From ab83d372ed5b1799d418afe83c468e4c5973cc34 Mon Sep 17 00:00:00 2001
From: Bram van den Heuvel <bram@vandenheuvel.online>
Date: Sat, 19 Sep 2020 12:06:17 +0200
Subject: [PATCH] Add an unused field of type `Option<DefId>` to `ParamEnv`
 struct.

---
 compiler/rustc_infer/src/traits/mod.rs               | 2 +-
 compiler/rustc_middle/src/ty/mod.rs                  | 5 ++++-
 compiler/rustc_trait_selection/src/traits/fulfill.rs | 2 +-
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/compiler/rustc_infer/src/traits/mod.rs b/compiler/rustc_infer/src/traits/mod.rs
index a3c4920fa8a..7e7c8588ffb 100644
--- a/compiler/rustc_infer/src/traits/mod.rs
+++ b/compiler/rustc_infer/src/traits/mod.rs
@@ -57,7 +57,7 @@ pub struct Obligation<'tcx, T> {
 
 // `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(target_arch = "x86_64")]
-static_assert_size!(PredicateObligation<'_>, 32);
+static_assert_size!(PredicateObligation<'_>, 40);
 
 pub type Obligations<'tcx, O> = Vec<Obligation<'tcx, O>>;
 pub type PredicateObligations<'tcx> = Vec<PredicateObligation<'tcx>>;
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index f23d666cfcf..637ef4c17eb 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -1745,6 +1745,9 @@ pub struct ParamEnv<'tcx> {
     ///
     /// Note: This is packed, use the reveal() method to access it.
     packed: CopyTaggedPtr<&'tcx List<Predicate<'tcx>>, traits::Reveal, true>,
+
+    /// FIXME: This field is not used, but removing it causes a performance degradation. See #76913.
+    unused_field: Option<DefId>,
 }
 
 unsafe impl rustc_data_structures::tagged_ptr::Tag for traits::Reveal {
@@ -1825,7 +1828,7 @@ pub fn reveal_all() -> Self {
     /// Construct a trait environment with the given set of predicates.
     #[inline]
     pub fn new(caller_bounds: &'tcx List<Predicate<'tcx>>, reveal: Reveal) -> Self {
-        ty::ParamEnv { packed: CopyTaggedPtr::new(caller_bounds, reveal) }
+        ty::ParamEnv { packed: CopyTaggedPtr::new(caller_bounds, reveal), unused_field: None }
     }
 
     pub fn with_user_facing(mut self) -> Self {
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index 5b4314598de..77073f51eb7 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -87,7 +87,7 @@ pub struct PendingPredicateObligation<'tcx> {
 
 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(target_arch = "x86_64")]
-static_assert_size!(PendingPredicateObligation<'_>, 56);
+static_assert_size!(PendingPredicateObligation<'_>, 64);
 
 impl<'a, 'tcx> FulfillmentContext<'tcx> {
     /// Creates a new fulfillment context.
-- 
2.44.0