Auto merge of #72962 - lcnr:ObligationCause-lrc, r=ecstatic-morse

store `ObligationCause` on the heap

Stores `ObligationCause` on the heap using an `Rc`.

This PR trades off some transient memory allocations to reduce the size of–and thus the number of instructions required to memcpy–a few widely used data structures in trait solving.

3 files changed, 6 insertions, 5 deletions

diff --git a/src/librustc_trait_selection/traits/fulfill.rs b/src/librustc_trait_selection/traits/fulfill.rs
index 4a08a3426c1..106753ed809 100644
--- a/src/librustc_trait_selection/traits/fulfill.rs
+++ b/src/librustc_trait_selection/traits/fulfill.rs
@@ -84,7 +84,7 @@ 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<'_>, 112);
+static_assert_size!(PendingPredicateObligation<'_>, 72);
 
 impl<'a, 'tcx> FulfillmentContext<'tcx> {
     /// Creates a new fulfillment context.
diff --git a/src/librustc_trait_selection/traits/misc.rs b/src/librustc_trait_selection/traits/misc.rs
index 7403b936391..61567aeb57c 100644
--- a/src/librustc_trait_selection/traits/misc.rs
+++ b/src/librustc_trait_selection/traits/misc.rs
@@ -48,7 +48,7 @@ pub fn can_type_implement_copy(
                     continue;
                 }
                 let span = tcx.def_span(field.did);
-                let cause = ObligationCause { span, ..ObligationCause::dummy() };
+                let cause = ObligationCause::dummy_with_span(span);
                 let ctx = traits::FulfillmentContext::new();
                 match traits::fully_normalize(&infcx, ctx, cause, param_env, &ty) {
                     Ok(ty) => {
diff --git a/src/librustc_trait_selection/traits/wf.rs b/src/librustc_trait_selection/traits/wf.rs
index 5024392a0f9..90a9b876d8d 100644
--- a/src/librustc_trait_selection/traits/wf.rs
+++ b/src/librustc_trait_selection/traits/wf.rs
@@ -205,7 +205,7 @@ fn extend_cause_with_original_assoc_item_obligation<'tcx>(
                 if let Some(impl_item_span) =
                     items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
                 {
-                    cause.span = impl_item_span;
+                    cause.make_mut().span = impl_item_span;
                 }
             }
         }
@@ -222,7 +222,7 @@ fn extend_cause_with_original_assoc_item_obligation<'tcx>(
                         items.iter().find(|i| i.ident == trait_assoc_item.ident).map(fix_span)
                     })
                 {
-                    cause.span = impl_item_span;
+                    cause.make_mut().span = impl_item_span;
                 }
             }
         }
@@ -273,7 +273,8 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
                     parent_trait_ref,
                     parent_code: Rc::new(obligation.cause.code.clone()),
                 };
-                cause.code = traits::ObligationCauseCode::DerivedObligation(derived_cause);
+                cause.make_mut().code =
+                    traits::ObligationCauseCode::DerivedObligation(derived_cause);
             }
             extend_cause_with_original_assoc_item_obligation(
                 tcx,