(DefId, SubstsRef<'tcx>)>>>
+    -> &'tcx [Option<(DefId, SubstsRef<'tcx>)>]
 {
     debug!("vtable_methods({:?})", trait_ref);
 
-    Lrc::new(
+    tcx.arena.alloc_from_iter(
         supertraits(tcx, trait_ref).flat_map(move |trait_ref| {
             let trait_methods = tcx.associated_items(trait_ref.def_id())
                 .filter(|item| item.kind == ty::AssociatedKind::Method);
@@ -1039,7 +1038,7 @@ fn vtable_methods<'a, 'tcx>(
 
                 Some((def_id, substs))
             })
-        }).collect()
+        })
     )
 }
 

src/librustc/traits/specialize/mod.rs

@@ -16,7 +16,6 @@ use crate::infer::{InferCtxt, InferOk};
 use crate::lint;
 use crate::traits::{self, coherence, FutureCompatOverlapErrorKind, ObligationCause, TraitEngine};
 use rustc_data_structures::fx::FxHashSet;
-use rustc_data_structures::sync::Lrc;
 use syntax_pos::DUMMY_SP;
 use crate::traits::select::IntercrateAmbiguityCause;
 use crate::ty::{self, TyCtxt, TypeFoldable};
@@ -289,7 +288,7 @@ fn fulfill_implication<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 pub(super) fn specialization_graph_provider<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     trait_id: DefId,
-) -> Lrc<specialization_graph::Graph> {
+) -> &'tcx specialization_graph::Graph {
     let mut sg = specialization_graph::Graph::new();
 
     let mut trait_impls = tcx.all_impls(trait_id);
@@ -383,7 +382,7 @@ pub(super) fn specialization_graph_provider<'a, 'tcx>(
         }
     }
 
-    Lrc::new(sg)
+    tcx.arena.alloc(sg)
 }
 
 /// Recovers the "impl X for Y" signature from `impl_def_id` and returns it as a

src/librustc/traits/specialize/specialization_graph.rs

@@ -7,7 +7,6 @@ use rustc_data_structures::stable_hasher::{HashStable, StableHasher,
 use crate::traits;
 use crate::ty::{self, TyCtxt, TypeFoldable};
 use crate::ty::fast_reject::{self, SimplifiedType};
-use rustc_data_structures::sync::Lrc;
 use syntax::ast::Ident;
 use crate::util::captures::Captures;
 use crate::util::nodemap::{DefIdMap, FxHashMap};
@@ -439,13 +438,13 @@ impl<'a, 'gcx, 'tcx> Node {
     }
 }
 
-pub struct Ancestors {
+pub struct Ancestors<'tcx> {
     trait_def_id: DefId,
-    specialization_graph: Lrc<Graph>,
+    specialization_graph: &'tcx Graph,
     current_source: Option<Node>,
 }
 
-impl Iterator for Ancestors {
+impl Iterator for Ancestors<'_> {
     type Item = Node;
     fn next(&mut self) -> Option<Node> {
         let cur = self.current_source.take();
@@ -476,7 +475,7 @@ impl<T> NodeItem<T> {
     }
 }
 
-impl<'a, 'gcx, 'tcx> Ancestors {
+impl<'a, 'gcx, 'tcx> Ancestors<'gcx> {
     /// Search the items from the given ancestors, returning each definition
     /// with the given name and the given kind.
     // FIXME(#35870): avoid closures being unexported due to `impl Trait`.
@@ -509,10 +508,10 @@ impl<'a, 'gcx, 'tcx> Ancestors {
 
 /// Walk up the specialization ancestors of a given impl, starting with that
 /// impl itself.
-pub fn ancestors(tcx: TyCtxt<'_, '_, '_>,
+pub fn ancestors(tcx: TyCtxt<'_, 'tcx, '_>,
                  trait_def_id: DefId,
                  start_from_impl: DefId)
-                 -> Ancestors {
+                 -> Ancestors<'tcx> {
     let specialization_graph = tcx.specialization_graph_of(trait_def_id);
     Ancestors {
         trait_def_id,

src/librustc/ty/codec.rs

@@ -6,6 +6,7 @@
 // The functionality in here is shared between persisting to crate metadata and
 // persisting to incr. comp. caches.
 
+use crate::arena::ArenaAllocatable;
 use crate::hir::def_id::{DefId, CrateNum};
 use crate::infer::canonical::{CanonicalVarInfo, CanonicalVarInfos};
 use rustc_data_structures::fx::FxHashMap;
@@ -130,6 +131,26 @@ pub trait TyDecoder<'a, 'tcx: 'a>: Decoder {
     }
 }
 
+#[inline]
+pub fn decode_arena_allocable<'a, 'tcx, D, T: ArenaAllocatable + Decodable>(
+    decoder: &mut D
+) -> Result<&'tcx T, D::Error>
+    where D: TyDecoder<'a, 'tcx>,
+          'tcx: 'a,
+{
+    Ok(decoder.tcx().arena.alloc(Decodable::decode(decoder)?))
+}
+
+#[inline]
+pub fn decode_arena_allocable_slice<'a, 'tcx, D, T: ArenaAllocatable + Decodable>(
+    decoder: &mut D
+) -> Result<&'tcx [T], D::Error>
+    where D: TyDecoder<'a, 'tcx>,
+          'tcx: 'a,
+{
+    Ok(decoder.tcx().arena.alloc_from_iter(<Vec<T> as Decodable>::decode(decoder)?))
+}
+
 #[inline]
 pub fn decode_cnum<'a, 'tcx, D>(decoder: &mut D) -> Result<CrateNum, D::Error>
     where D: TyDecoder<'a, 'tcx>,
@@ -273,6 +294,35 @@ macro_rules! __impl_decoder_methods {
     }
 }
 
+#[macro_export]
+macro_rules! impl_arena_allocatable_decoder {
+    ([$DecoderName:ident [$($typaram:tt),*]], [[decode] $name:ident: $ty:ty], $tcx:lifetime) => {
+        impl<$($typaram),*> SpecializedDecoder<&$tcx $ty> for $DecoderName<$($typaram),*> {
+            #[inline]
+            fn specialized_decode(&mut self) -> Result<&$tcx $ty, Self::Error> {
+                decode_arena_allocable(self)
+            }
+        }
+
+        impl<$($typaram),*> SpecializedDecoder<&$tcx [$ty]> for $DecoderName<$($typaram),*> {
+            #[inline]
+            fn specialized_decode(&mut self) -> Result<&$tcx [$ty], Self::Error> {
+                decode_arena_allocable_slice(self)
+            }
+        }
+    };
+    ([$DecoderName:ident [$($typaram:tt),*]], [[] $name:ident: $ty:ty], $tcx:lifetime) => {};
+}
+
+#[macro_export]
+macro_rules! impl_arena_allocatable_decoders {
+    ($args:tt, [$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) => {
+        $(
+            impl_arena_allocatable_decoder!($args, [$a $name: $ty], $tcx);
+        )*
+    }
+}
+
 #[macro_export]
 macro_rules! implement_ty_decoder {
     ($DecoderName:ident <$($typaram:tt),*>) => {
@@ -322,6 +372,8 @@ macro_rules! implement_ty_decoder {
             // the caller to pick any lifetime for 'tcx, including 'static,
             // by using the unspecialized proxies to them.
 
+            arena_types!(impl_arena_allocatable_decoders, [$DecoderName [$($typaram),*]], 'tcx);
+
             impl<$($typaram),*> SpecializedDecoder<CrateNum>
             for $DecoderName<$($typaram),*> {
                 fn specialized_decode(&mut self) -> Result<CrateNum, Self::Error> {

src/librustc/ty/context.rs

@@ -1,5 +1,6 @@
 //! Type context book-keeping.
 
+use crate::arena::Arena;
 use crate::dep_graph::DepGraph;
 use crate::dep_graph::{self, DepNode, DepConstructor};
 use crate::session::Session;
@@ -1003,6 +1004,7 @@ impl<'gcx> Deref for TyCtxt<'_, 'gcx, '_> {
 }
 
 pub struct GlobalCtxt<'tcx> {
+    pub arena: WorkerLocal<Arena<'tcx>>,
     global_arenas: &'tcx WorkerLocal<GlobalArenas<'tcx>>,
     global_interners: CtxtInterners<'tcx>,
 
@@ -1262,6 +1264,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         GlobalCtxt {
             sess: s,
             cstore,
+            arena: WorkerLocal::new(|_| Arena::default()),
             global_arenas: &arenas.global,
             global_interners: interners,
             dep_graph,

src/librustc/ty/trait_def.rs

@@ -67,7 +67,7 @@ impl<'a, 'gcx, 'tcx> TraitDef {
 
     pub fn ancestors(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>,
                      of_impl: DefId)
-                     -> specialization_graph::Ancestors {
+                     -> specialization_graph::Ancestors<'gcx> {
         specialization_graph::ancestors(tcx, self.def_id, of_impl)
     }
 }

src/librustc_mir/interpret/traits.rs

@@ -1,4 +1,3 @@
-use rustc_data_structures::sync::Lrc;
 use rustc::ty::{self, Ty};
 use rustc::ty::layout::{Size, Align, LayoutOf};
 use rustc::mir::interpret::{Scalar, Pointer, EvalResult, PointerArithmetic};
@@ -35,7 +34,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> InterpretCx<'a, 'mir, 'tcx, M>
 
             self.tcx.vtable_methods(trait_ref)
         } else {
-            Lrc::new(Vec::new())
+            &[]
         };
 
         let layout = self.layout_of(ty)?;

src/libserialize/serialize.rs

@@ -911,4 +911,5 @@ impl<T: UseSpecializedDecodable> Decodable for T {
 impl<'a, T: ?Sized + Encodable> UseSpecializedEncodable for &'a T {}
 impl<T: ?Sized + Encodable> UseSpecializedEncodable for Box<T> {}
 impl<T: Decodable> UseSpecializedDecodable for Box<T> {}
-
+impl<'a, T: ?Sized + Decodable> UseSpecializedDecodable for &'a T {}
+impl<'a, T: ?Sized + Decodable> UseSpecializedDecodable for &'a [T] {}