Shrink ObligationCauseCode by boxing MatchExpressionArm.

The reduction in `memcpy` calls greatly outweighs the cost of the extra
allocations, for a net performance win.

src/librustc/infer/error_reporting/mod.rs

@@ -55,7 +55,7 @@ use crate::hir::def_id::DefId;
 use crate::hir::Node;
 use crate::infer::opaque_types;
 use crate::middle::region;
-use crate::traits::{ObligationCause, ObligationCauseCode};
+use crate::traits::{MatchExpressionArmCause, ObligationCause, ObligationCauseCode};
 use crate::ty::error::TypeError;
 use crate::ty::{self, subst::{Subst, SubstsRef}, Region, Ty, TyCtxt, TypeFoldable};
 use errors::{Applicability, DiagnosticBuilder, DiagnosticStyledString};
@@ -624,13 +624,13 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     }
                 }
             }
-            ObligationCauseCode::MatchExpressionArm {
+            ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
                 source,
                 ref prior_arms,
                 last_ty,
                 discrim_hir_id,
                 ..
-            } => match source {
+            }) => match source {
                 hir::MatchSource::IfLetDesugar { .. } => {
                     let msg = "`if let` arms have incompatible types";
                     err.span_label(cause.span, msg);
@@ -1622,13 +1622,15 @@ impl<'tcx> ObligationCause<'tcx> {
         use crate::traits::ObligationCauseCode::*;
         match self.code {
             CompareImplMethodObligation { .. } => Error0308("method not compatible with trait"),
-            MatchExpressionArm { source, .. } => Error0308(match source {
-                hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have incompatible types",
-                hir::MatchSource::TryDesugar => {
-                    "try expression alternatives have incompatible types"
-                }
-                _ => "match arms have incompatible types",
-            }),
+            MatchExpressionArm(box MatchExpressionArmCause { source, .. }) =>
+                Error0308(match source {
+                    hir::MatchSource::IfLetDesugar { .. } =>
+                        "`if let` arms have incompatible types",
+                    hir::MatchSource::TryDesugar => {
+                        "try expression alternatives have incompatible types"
+                    }
+                    _ => "match arms have incompatible types",
+                }),
             IfExpression { .. } => Error0308("if and else have incompatible types"),
             IfExpressionWithNoElse => Error0317("if may be missing an else clause"),
             MainFunctionType => Error0580("main function has wrong type"),
@@ -1656,7 +1658,7 @@ impl<'tcx> ObligationCause<'tcx> {
         match self.code {
             CompareImplMethodObligation { .. } => "method type is compatible with trait",
             ExprAssignable => "expression is assignable",
-            MatchExpressionArm { source, .. } => match source {
+            MatchExpressionArm(box MatchExpressionArmCause { source, .. }) => match source {
                 hir::MatchSource::IfLetDesugar { .. } => "`if let` arms have compatible types",
                 _ => "match arms have compatible types",
             },

src/librustc/traits/fulfill.rs

@@ -70,7 +70,7 @@ 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<'_>, 160);
+static_assert_size!(PendingPredicateObligation<'_>, 144);
 
 impl<'a, 'tcx> FulfillmentContext<'tcx> {
     /// Creates a new fulfillment context.

src/librustc/traits/mod.rs

@@ -125,7 +125,7 @@ pub type TraitObligation<'tcx> = Obligation<'tcx, ty::PolyTraitPredicate<'tcx>>;
 
 // `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(target_arch = "x86_64")]
-static_assert_size!(PredicateObligation<'_>, 136);
+static_assert_size!(PredicateObligation<'_>, 120);
 
 /// The reason why we incurred this obligation; used for error reporting.
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
@@ -151,7 +151,8 @@ impl<'tcx> ObligationCause<'tcx> {
             ObligationCauseCode::StartFunctionType => {
                 tcx.sess.source_map().def_span(self.span)
             }
-            ObligationCauseCode::MatchExpressionArm { arm_span, .. } => arm_span,
+            ObligationCauseCode::MatchExpressionArm(
+                box MatchExpressionArmCause { arm_span, .. }) => arm_span,
             _ => self.span,
         }
     }
@@ -227,13 +228,7 @@ pub enum ObligationCauseCode<'tcx> {
     ExprAssignable,
 
     /// Computing common supertype in the arms of a match expression
-    MatchExpressionArm {
-        arm_span: Span,
-        source: hir::MatchSource,
-        prior_arms: Vec<Span>,
-        last_ty: Ty<'tcx>,
-        discrim_hir_id: hir::HirId,
-    },
+    MatchExpressionArm(Box<MatchExpressionArmCause<'tcx>>),
 
     /// Computing common supertype in the pattern guard for the arms of a match expression
     MatchExpressionArmPattern { span: Span, ty: Ty<'tcx> },
@@ -275,7 +270,16 @@ pub enum ObligationCauseCode<'tcx> {
 
 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(target_arch = "x86_64")]
-static_assert_size!(ObligationCauseCode<'_>, 56);
+static_assert_size!(ObligationCauseCode<'_>, 40);
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct MatchExpressionArmCause<'tcx> {
+    pub arm_span: Span,
+    pub source: hir::MatchSource,
+    pub prior_arms: Vec<Span>,
+    pub last_ty: Ty<'tcx>,
+    pub discrim_hir_id: hir::HirId,
+}
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct DerivedObligationCause<'tcx> {

src/librustc/traits/structural_impls.rs

@@ -508,21 +508,21 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
                 trait_item_def_id,
             }),
             super::ExprAssignable => Some(super::ExprAssignable),
-            super::MatchExpressionArm {
+            super::MatchExpressionArm(box super::MatchExpressionArmCause {
                 arm_span,
                 source,
                 ref prior_arms,
                 last_ty,
                 discrim_hir_id,
-            } => {
+            }) => {
                 tcx.lift(&last_ty).map(|last_ty| {
-                    super::MatchExpressionArm {
+                    super::MatchExpressionArm(box super::MatchExpressionArmCause {
                         arm_span,
                         source,
                         prior_arms: prior_arms.clone(),
                         last_ty,
                         discrim_hir_id,
-                    }
+                    })
                 })
             }
             super::MatchExpressionArmPattern { span, ty } => {

src/librustc_typeck/check/_match.rs

@@ -2,7 +2,7 @@ use crate::check::{FnCtxt, Expectation, Diverges, Needs};
 use crate::check::coercion::CoerceMany;
 use rustc::hir::{self, ExprKind};
 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
-use rustc::traits::{ObligationCause, ObligationCauseCode};
+use rustc::traits::{MatchExpressionArmCause, ObligationCause, ObligationCauseCode};
 use rustc::ty::Ty;
 use syntax_pos::Span;
 
@@ -146,13 +146,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // The reason for the first arm to fail is not that the match arms diverge,
                     // but rather that there's a prior obligation that doesn't hold.
                     0 => (arm_span, ObligationCauseCode::BlockTailExpression(arm.body.hir_id)),
-                    _ => (expr.span, ObligationCauseCode::MatchExpressionArm {
-                        arm_span,
-                        source: match_src,
-                        prior_arms: other_arms.clone(),
-                        last_ty: prior_arm_ty.unwrap(),
-                        discrim_hir_id: discrim.hir_id,
-                    }),
+                    _ => (expr.span,
+                          ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
+                            arm_span,
+                            source: match_src,
+                            prior_arms: other_arms.clone(),
+                            last_ty: prior_arm_ty.unwrap(),
+                            discrim_hir_id: discrim.hir_id,
+                          })
+                         ),
                 };
                 let cause = self.cause(span, code);
                 coercion.coerce(self, &cause, &arm.body, arm_ty);