From d1a9a9551741c3e888d350d8d4f4821a5addccb2 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Sun, 22 May 2022 12:34:34 -0700
Subject: [PATCH] Make Lazy not care about lifetimes until decode

---
 compiler/rustc_metadata/src/creader.rs        |   6 +-
 compiler/rustc_metadata/src/lib.rs            |   1 +
 compiler/rustc_metadata/src/rmeta/decoder.rs  |  34 +++--
 .../src/rmeta/def_path_hash_map.rs            |   5 +
 compiler/rustc_metadata/src/rmeta/encoder.rs  |  27 ++--
 compiler/rustc_metadata/src/rmeta/mod.rs      |  67 ++++++----
 compiler/rustc_metadata/src/rmeta/table.rs    |   9 +-
 compiler/rustc_middle/src/lib.rs              |   1 +
 compiler/rustc_middle/src/ty/mod.rs           |   2 +
 compiler/rustc_middle/src/ty/parameterized.rs | 119 ++++++++++++++++++
 10 files changed, 217 insertions(+), 54 deletions(-)
 create mode 100644 compiler/rustc_middle/src/ty/parameterized.rs

diff --git a/compiler/rustc_metadata/src/creader.rs b/compiler/rustc_metadata/src/creader.rs
index dfc675a0494b1..947d563ae3cd1 100644
--- a/compiler/rustc_metadata/src/creader.rs
+++ b/compiler/rustc_metadata/src/creader.rs
@@ -323,7 +323,7 @@ impl<'a> CrateLoader<'a> {
         None
     }
 
-    fn verify_no_symbol_conflicts(&self, root: &CrateRoot<'_>) -> Result<(), CrateError> {
+    fn verify_no_symbol_conflicts(&self, root: &CrateRoot) -> Result<(), CrateError> {
         // Check for (potential) conflicts with the local crate
         if self.sess.local_stable_crate_id() == root.stable_crate_id() {
             return Err(CrateError::SymbolConflictsCurrent(root.name()));
@@ -342,7 +342,7 @@ impl<'a> CrateLoader<'a> {
 
     fn verify_no_stable_crate_id_hash_conflicts(
         &mut self,
-        root: &CrateRoot<'_>,
+        root: &CrateRoot,
         cnum: CrateNum,
     ) -> Result<(), CrateError> {
         if let Some(existing) = self.cstore.stable_crate_ids.insert(root.stable_crate_id(), cnum) {
@@ -623,7 +623,7 @@ impl<'a> CrateLoader<'a> {
     fn resolve_crate_deps(
         &mut self,
         root: &CratePaths,
-        crate_root: &CrateRoot<'_>,
+        crate_root: &CrateRoot,
         metadata: &MetadataBlob,
         krate: CrateNum,
         dep_kind: CrateDepKind,
diff --git a/compiler/rustc_metadata/src/lib.rs b/compiler/rustc_metadata/src/lib.rs
index 5e4c1284e241e..4c5d13dfa6c8c 100644
--- a/compiler/rustc_metadata/src/lib.rs
+++ b/compiler/rustc_metadata/src/lib.rs
@@ -2,6 +2,7 @@
 #![feature(decl_macro)]
 #![feature(drain_filter)]
 #![feature(generators)]
+#![feature(generic_associated_types)]
 #![feature(let_chains)]
 #![feature(let_else)]
 #![feature(nll)]
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 72b7c08f04bc2..1420871b0256e 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -25,7 +25,7 @@ use rustc_middle::thir;
 use rustc_middle::ty::codec::TyDecoder;
 use rustc_middle::ty::fast_reject::SimplifiedType;
 use rustc_middle::ty::GeneratorDiagnosticData;
-use rustc_middle::ty::{self, Ty, TyCtxt, Visibility};
+use rustc_middle::ty::{self, ParameterizedOverTcx, Ty, TyCtxt, Visibility};
 use rustc_serialize::{opaque, Decodable, Decoder};
 use rustc_session::cstore::{
     CrateSource, ExternCrate, ForeignModule, LinkagePreference, NativeLib,
@@ -86,7 +86,7 @@ pub(crate) struct CrateMetadata {
     /// lifetime is only used behind `Lazy`, and therefore acts like a
     /// universal (`for<'tcx>`), that is paired up with whichever `TyCtxt`
     /// is being used to decode those values.
-    root: CrateRoot<'static>,
+    root: CrateRoot,
     /// Trait impl data.
     /// FIXME: Used only from queries and can use query cache,
     /// so pre-decoding can probably be avoided.
@@ -261,11 +261,14 @@ impl<'a, 'tcx> Metadata<'a, 'tcx> for (CrateMetadataRef<'a>, TyCtxt<'tcx>) {
     }
 }
 
-impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> LazyValue<T> {
-    fn decode<M: Metadata<'a, 'tcx>>(self, metadata: M) -> T {
+impl<T: ParameterizedOverTcx> LazyValue<T> {
+    fn decode<'a, 'tcx, M: Metadata<'a, 'tcx>>(self, metadata: M) -> T::Value<'tcx>
+    where
+        T::Value<'tcx>: Decodable<DecodeContext<'a, 'tcx>>,
+    {
         let mut dcx = metadata.decoder(self.position.get());
         dcx.lazy_state = LazyState::NodeStart(self.position);
-        T::decode(&mut dcx)
+        T::Value::decode(&mut dcx)
     }
 }
 
@@ -292,6 +295,9 @@ impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> Iterator for DecodeIterato
 impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> ExactSizeIterator
     for DecodeIterator<'a, 'tcx, T>
 {
+    fn len(&self) -> usize {
+        self.elem_counter.len()
+    }
 }
 
 unsafe impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> TrustedLen
@@ -299,8 +305,14 @@ unsafe impl<'a, 'tcx, T: Decodable<DecodeContext<'a, 'tcx>>> TrustedLen
 {
 }
 
-impl<'a: 'x, 'tcx: 'x, 'x, T: Decodable<DecodeContext<'a, 'tcx>>> LazyArray<T> {
-    fn decode<M: Metadata<'a, 'tcx>>(self, metadata: M) -> DecodeIterator<'a, 'tcx, T> {
+impl<T: ParameterizedOverTcx> LazyArray<T> {
+    fn decode<'a, 'tcx, M: Metadata<'a, 'tcx>>(
+        self,
+        metadata: M,
+    ) -> DecodeIterator<'a, 'tcx, T::Value<'tcx>>
+    where
+        T::Value<'tcx>: Decodable<DecodeContext<'a, 'tcx>>,
+    {
         let mut dcx = metadata.decoder(self.position.get());
         dcx.lazy_state = LazyState::NodeStart(self.position);
         DecodeIterator { elem_counter: (0..self.num_elems), dcx, _phantom: PhantomData }
@@ -671,14 +683,14 @@ impl MetadataBlob {
             .decode(self)
     }
 
-    pub(crate) fn get_root<'tcx>(&self) -> CrateRoot<'tcx> {
+    pub(crate) fn get_root(&self) -> CrateRoot {
         let slice = &self.blob()[..];
         let offset = METADATA_HEADER.len();
         let pos = (((slice[offset + 0] as u32) << 24)
             | ((slice[offset + 1] as u32) << 16)
             | ((slice[offset + 2] as u32) << 8)
             | ((slice[offset + 3] as u32) << 0)) as usize;
-        LazyValue::<CrateRoot<'tcx>>::from_position(NonZeroUsize::new(pos).unwrap()).decode(self)
+        LazyValue::<CrateRoot>::from_position(NonZeroUsize::new(pos).unwrap()).decode(self)
     }
 
     pub(crate) fn list_crate_metadata(&self, out: &mut dyn io::Write) -> io::Result<()> {
@@ -705,7 +717,7 @@ impl MetadataBlob {
     }
 }
 
-impl CrateRoot<'_> {
+impl CrateRoot {
     pub(crate) fn is_proc_macro_crate(&self) -> bool {
         self.proc_macro_data.is_some()
     }
@@ -1677,7 +1689,7 @@ impl CrateMetadata {
         sess: &Session,
         cstore: &CStore,
         blob: MetadataBlob,
-        root: CrateRoot<'static>,
+        root: CrateRoot,
         raw_proc_macros: Option<&'static [ProcMacro]>,
         cnum: CrateNum,
         cnum_map: CrateNumMap,
diff --git a/compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs b/compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs
index 06045bb3e3d97..15fd190b0492a 100644
--- a/compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs
+++ b/compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs
@@ -3,6 +3,7 @@ use crate::rmeta::EncodeContext;
 use crate::rmeta::MetadataBlob;
 use rustc_data_structures::owning_ref::OwningRef;
 use rustc_hir::def_path_hash_map::{Config as HashMapConfig, DefPathHashMap};
+use rustc_middle::parameterized_over_tcx;
 use rustc_serialize::{opaque, Decodable, Decoder, Encodable, Encoder};
 use rustc_span::def_id::{DefIndex, DefPathHash};
 
@@ -11,6 +12,10 @@ pub(crate) enum DefPathHashMapRef<'tcx> {
     BorrowedFromTcx(&'tcx DefPathHashMap),
 }
 
+parameterized_over_tcx! {
+    DefPathHashMapRef,
+}
+
 impl DefPathHashMapRef<'_> {
     #[inline]
     pub fn def_path_hash_to_def_index(&self, def_path_hash: &DefPathHash) -> DefIndex {
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 28f289f06ecb2..1e83434b9d062 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -47,7 +47,7 @@ pub(super) struct EncodeContext<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     feat: &'tcx rustc_feature::Features,
 
-    tables: TableBuilders<'tcx>,
+    tables: TableBuilders,
 
     lazy_state: LazyState,
     type_shorthands: FxHashMap<Ty<'tcx>, usize>,
@@ -388,10 +388,10 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         self.emit_usize(distance)
     }
 
-    fn lazy<T: Encodable<EncodeContext<'a, 'tcx>>, B: Borrow<T>>(
-        &mut self,
-        value: B,
-    ) -> LazyValue<T> {
+    fn lazy<T: ParameterizedOverTcx, B: Borrow<T::Value<'tcx>>>(&mut self, value: B) -> LazyValue<T>
+    where
+        T::Value<'tcx>: Encodable<EncodeContext<'a, 'tcx>>,
+    {
         let pos = NonZeroUsize::new(self.position()).unwrap();
 
         assert_eq!(self.lazy_state, LazyState::NoNode);
@@ -404,14 +404,13 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         LazyValue::from_position(pos)
     }
 
-    fn lazy_array<
-        T: Encodable<EncodeContext<'a, 'tcx>>,
-        I: IntoIterator<Item = B>,
-        B: Borrow<T>,
-    >(
+    fn lazy_array<T: ParameterizedOverTcx, I: IntoIterator<Item = B>, B: Borrow<T::Value<'tcx>>>(
         &mut self,
         values: I,
-    ) -> LazyArray<T> {
+    ) -> LazyArray<T>
+    where
+        T::Value<'tcx>: Encodable<EncodeContext<'a, 'tcx>>,
+    {
         let pos = NonZeroUsize::new(self.position()).unwrap();
 
         assert_eq!(self.lazy_state, LazyState::NoNode);
@@ -456,7 +455,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         }
     }
 
-    fn encode_def_path_hash_map(&mut self) -> LazyValue<DefPathHashMapRef<'tcx>> {
+    fn encode_def_path_hash_map(&mut self) -> LazyValue<DefPathHashMapRef<'static>> {
         self.lazy(DefPathHashMapRef::BorrowedFromTcx(
             self.tcx.resolutions(()).definitions.def_path_hash_to_def_index_map(),
         ))
@@ -535,7 +534,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         self.lazy_array(adapted.iter().map(|rc| &**rc))
     }
 
-    fn encode_crate_root(&mut self) -> LazyValue<CrateRoot<'tcx>> {
+    fn encode_crate_root(&mut self) -> LazyValue<CrateRoot> {
         let tcx = self.tcx;
         let mut i = self.position();
 
@@ -1859,7 +1858,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
     fn encode_exported_symbols(
         &mut self,
         exported_symbols: &[(ExportedSymbol<'tcx>, SymbolExportInfo)],
-    ) -> LazyArray<(ExportedSymbol<'tcx>, SymbolExportInfo)> {
+    ) -> LazyArray<(ExportedSymbol<'static>, SymbolExportInfo)> {
         empty_proc_macro!(self);
         // The metadata symbol name is special. It should not show up in
         // downstream crates.
diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index 00c4dc3955da0..fb2ffe1d73d96 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -21,7 +21,7 @@ use rustc_middle::thir;
 use rustc_middle::ty::fast_reject::SimplifiedType;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, ReprOptions, Ty};
-use rustc_middle::ty::{GeneratorDiagnosticData, TyCtxt};
+use rustc_middle::ty::{GeneratorDiagnosticData, ParameterizedOverTcx, TyCtxt};
 use rustc_serialize::opaque::Encoder;
 use rustc_session::config::SymbolManglingVersion;
 use rustc_session::cstore::{CrateDepKind, ForeignModule, LinkagePreference, NativeLib};
@@ -83,6 +83,10 @@ struct LazyValue<T> {
     _marker: PhantomData<fn() -> T>,
 }
 
+impl<T: ParameterizedOverTcx> ParameterizedOverTcx for LazyValue<T> {
+    type Value<'tcx> = LazyValue<T::Value<'tcx>>;
+}
+
 impl<T> LazyValue<T> {
     fn from_position(position: NonZeroUsize) -> LazyValue<T> {
         LazyValue { position, _marker: PhantomData }
@@ -105,6 +109,10 @@ struct LazyArray<T> {
     _marker: PhantomData<fn() -> T>,
 }
 
+impl<T: ParameterizedOverTcx> ParameterizedOverTcx for LazyArray<T> {
+    type Value<'tcx> = LazyArray<T::Value<'tcx>>;
+}
+
 impl<T> LazyArray<T> {
     fn from_position_and_num_elems(position: NonZeroUsize, num_elems: usize) -> LazyArray<T> {
         LazyArray { position, num_elems, _marker: PhantomData }
@@ -126,6 +134,10 @@ struct LazyTable<I, T> {
     _marker: PhantomData<fn(I) -> T>,
 }
 
+impl<I: 'static, T: ParameterizedOverTcx> ParameterizedOverTcx for LazyTable<I, T> {
+    type Value<'tcx> = LazyTable<I, T::Value<'tcx>>;
+}
+
 impl<I, T> LazyTable<I, T> {
     fn from_position_and_encoded_size(
         position: NonZeroUsize,
@@ -199,7 +211,7 @@ pub(crate) struct ProcMacroData {
 /// a normal crate, much of what we serialized would be unusable in addition
 /// to being unused.
 #[derive(MetadataEncodable, MetadataDecodable)]
-pub(crate) struct CrateRoot<'tcx> {
+pub(crate) struct CrateRoot {
     name: Symbol,
     triple: TargetTriple,
     extra_filename: String,
@@ -226,16 +238,16 @@ pub(crate) struct CrateRoot<'tcx> {
     interpret_alloc_index: LazyArray<u32>,
     proc_macro_data: Option<ProcMacroData>,
 
-    tables: LazyTables<'tcx>,
+    tables: LazyTables,
     debugger_visualizers: LazyArray<rustc_span::DebuggerVisualizerFile>,
 
-    exported_symbols: LazyArray<(ExportedSymbol<'tcx>, SymbolExportInfo)>,
+    exported_symbols: LazyArray<(ExportedSymbol<'static>, SymbolExportInfo)>,
 
     syntax_contexts: SyntaxContextTable,
     expn_data: ExpnDataTable,
     expn_hashes: ExpnHashTable,
 
-    def_path_hash_map: LazyValue<DefPathHashMapRef<'tcx>>,
+    def_path_hash_map: LazyValue<DefPathHashMapRef<'static>>,
 
     source_map: LazyArray<rustc_span::SourceFile>,
 
@@ -301,17 +313,17 @@ pub(crate) struct IncoherentImpls {
 macro_rules! define_tables {
     ($($name:ident: Table<$IDX:ty, $T:ty>),+ $(,)?) => {
         #[derive(MetadataEncodable, MetadataDecodable)]
-        pub(crate) struct LazyTables<'tcx> {
+        pub(crate) struct LazyTables {
             $($name: LazyTable<$IDX, $T>),+
         }
 
         #[derive(Default)]
-        struct TableBuilders<'tcx> {
+        struct TableBuilders {
             $($name: TableBuilder<$IDX, $T>),+
         }
 
-        impl<'tcx> TableBuilders<'tcx> {
-            fn encode(&self, buf: &mut Encoder) -> LazyTables<'tcx> {
+        impl TableBuilders {
+            fn encode(&self, buf: &mut Encoder) -> LazyTables {
                 LazyTables {
                     $($name: self.$name.encode(buf)),+
                 }
@@ -333,23 +345,23 @@ define_tables! {
     lookup_const_stability: Table<DefIndex, LazyValue<attr::ConstStability>>,
     lookup_deprecation_entry: Table<DefIndex, LazyValue<attr::Deprecation>>,
     // As an optimization, a missing entry indicates an empty `&[]`.
-    explicit_item_bounds: Table<DefIndex, LazyArray<(ty::Predicate<'tcx>, Span)>>,
-    explicit_predicates_of: Table<DefIndex, LazyValue<ty::GenericPredicates<'tcx>>>,
+    explicit_item_bounds: Table<DefIndex, LazyArray<(ty::Predicate<'static>, Span)>>,
+    explicit_predicates_of: Table<DefIndex, LazyValue<ty::GenericPredicates<'static>>>,
     generics_of: Table<DefIndex, LazyValue<ty::Generics>>,
     // As an optimization, a missing entry indicates an empty `&[]`.
-    inferred_outlives_of: Table<DefIndex, LazyArray<(ty::Predicate<'tcx>, Span)>>,
-    super_predicates_of: Table<DefIndex, LazyValue<ty::GenericPredicates<'tcx>>>,
-    type_of: Table<DefIndex, LazyValue<Ty<'tcx>>>,
+    inferred_outlives_of: Table<DefIndex, LazyArray<(ty::Predicate<'static>, Span)>>,
+    super_predicates_of: Table<DefIndex, LazyValue<ty::GenericPredicates<'static>>>,
+    type_of: Table<DefIndex, LazyValue<Ty<'static>>>,
     variances_of: Table<DefIndex, LazyArray<ty::Variance>>,
-    fn_sig: Table<DefIndex, LazyValue<ty::PolyFnSig<'tcx>>>,
+    fn_sig: Table<DefIndex, LazyValue<ty::PolyFnSig<'static>>>,
     codegen_fn_attrs: Table<DefIndex, LazyValue<CodegenFnAttrs>>,
-    impl_trait_ref: Table<DefIndex, LazyValue<ty::TraitRef<'tcx>>>,
-    const_param_default: Table<DefIndex, LazyValue<rustc_middle::ty::Const<'tcx>>>,
-    optimized_mir: Table<DefIndex, LazyValue<mir::Body<'tcx>>>,
-    mir_for_ctfe: Table<DefIndex, LazyValue<mir::Body<'tcx>>>,
-    promoted_mir: Table<DefIndex, LazyValue<IndexVec<mir::Promoted, mir::Body<'tcx>>>>,
+    impl_trait_ref: Table<DefIndex, LazyValue<ty::TraitRef<'static>>>,
+    const_param_default: Table<DefIndex, LazyValue<rustc_middle::ty::Const<'static>>>,
+    optimized_mir: Table<DefIndex, LazyValue<mir::Body<'static>>>,
+    mir_for_ctfe: Table<DefIndex, LazyValue<mir::Body<'static>>>,
+    promoted_mir: Table<DefIndex, LazyValue<IndexVec<mir::Promoted, mir::Body<'static>>>>,
     // FIXME(compiler-errors): Why isn't this a LazyArray?
-    thir_abstract_const: Table<DefIndex, LazyValue<&'tcx [thir::abstract_const::Node<'tcx>]>>,
+    thir_abstract_const: Table<DefIndex, LazyValue<&'static [thir::abstract_const::Node<'static>]>>,
     impl_parent: Table<DefIndex, RawDefId>,
     impl_polarity: Table<DefIndex, ty::ImplPolarity>,
     impl_constness: Table<DefIndex, hir::Constness>,
@@ -376,7 +388,7 @@ define_tables! {
     def_keys: Table<DefIndex, LazyValue<DefKey>>,
     def_path_hashes: Table<DefIndex, DefPathHash>,
     proc_macro_quoted_spans: Table<usize, LazyValue<Span>>,
-    generator_diagnostic_data: Table<DefIndex, LazyValue<GeneratorDiagnosticData<'tcx>>>,
+    generator_diagnostic_data: Table<DefIndex, LazyValue<GeneratorDiagnosticData<'static>>>,
     may_have_doc_links: Table<DefIndex, ()>,
 }
 
@@ -477,3 +489,14 @@ pub fn provide(providers: &mut Providers) {
     encoder::provide(providers);
     decoder::provide(providers);
 }
+
+trivially_parameterized_over_tcx! {
+    VariantData,
+    AssocFnData,
+    EntryKind,
+    RawDefId,
+    TraitImpls,
+    IncoherentImpls,
+    CrateRoot,
+    CrateDep,
+}
diff --git a/compiler/rustc_metadata/src/rmeta/table.rs b/compiler/rustc_metadata/src/rmeta/table.rs
index 60c5143a6c377..100bac15b80a4 100644
--- a/compiler/rustc_metadata/src/rmeta/table.rs
+++ b/compiler/rustc_metadata/src/rmeta/table.rs
@@ -3,6 +3,7 @@ use crate::rmeta::*;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_hir::def::{CtorKind, CtorOf};
 use rustc_index::vec::Idx;
+use rustc_middle::ty::ParameterizedOverTcx;
 use rustc_serialize::opaque::Encoder;
 use rustc_serialize::Encoder as _;
 use rustc_span::hygiene::MacroKind;
@@ -296,7 +297,7 @@ where
     }
 }
 
-impl<I: Idx, T> LazyTable<I, T>
+impl<I: Idx, T: ParameterizedOverTcx> LazyTable<I, T>
 where
     Option<T>: FixedSizeEncoding,
 {
@@ -306,9 +307,9 @@ where
         &self,
         metadata: M,
         i: I,
-    ) -> Option<T>
+    ) -> Option<T::Value<'tcx>>
     where
-        Option<T>: FixedSizeEncoding<ByteArray = [u8; N]>,
+        Option<T::Value<'tcx>>: FixedSizeEncoding<ByteArray = [u8; N]>,
     {
         debug!("LazyTable::lookup: index={:?} len={:?}", i, self.encoded_size);
 
@@ -322,7 +323,7 @@ where
     /// Size of the table in entries, including possible gaps.
     pub(super) fn size<const N: usize>(&self) -> usize
     where
-        Option<T>: FixedSizeEncoding<ByteArray = [u8; N]>,
+        for<'tcx> Option<T::Value<'tcx>>: FixedSizeEncoding<ByteArray = [u8; N]>,
     {
         self.encoded_size / N
     }
diff --git a/compiler/rustc_middle/src/lib.rs b/compiler/rustc_middle/src/lib.rs
index b4dd253b83914..3a9fe31735fe2 100644
--- a/compiler/rustc_middle/src/lib.rs
+++ b/compiler/rustc_middle/src/lib.rs
@@ -33,6 +33,7 @@
 #![feature(discriminant_kind)]
 #![feature(exhaustive_patterns)]
 #![feature(get_mut_unchecked)]
+#![feature(generic_associated_types)]
 #![feature(if_let_guard)]
 #![feature(map_first_last)]
 #![feature(negative_impls)]
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index 775b06c6c3832..b61a3827602fa 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -72,6 +72,7 @@ pub use self::context::{
 };
 pub use self::instance::{Instance, InstanceDef};
 pub use self::list::List;
+pub use self::parameterized::ParameterizedOverTcx;
 pub use self::rvalue_scopes::RvalueScopes;
 pub use self::sty::BoundRegionKind::*;
 pub use self::sty::RegionKind::*;
@@ -119,6 +120,7 @@ mod generics;
 mod impls_ty;
 mod instance;
 mod list;
+mod parameterized;
 mod rvalue_scopes;
 mod structural_impls;
 mod sty;
diff --git a/compiler/rustc_middle/src/ty/parameterized.rs b/compiler/rustc_middle/src/ty/parameterized.rs
new file mode 100644
index 0000000000000..54ba9e84fdb7b
--- /dev/null
+++ b/compiler/rustc_middle/src/ty/parameterized.rs
@@ -0,0 +1,119 @@
+use rustc_hir::def_id::DefId;
+use rustc_index::vec::{Idx, IndexVec};
+
+use crate::middle::exported_symbols::ExportedSymbol;
+use crate::mir::Body;
+use crate::thir::abstract_const::Node;
+use crate::ty::{
+    self, Const, FnSig, GeneratorDiagnosticData, GenericPredicates, Predicate, TraitRef, Ty,
+};
+
+pub trait ParameterizedOverTcx: 'static {
+    #[allow(unused_lifetimes)]
+    type Value<'tcx>;
+}
+
+impl<T: ParameterizedOverTcx> ParameterizedOverTcx for &'static [T] {
+    type Value<'tcx> = &'tcx [T::Value<'tcx>];
+}
+
+impl<T: ParameterizedOverTcx> ParameterizedOverTcx for Option<T> {
+    type Value<'tcx> = Option<T::Value<'tcx>>;
+}
+
+impl<A: ParameterizedOverTcx, B: ParameterizedOverTcx> ParameterizedOverTcx for (A, B) {
+    type Value<'tcx> = (A::Value<'tcx>, B::Value<'tcx>);
+}
+
+impl<I: Idx + 'static, T: ParameterizedOverTcx> ParameterizedOverTcx for IndexVec<I, T> {
+    type Value<'tcx> = IndexVec<I, T::Value<'tcx>>;
+}
+
+impl<T: ParameterizedOverTcx> ParameterizedOverTcx for ty::Binder<'static, T> {
+    type Value<'tcx> = ty::Binder<'tcx, T::Value<'tcx>>;
+}
+
+#[macro_export]
+macro_rules! trivially_parameterized_over_tcx {
+    ($($ty:ty),+ $(,)?) => {
+        $(
+            impl $crate::ty::ParameterizedOverTcx for $ty {
+                #[allow(unused_lifetimes)]
+                type Value<'tcx> = $ty;
+            }
+        )*
+    }
+}
+
+trivially_parameterized_over_tcx! {
+    usize,
+    (),
+    u32,
+    std::string::String,
+    crate::metadata::ModChild,
+    crate::middle::codegen_fn_attrs::CodegenFnAttrs,
+    crate::middle::exported_symbols::SymbolExportInfo,
+    crate::mir::ConstQualifs,
+    ty::Generics,
+    ty::ImplPolarity,
+    ty::ReprOptions,
+    ty::TraitDef,
+    ty::Visibility,
+    ty::adjustment::CoerceUnsizedInfo,
+    ty::fast_reject::SimplifiedTypeGen<DefId>,
+    rustc_ast::Attribute,
+    rustc_ast::MacArgs,
+    rustc_attr::ConstStability,
+    rustc_attr::Deprecation,
+    rustc_attr::Stability,
+    rustc_hir::Constness,
+    rustc_hir::Defaultness,
+    rustc_hir::GeneratorKind,
+    rustc_hir::IsAsync,
+    rustc_hir::LangItem,
+    rustc_hir::def::DefKind,
+    rustc_hir::def_id::DefIndex,
+    rustc_hir::definitions::DefKey,
+    rustc_index::bit_set::FiniteBitSet<u32>,
+    rustc_session::cstore::ForeignModule,
+    rustc_session::cstore::LinkagePreference,
+    rustc_session::cstore::NativeLib,
+    rustc_span::DebuggerVisualizerFile,
+    rustc_span::ExpnData,
+    rustc_span::ExpnHash,
+    rustc_span::ExpnId,
+    rustc_span::SourceFile,
+    rustc_span::Span,
+    rustc_span::Symbol,
+    rustc_span::def_id::DefPathHash,
+    rustc_span::hygiene::SyntaxContextData,
+    rustc_span::symbol::Ident,
+    rustc_type_ir::Variance,
+}
+
+// HACK(compiler-errors): This macro rule can only take an ident,
+// not a path, due to parsing ambiguity reasons. That means we gotta
+// import all of these types above.
+#[macro_export]
+macro_rules! parameterized_over_tcx {
+    ($($ident:ident),+ $(,)?) => {
+        $(
+            impl $crate::ty::ParameterizedOverTcx for $ident<'static> {
+                type Value<'tcx> = $ident<'tcx>;
+            }
+        )*
+    }
+}
+
+parameterized_over_tcx! {
+    Ty,
+    FnSig,
+    GenericPredicates,
+    TraitRef,
+    Const,
+    Predicate,
+    GeneratorDiagnosticData,
+    Body,
+    Node,
+    ExportedSymbol,
+}