Eliminate ObligationCauseData.

This makes `Obligation` two words bigger, but avoids allocating a lot of
the time.

I previously tried this in #73983 and it didn't help much, but local
timings look more promising now.

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -604,7 +604,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
         terr: &TypeError<'tcx>,
     ) {
-        match cause.code {
+        match *cause.code() {
             ObligationCauseCode::Pattern { origin_expr: true, span: Some(span), root_ty } => {
                 let ty = self.resolve_vars_if_possible(root_ty);
                 if ty.is_suggestable() {
@@ -781,7 +781,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             }
             _ => {
                 if let ObligationCauseCode::BindingObligation(_, binding_span) =
-                    cause.code.peel_derives()
+                    cause.code().peel_derives()
                 {
                     if matches!(terr, TypeError::RegionsPlaceholderMismatch) {
                         err.span_note(*binding_span, "the lifetime requirement is introduced here");
@@ -1729,10 +1729,10 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             }
             _ => exp_found,
         };
-        debug!("exp_found {:?} terr {:?} cause.code {:?}", exp_found, terr, cause.code);
+        debug!("exp_found {:?} terr {:?} cause.code {:?}", exp_found, terr, cause.code());
         if let Some(exp_found) = exp_found {
             let should_suggest_fixes = if let ObligationCauseCode::Pattern { root_ty, .. } =
-                &cause.code
+                cause.code()
             {
                 // Skip if the root_ty of the pattern is not the same as the expected_ty.
                 // If these types aren't equal then we've probably peeled off a layer of arrays.
@@ -1827,15 +1827,15 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             exp_span, exp_found.expected, exp_found.found,
         );
 
-        if let ObligationCauseCode::CompareImplMethodObligation { .. } = &cause.code {
+        if let ObligationCauseCode::CompareImplMethodObligation { .. } = cause.code() {
             return;
         }
 
         match (
             self.get_impl_future_output_ty(exp_found.expected),
             self.get_impl_future_output_ty(exp_found.found),
         ) {
-            (Some(exp), Some(found)) if same_type_modulo_infer(exp, found) => match &cause.code {
+            (Some(exp), Some(found)) if same_type_modulo_infer(exp, found) => match cause.code() {
                 ObligationCauseCode::IfExpression(box IfExpressionCause { then, .. }) => {
                     diag.multipart_suggestion(
                         "consider `await`ing on both `Future`s",
@@ -1875,7 +1875,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     Applicability::MaybeIncorrect,
                 );
             }
-            (Some(ty), _) if same_type_modulo_infer(ty, exp_found.found) => match cause.code {
+            (Some(ty), _) if same_type_modulo_infer(ty, exp_found.found) => match cause.code() {
                 ObligationCauseCode::Pattern { span: Some(span), .. }
                 | ObligationCauseCode::IfExpression(box IfExpressionCause { then: span, .. }) => {
                     diag.span_suggestion_verbose(
@@ -1927,7 +1927,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 .map(|field| (field.ident.name, field.ty(self.tcx, expected_substs)))
                 .find(|(_, ty)| same_type_modulo_infer(ty, exp_found.found))
             {
-                if let ObligationCauseCode::Pattern { span: Some(span), .. } = cause.code {
+                if let ObligationCauseCode::Pattern { span: Some(span), .. } = *cause.code() {
                     if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
                         let suggestion = if expected_def.is_struct() {
                             format!("{}.{}", snippet, name)
@@ -2064,7 +2064,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     }
                 }
                 if let MatchExpressionArm(box MatchExpressionArmCause { source, .. }) =
-                    trace.cause.code
+                    *trace.cause.code()
                 {
                     if let hir::MatchSource::TryDesugar = source {
                         if let Some((expected_ty, found_ty)) = self.values_str(trace.values) {
@@ -2659,7 +2659,7 @@ impl<'tcx> ObligationCauseExt<'tcx> for ObligationCause<'tcx> {
     fn as_failure_code(&self, terr: &TypeError<'tcx>) -> FailureCode {
         use self::FailureCode::*;
         use crate::traits::ObligationCauseCode::*;
-        match self.code {
+        match self.code() {
             CompareImplMethodObligation { .. } => Error0308("method not compatible with trait"),
             CompareImplTypeObligation { .. } => Error0308("type not compatible with trait"),
             MatchExpressionArm(box MatchExpressionArmCause { source, .. }) => {
@@ -2694,7 +2694,7 @@ impl<'tcx> ObligationCauseExt<'tcx> for ObligationCause<'tcx> {
 
     fn as_requirement_str(&self) -> &'static str {
         use crate::traits::ObligationCauseCode::*;
-        match self.code {
+        match self.code() {
             CompareImplMethodObligation { .. } => "method type is compatible with trait",
             CompareImplTypeObligation { .. } => "associated type is compatible with trait",
             ExprAssignable => "expression is assignable",

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mismatched_static_lifetime.rs

@@ -31,15 +31,15 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         };
         // If we added a "points at argument expression" obligation, we remove it here, we care
         // about the original obligation only.
-        let code = match &cause.code {
+        let code = match cause.code() {
             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => &*parent_code,
-            _ => &cause.code,
+            _ => cause.code(),
         };
         let (parent, impl_def_id) = match code {
             ObligationCauseCode::MatchImpl(parent, impl_def_id) => (parent, impl_def_id),
             _ => return None,
         };
-        let binding_span = match parent.code {
+        let binding_span = match *parent.code() {
             ObligationCauseCode::BindingObligation(_def_id, binding_span) => binding_span,
             _ => return None,
         };

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs

@@ -208,7 +208,7 @@ impl<'tcx> NiceRegionError<'_, 'tcx> {
         );
         let mut err = self.tcx().sess.struct_span_err(span, &msg);
 
-        let leading_ellipsis = if let ObligationCauseCode::ItemObligation(def_id) = cause.code {
+        let leading_ellipsis = if let ObligationCauseCode::ItemObligation(def_id) = *cause.code() {
             err.span_label(span, "doesn't satisfy where-clause");
             err.span_label(
                 self.tcx().def_span(def_id),

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -42,7 +42,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                 sup_r,
             ) if **sub_r == RegionKind::ReStatic => {
                 // This is for an implicit `'static` requirement coming from `impl dyn Trait {}`.
-                if let ObligationCauseCode::UnifyReceiver(ctxt) = &cause.code {
+                if let ObligationCauseCode::UnifyReceiver(ctxt) = cause.code() {
                     // This may have a closure and it would cause ICE
                     // through `find_param_with_region` (#78262).
                     let anon_reg_sup = tcx.is_suitable_region(sup_r)?;
@@ -184,7 +184,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         }
         if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = sub_origin {
             if let ObligationCauseCode::ReturnValue(hir_id)
-            | ObligationCauseCode::BlockTailExpression(hir_id) = &cause.code
+            | ObligationCauseCode::BlockTailExpression(hir_id) = cause.code()
             {
                 let parent_id = tcx.hir().get_parent_item(*hir_id);
                 if let Some(fn_decl) = tcx.hir().fn_decl_by_hir_id(parent_id) {
@@ -226,7 +226,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
 
         let mut override_error_code = None;
         if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = &sup_origin {
-            if let ObligationCauseCode::UnifyReceiver(ctxt) = &cause.code {
+            if let ObligationCauseCode::UnifyReceiver(ctxt) = cause.code() {
                 // Handle case of `impl Foo for dyn Bar { fn qux(&self) {} }` introducing a
                 // `'static` lifetime when called as a method on a binding: `bar.qux()`.
                 if self.find_impl_on_dyn_trait(&mut err, param.param_ty, &ctxt) {
@@ -235,9 +235,9 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             }
         }
         if let SubregionOrigin::Subtype(box TypeTrace { cause, .. }) = &sub_origin {
-            let code = match &cause.code {
-                ObligationCauseCode::MatchImpl(parent, ..) => &parent.code,
-                _ => &cause.code,
+            let code = match cause.code() {
+                ObligationCauseCode::MatchImpl(parent, ..) => parent.code(),
+                _ => cause.code(),
             };
             if let (ObligationCauseCode::ItemObligation(item_def_id), None) =
                 (code, override_error_code)

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs

@@ -36,7 +36,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                     ValuePairs::Types(sub_expected_found),
                     ValuePairs::Types(sup_expected_found),
                     CompareImplMethodObligation { trait_item_def_id, .. },
-                ) = (&sub_trace.values, &sup_trace.values, &sub_trace.cause.code)
+                ) = (&sub_trace.values, &sup_trace.values, sub_trace.cause.code())
                 {
                     if sup_expected_found == sub_expected_found {
                         self.emit_err(

compiler/rustc_infer/src/infer/error_reporting/note.rs

@@ -359,13 +359,13 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         match placeholder_origin {
             infer::Subtype(box ref trace)
                 if matches!(
-                    &trace.cause.code.peel_derives(),
+                    &trace.cause.code().peel_derives(),
                     ObligationCauseCode::BindingObligation(..)
                 ) =>
             {
                 // Hack to get around the borrow checker because trace.cause has an `Rc`.
                 if let ObligationCauseCode::BindingObligation(_, span) =
-                    &trace.cause.code.peel_derives()
+                    &trace.cause.code().peel_derives()
                 {
                     let span = *span;
                     let mut err = self.report_concrete_failure(placeholder_origin, sub, sup);

compiler/rustc_infer/src/infer/mod.rs

@@ -1824,7 +1824,7 @@ impl<'tcx> SubregionOrigin<'tcx> {
     where
         F: FnOnce() -> Self,
     {
-        match cause.code {
+        match *cause.code() {
             traits::ObligationCauseCode::ReferenceOutlivesReferent(ref_type) => {
                 SubregionOrigin::ReferenceOutlivesReferent(ref_type, cause.span)
             }

compiler/rustc_infer/src/infer/outlives/obligations.rs

@@ -102,7 +102,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
             infer::RelateParamBound(
                 cause.span,
                 sup_type,
-                match cause.code.peel_derives() {
+                match cause.code().peel_derives() {
                     ObligationCauseCode::BindingObligation(_, span) => Some(*span),
                     _ => None,
                 },

compiler/rustc_infer/src/traits/mod.rs

@@ -81,7 +81,7 @@ impl TraitObligation<'_> {
 
 // `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-static_assert_size!(PredicateObligation<'_>, 32);
+static_assert_size!(PredicateObligation<'_>, 48);
 
 pub type PredicateObligations<'tcx> = Vec<PredicateObligation<'tcx>>;
 

compiler/rustc_middle/src/traits/mod.rs

@@ -23,9 +23,7 @@ use rustc_span::{Span, DUMMY_SP};
 use smallvec::SmallVec;
 
 use std::borrow::Cow;
-use std::fmt;
 use std::hash::{Hash, Hasher};
-use std::ops::Deref;
 
 pub use self::select::{EvaluationCache, EvaluationResult, OverflowError, SelectionCache};
 
@@ -80,38 +78,14 @@ pub enum Reveal {
 
 /// The reason why we incurred this obligation; used for error reporting.
 ///
-/// As the happy path does not care about this struct, storing this on the heap
-/// ends up increasing performance.
+/// Non-misc `ObligationCauseCode`s are stored on the heap. This gives the
+/// best trade-off between keeping the type small (which makes copies cheaper)
+/// while not doing too many heap allocations.
 ///
 /// We do not want to intern this as there are a lot of obligation causes which
 /// only live for a short period of time.
-#[derive(Clone, PartialEq, Eq, Hash, Lift)]
-pub struct ObligationCause<'tcx> {
-    /// `None` for `ObligationCause::dummy`, `Some` otherwise.
-    data: Option<Lrc<ObligationCauseData<'tcx>>>,
-}
-
-const DUMMY_OBLIGATION_CAUSE_DATA: ObligationCauseData<'static> =
-    ObligationCauseData { span: DUMMY_SP, body_id: hir::CRATE_HIR_ID, code: MiscObligation };
-
-// Correctly format `ObligationCause::dummy`.
-impl<'tcx> fmt::Debug for ObligationCause<'tcx> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        ObligationCauseData::fmt(self, f)
-    }
-}
-
-impl<'tcx> Deref for ObligationCause<'tcx> {
-    type Target = ObligationCauseData<'tcx>;
-
-    #[inline(always)]
-    fn deref(&self) -> &Self::Target {
-        self.data.as_deref().unwrap_or(&DUMMY_OBLIGATION_CAUSE_DATA)
-    }
-}
-
 #[derive(Clone, Debug, PartialEq, Eq, Lift)]
-pub struct ObligationCauseData<'tcx> {
+pub struct ObligationCause<'tcx> {
     pub span: Span,
 
     /// The ID of the fn body that triggered this obligation. This is
@@ -122,46 +96,58 @@ pub struct ObligationCauseData<'tcx> {
     /// information.
     pub body_id: hir::HirId,
 
-    pub code: ObligationCauseCode<'tcx>,
+    /// `None` for `MISC_OBLIGATION_CAUSE_CODE` (a common case, occurs ~60% of
+    /// the time). `Some` otherwise.
+    code: Option<Lrc<ObligationCauseCode<'tcx>>>,
 }
 
-impl Hash for ObligationCauseData<'_> {
+// This custom hash function speeds up hashing for `Obligation` deduplication
+// greatly by skipping the `code` field, which can be large and complex. That
+// shouldn't affect hash quality much since there are several other fields in
+// `Obligation` which should be unique enough, especially the predicate itself
+// which is hashed as an interned pointer. See #90996.
+impl Hash for ObligationCause<'_> {
     fn hash<H: Hasher>(&self, state: &mut H) {
         self.body_id.hash(state);
         self.span.hash(state);
-        std::mem::discriminant(&self.code).hash(state);
     }
 }
 
+const MISC_OBLIGATION_CAUSE_CODE: ObligationCauseCode<'static> = MiscObligation;
+
 impl<'tcx> ObligationCause<'tcx> {
     #[inline]
     pub fn new(
         span: Span,
         body_id: hir::HirId,
         code: ObligationCauseCode<'tcx>,
     ) -> ObligationCause<'tcx> {
-        ObligationCause { data: Some(Lrc::new(ObligationCauseData { span, body_id, code })) }
+        ObligationCause {
+            span,
+            body_id,
+            code: if code == MISC_OBLIGATION_CAUSE_CODE { None } else { Some(Lrc::new(code)) },
+        }
     }
 
     pub fn misc(span: Span, body_id: hir::HirId) -> ObligationCause<'tcx> {
         ObligationCause::new(span, body_id, MiscObligation)
     }
 
-    pub fn dummy_with_span(span: Span) -> ObligationCause<'tcx> {
-        ObligationCause::new(span, hir::CRATE_HIR_ID, MiscObligation)
-    }
-
     #[inline(always)]
     pub fn dummy() -> ObligationCause<'tcx> {
-        ObligationCause { data: None }
+        ObligationCause { span: DUMMY_SP, body_id: hir::CRATE_HIR_ID, code: None }
+    }
+
+    pub fn dummy_with_span(span: Span) -> ObligationCause<'tcx> {
+        ObligationCause { span, body_id: hir::CRATE_HIR_ID, code: None }
     }
 
-    pub fn make_mut(&mut self) -> &mut ObligationCauseData<'tcx> {
-        Lrc::make_mut(self.data.get_or_insert_with(|| Lrc::new(DUMMY_OBLIGATION_CAUSE_DATA)))
+    pub fn make_mut_code(&mut self) -> &mut ObligationCauseCode<'tcx> {
+        Lrc::make_mut(self.code.get_or_insert_with(|| Lrc::new(MISC_OBLIGATION_CAUSE_CODE)))
     }
 
     pub fn span(&self, tcx: TyCtxt<'tcx>) -> Span {
-        match self.code {
+        match *self.code() {
             ObligationCauseCode::CompareImplMethodObligation { .. }
             | ObligationCauseCode::MainFunctionType
             | ObligationCauseCode::StartFunctionType => {
@@ -174,6 +160,18 @@ impl<'tcx> ObligationCause<'tcx> {
             _ => self.span,
         }
     }
+
+    #[inline]
+    pub fn code(&self) -> &ObligationCauseCode<'tcx> {
+        self.code.as_deref().unwrap_or(&MISC_OBLIGATION_CAUSE_CODE)
+    }
+
+    pub fn clone_code(&self) -> Lrc<ObligationCauseCode<'tcx>> {
+        match &self.code {
+            Some(code) => code.clone(),
+            None => Lrc::new(MISC_OBLIGATION_CAUSE_CODE),
+        }
+    }
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]

compiler/rustc_middle/src/ty/error.rs

@@ -519,7 +519,7 @@ impl<T> Trait<T> for X {
                             proj_ty,
                             values,
                             body_owner_def_id,
-                            &cause.code,
+                            cause.code(),
                         );
                     }
                     (_, ty::Projection(proj_ty)) => {