Auto merge of #91844 - nnethercote:rm-ObligationCauseData-2, r=Mark-Simulacrum

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.

r? `@ghost`

22 files changed, 135 insertions, 139 deletions

diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index a292bcd4f20..9a76c05e4f6 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/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.
@@ -1827,7 +1827,7 @@ 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;
         }
 
@@ -1835,7 +1835,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             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",
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mismatched_static_lifetime.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mismatched_static_lifetime.rs
index 35c8786dcd3..d3b47e396ec 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mismatched_static_lifetime.rs
+++ b/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,
         };
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs
index 24652a3a6bc..7178bfa525b 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs
+++ b/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),
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs
index 51f33a206f4..b6dff2e53e9 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs
+++ b/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)
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs
index c12367409a2..f5fb82dbf31 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs
+++ b/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(
diff --git a/compiler/rustc_infer/src/infer/error_reporting/note.rs b/compiler/rustc_infer/src/infer/error_reporting/note.rs
index 6600c18351f..82bd8890dda 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/note.rs
+++ b/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);
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index 9db815548d8..04e04e297cd 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/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)
             }
diff --git a/compiler/rustc_infer/src/infer/outlives/obligations.rs b/compiler/rustc_infer/src/infer/outlives/obligations.rs
index 91a22ecc5a9..74eb263a633 100644
--- a/compiler/rustc_infer/src/infer/outlives/obligations.rs
+++ b/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,
                 },
diff --git a/compiler/rustc_infer/src/traits/mod.rs b/compiler/rustc_infer/src/traits/mod.rs
index b4fb10370d4..e1f3b548e97 100644
--- a/compiler/rustc_infer/src/traits/mod.rs
+++ b/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>>;
 
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 3e6a6b3ba42..de5beffb5c5 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/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
@@ -122,17 +96,25 @@ 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(
@@ -140,28 +122,32 @@ impl<'tcx> ObligationCause<'tcx> {
         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)]
diff --git a/compiler/rustc_middle/src/ty/error.rs b/compiler/rustc_middle/src/ty/error.rs
index 1b4566fd026..df6e739dc20 100644
--- a/compiler/rustc_middle/src/ty/error.rs
+++ b/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)) => {
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 8833805d35c..b5c5724f56e 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -205,7 +205,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         self.note_obligation_cause_code(
             &mut err,
             &obligation.predicate,
-            &obligation.cause.code,
+            obligation.cause.code(),
             &mut vec![],
             &mut Default::default(),
         );
@@ -255,7 +255,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 // If this obligation was generated as a result of well-formedness checking, see if we
                 // can get a better error message by performing HIR-based well-formedness checking.
                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
-                    root_obligation.cause.code.peel_derives()
+                    root_obligation.cause.code().peel_derives()
                 {
                     if let Some(cause) = self
                         .tcx
@@ -272,7 +272,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 | ObligationCauseCode::CompareImplTypeObligation {
                     impl_item_def_id,
                     trait_item_def_id,
-                } = obligation.cause.code
+                } = *obligation.cause.code()
                 {
                     self.report_extra_impl_obligation(
                         span,
@@ -295,7 +295,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         }
                         let trait_ref = trait_predicate.to_poly_trait_ref();
                         let (post_message, pre_message, type_def) = self
-                            .get_parent_trait_ref(&obligation.cause.code)
+                            .get_parent_trait_ref(obligation.cause.code())
                             .map(|(t, s)| {
                                 (
                                     format!(" in `{}`", t),
@@ -376,17 +376,18 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             }
                         }
 
-                        let explanation =
-                            if obligation.cause.code == ObligationCauseCode::MainFunctionType {
-                                "consider using `()`, or a `Result`".to_owned()
-                            } else {
-                                format!(
-                                    "{}the trait `{}` is not implemented for `{}`",
-                                    pre_message,
-                                    trait_ref.print_only_trait_path(),
-                                    trait_ref.skip_binder().self_ty(),
-                                )
-                            };
+                        let explanation = if let ObligationCauseCode::MainFunctionType =
+                            obligation.cause.code()
+                        {
+                            "consider using `()`, or a `Result`".to_owned()
+                        } else {
+                            format!(
+                                "{}the trait `{}` is not implemented for `{}`",
+                                pre_message,
+                                trait_ref.print_only_trait_path(),
+                                trait_ref.skip_binder().self_ty(),
+                            )
+                        };
 
                         if self.suggest_add_reference_to_arg(
                             &obligation,
@@ -1305,7 +1306,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 );
 
                 let is_normalized_ty_expected = !matches!(
-                    obligation.cause.code.peel_derives(),
+                    obligation.cause.code().peel_derives(),
                     ObligationCauseCode::ItemObligation(_)
                         | ObligationCauseCode::BindingObligation(_, _)
                         | ObligationCauseCode::ObjectCastObligation(_)
@@ -1620,9 +1621,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
         let predicate = self.resolve_vars_if_possible(obligation.predicate);
         let span = obligation.cause.span;
 
-        debug!(
-            ?predicate, ?obligation.cause.code,
-        );
+        debug!(?predicate, obligation.cause.code = tracing::field::debug(&obligation.cause.code()));
 
         // Ambiguity errors are often caused as fallout from earlier errors.
         // We ignore them if this `infcx` is tainted in some cases below.
@@ -1717,13 +1716,13 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                     }
                 }
 
-                if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
+                if let ObligationCauseCode::ItemObligation(def_id) = *obligation.cause.code() {
                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
                 } else if let (
                     Ok(ref snippet),
                     ObligationCauseCode::BindingObligation(ref def_id, _),
                 ) =
-                    (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
+                    (self.tcx.sess.source_map().span_to_snippet(span), obligation.cause.code())
                 {
                     let generics = self.tcx.generics_of(*def_id);
                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
@@ -2006,7 +2005,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
             self.note_obligation_cause_code(
                 err,
                 &obligation.predicate,
-                &obligation.cause.code,
+                obligation.cause.code(),
                 &mut vec![],
                 &mut Default::default(),
             );
@@ -2019,15 +2018,16 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
         err: &mut DiagnosticBuilder<'tcx>,
         obligation: &PredicateObligation<'tcx>,
     ) {
-        let (pred, item_def_id, span) =
-            match (obligation.predicate.kind().skip_binder(), obligation.cause.code.peel_derives())
-            {
-                (
-                    ty::PredicateKind::Trait(pred),
-                    &ObligationCauseCode::BindingObligation(item_def_id, span),
-                ) => (pred, item_def_id, span),
-                _ => return,
-            };
+        let (pred, item_def_id, span) = match (
+            obligation.predicate.kind().skip_binder(),
+            obligation.cause.code().peel_derives(),
+        ) {
+            (
+                ty::PredicateKind::Trait(pred),
+                &ObligationCauseCode::BindingObligation(item_def_id, span),
+            ) => (pred, item_def_id, span),
+            _ => return,
+        };
         debug!(
             "suggest_unsized_bound_if_applicable: pred={:?} item_def_id={:?} span={:?}",
             pred, item_def_id, span
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
index d9a5aea4d95..1540725246b 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
@@ -129,7 +129,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             self.describe_enclosure(obligation.cause.body_id).map(|s| s.to_owned()),
         )];
 
-        match obligation.cause.code {
+        match obligation.cause.code() {
             ObligationCauseCode::BuiltinDerivedObligation(..)
             | ObligationCauseCode::ImplDerivedObligation(..)
             | ObligationCauseCode::DerivedObligation(..) => {}
@@ -141,7 +141,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         }
 
         if let ObligationCauseCode::ItemObligation(item)
-        | ObligationCauseCode::BindingObligation(item, _) = obligation.cause.code
+        | ObligationCauseCode::BindingObligation(item, _) = *obligation.cause.code()
         {
             // FIXME: maybe also have some way of handling methods
             // from other traits? That would require name resolution,
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 9c10823a844..0f276718c16 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -9,7 +9,6 @@ use crate::traits::normalize_projection_type;
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_data_structures::sync::Lrc;
 use rustc_errors::{
     error_code, pluralize, struct_span_err, Applicability, DiagnosticBuilder, Style,
 };
@@ -497,7 +496,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
     ) {
         // It only make sense when suggesting dereferences for arguments
         let code = if let ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } =
-            &obligation.cause.code
+            obligation.cause.code()
         {
             parent_code.clone()
         } else {
@@ -662,7 +661,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             }
             _ => return,
         };
-        if matches!(obligation.cause.code, ObligationCauseCode::FunctionArgumentObligation { .. }) {
+        if matches!(obligation.cause.code(), ObligationCauseCode::FunctionArgumentObligation { .. })
+        {
             // When the obligation error has been ensured to have been caused by
             // an argument, the `obligation.cause.span` points at the expression
             // of the argument, so we can provide a suggestion. Otherwise, we give
@@ -688,13 +688,13 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let span = obligation.cause.span;
 
         let code = if let ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } =
-            &obligation.cause.code
+            obligation.cause.code()
         {
-            parent_code.clone()
+            &parent_code
         } else if let ExpnKind::Desugaring(DesugaringKind::ForLoop) =
             span.ctxt().outer_expn_data().kind
         {
-            Lrc::new(obligation.cause.code.clone())
+            obligation.cause.code()
         } else {
             return false;
         };
@@ -805,10 +805,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             return false;
         };
 
-        if let ObligationCauseCode::ImplDerivedObligation(obligation) = &*code {
+        if let ObligationCauseCode::ImplDerivedObligation(obligation) = code {
             try_borrowing(obligation.parent_trait_ref, &[])
         } else if let ObligationCauseCode::BindingObligation(_, _)
-        | ObligationCauseCode::ItemObligation(_) = &*code
+        | ObligationCauseCode::ItemObligation(_) = code
         {
             try_borrowing(*poly_trait_ref, &never_suggest_borrow)
         } else {
@@ -886,7 +886,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
     ) {
         let span = obligation.cause.span;
 
-        if let ObligationCauseCode::AwaitableExpr(hir_id) = obligation.cause.code.peel_derives() {
+        if let ObligationCauseCode::AwaitableExpr(hir_id) = obligation.cause.code().peel_derives() {
             let hir = self.tcx.hir();
             if let Some(node) = hir_id.and_then(|hir_id| hir.find(hir_id)) {
                 if let hir::Node::Expr(expr) = node {
@@ -945,7 +945,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
     ) {
         let points_at_arg = matches!(
-            obligation.cause.code,
+            obligation.cause.code(),
             ObligationCauseCode::FunctionArgumentObligation { .. },
         );
 
@@ -1072,7 +1072,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         obligation: &PredicateObligation<'tcx>,
         trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
     ) -> bool {
-        match obligation.cause.code.peel_derives() {
+        match obligation.cause.code().peel_derives() {
             // Only suggest `impl Trait` if the return type is unsized because it is `dyn Trait`.
             ObligationCauseCode::SizedReturnType => {}
             _ => return false,
@@ -1267,7 +1267,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         err: &mut DiagnosticBuilder<'_>,
         obligation: &PredicateObligation<'tcx>,
     ) {
-        match obligation.cause.code.peel_derives() {
+        match obligation.cause.code().peel_derives() {
             ObligationCauseCode::SizedReturnType => {}
             _ => return,
         }
@@ -1461,7 +1461,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         };
         let mut generator = None;
         let mut outer_generator = None;
-        let mut next_code = Some(&obligation.cause.code);
+        let mut next_code = Some(obligation.cause.code());
 
         let mut seen_upvar_tys_infer_tuple = false;
 
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index 35bb7d6f06c..42e3f0db15e 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -96,7 +96,7 @@ pub struct PendingPredicateObligation<'tcx> {
 
 // `PendingPredicateObligation` 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!(PendingPredicateObligation<'_>, 56);
+static_assert_size!(PendingPredicateObligation<'_>, 72);
 
 impl<'a, 'tcx> FulfillmentContext<'tcx> {
     /// Creates a new fulfillment context.
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 77b8fc49a15..85016a701f3 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -29,7 +29,6 @@ use crate::traits::project::ProjectionCacheKeyExt;
 use crate::traits::ProjectionCacheKey;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_data_structures::sync::Lrc;
 use rustc_errors::ErrorReported;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
@@ -2384,7 +2383,7 @@ impl<'tcx> TraitObligationExt<'tcx> for TraitObligation<'tcx> {
         // by using -Z verbose or just a CLI argument.
         let derived_cause = DerivedObligationCause {
             parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
-            parent_code: Lrc::new(obligation.cause.code.clone()),
+            parent_code: obligation.cause.clone_code(),
         };
         let derived_code = variant(derived_cause);
         ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
index 3090e8a0428..b6e653c0eea 100644
--- a/compiler/rustc_trait_selection/src/traits/util.rs
+++ b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -232,7 +232,7 @@ pub fn predicates_for_generics<'tcx>(
     debug!("predicates_for_generics(generic_bounds={:?})", generic_bounds);
 
     iter::zip(generic_bounds.predicates, generic_bounds.spans).map(move |(predicate, span)| {
-        let cause = match cause.code {
+        let cause = match *cause.code() {
             traits::ItemObligation(def_id) if !span.is_dummy() => traits::ObligationCause::new(
                 cause.span,
                 cause.body_id,
diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index 5875b764e9f..4bd73ef68aa 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -1,7 +1,6 @@
 use crate::infer::InferCtxt;
 use crate::opaque_types::required_region_bounds;
 use crate::traits;
-use rustc_data_structures::sync::Lrc;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
@@ -227,7 +226,7 @@ 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.make_mut().span = impl_item_span;
+                    cause.span = impl_item_span;
                 }
             }
         }
@@ -242,7 +241,7 @@ fn extend_cause_with_original_assoc_item_obligation<'tcx>(
                         items.iter().find(|i| i.ident == trait_assoc_item.ident).map(fix_span)
                     })
                 {
-                    cause.make_mut().span = impl_item_span;
+                    cause.span = impl_item_span;
                 }
             }
         }
@@ -302,9 +301,9 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
                 let derived_cause = traits::DerivedObligationCause {
                     // FIXME(fee1-dead): when improving error messages, change this to PolyTraitPredicate
                     parent_trait_ref: parent_trait_ref.map_bound(|t| t.trait_ref),
-                    parent_code: Lrc::new(obligation.cause.code.clone()),
+                    parent_code: obligation.cause.clone_code(),
                 };
-                cause.make_mut().code =
+                *cause.make_mut_code() =
                     traits::ObligationCauseCode::DerivedObligation(derived_cause);
             }
             extend_cause_with_original_assoc_item_obligation(
@@ -343,7 +342,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
                             item.map(|i| &i.kind)
                         {
-                            new_cause.make_mut().span = self_ty.span;
+                            new_cause.span = self_ty.span;
                         }
                     }
                     traits::Obligation::with_depth(
diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index ac18908e95b..6192c77d6c6 100644
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/compiler/rustc_typeck/src/check/coercion.rs
@@ -1444,7 +1444,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
 
                 let mut err;
                 let mut unsized_return = false;
-                match cause.code {
+                match *cause.code() {
                     ObligationCauseCode::ReturnNoExpression => {
                         err = struct_span_err!(
                             fcx.tcx.sess,
diff --git a/compiler/rustc_typeck/src/check/compare_method.rs b/compiler/rustc_typeck/src/check/compare_method.rs
index 11560f51822..c942bafcf03 100644
--- a/compiler/rustc_typeck/src/check/compare_method.rs
+++ b/compiler/rustc_typeck/src/check/compare_method.rs
@@ -232,7 +232,7 @@ fn compare_predicate_entailment<'tcx>(
 
             inh.register_predicates(obligations);
             let mut cause = cause.clone();
-            cause.make_mut().span = span;
+            cause.span = span;
             inh.register_predicate(traits::Obligation::new(cause, param_env, predicate));
         }
 
@@ -293,7 +293,7 @@ fn compare_predicate_entailment<'tcx>(
             let (impl_err_span, trait_err_span) =
                 extract_spans_for_error_reporting(&infcx, &terr, &cause, impl_m, trait_m);
 
-            cause.make_mut().span = impl_err_span;
+            cause.span = impl_err_span;
 
             let mut diag = struct_span_err!(
                 tcx.sess,
@@ -1043,7 +1043,7 @@ crate fn compare_const_impl<'tcx>(
 
             // Locate the Span containing just the type of the offending impl
             match tcx.hir().expect_impl_item(impl_c.def_id.expect_local()).kind {
-                ImplItemKind::Const(ref ty, _) => cause.make_mut().span = ty.span,
+                ImplItemKind::Const(ref ty, _) => cause.span = ty.span,
                 _ => bug!("{:?} is not a impl const", impl_c),
             }
 
diff --git a/compiler/rustc_typeck/src/check/fn_ctxt/checks.rs b/compiler/rustc_typeck/src/check/fn_ctxt/checks.rs
index 38a8c1bb9f5..11b63a99043 100644
--- a/compiler/rustc_typeck/src/check/fn_ctxt/checks.rs
+++ b/compiler/rustc_typeck/src/check/fn_ctxt/checks.rs
@@ -997,7 +997,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
                 result_code
             }
-            let self_: ty::subst::GenericArg<'_> = match &*unpeel_to_top(Lrc::new(error.obligation.cause.code.clone())) {
+            let self_: ty::subst::GenericArg<'_> = match &*unpeel_to_top(error.obligation.cause.clone_code()) {
                 ObligationCauseCode::BuiltinDerivedObligation(code) |
                 ObligationCauseCode::ImplDerivedObligation(code) |
                 ObligationCauseCode::DerivedObligation(code) => {
@@ -1040,18 +1040,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
                 // We make sure that only *one* argument matches the obligation failure
                 // and we assign the obligation's span to its expression's.
-                error.obligation.cause.make_mut().span = args[ref_in].span;
-                let code = error.obligation.cause.code.clone();
-                error.obligation.cause.make_mut().code =
+                error.obligation.cause.span = args[ref_in].span;
+                let parent_code = error.obligation.cause.clone_code();
+                *error.obligation.cause.make_mut_code() =
                     ObligationCauseCode::FunctionArgumentObligation {
                         arg_hir_id: args[ref_in].hir_id,
                         call_hir_id: expr.hir_id,
-                        parent_code: Lrc::new(code),
+                        parent_code,
                     };
-            } else if error.obligation.cause.make_mut().span == call_sp {
+            } else if error.obligation.cause.span == call_sp {
                 // Make function calls point at the callee, not the whole thing.
                 if let hir::ExprKind::Call(callee, _) = expr.kind {
-                    error.obligation.cause.make_mut().span = callee.span;
+                    error.obligation.cause.span = callee.span;
                 }
             }
         }
@@ -1092,7 +1092,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                     let ty = <dyn AstConv<'_>>::ast_ty_to_ty(self, hir_ty);
                                     let ty = self.resolve_vars_if_possible(ty);
                                     if ty == predicate.self_ty() {
-                                        error.obligation.cause.make_mut().span = hir_ty.span;
+                                        error.obligation.cause.span = hir_ty.span;
                                     }
                                 }
                             }
diff --git a/compiler/rustc_typeck/src/check/method/suggest.rs b/compiler/rustc_typeck/src/check/method/suggest.rs
index 168bdce3210..1dcc20c29a3 100644
--- a/compiler/rustc_typeck/src/check/method/suggest.rs
+++ b/compiler/rustc_typeck/src/check/method/suggest.rs
@@ -832,7 +832,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     for (data, p, parent_p) in unsatisfied_predicates
                         .iter()
                         .filter_map(|(p, parent, c)| c.as_ref().map(|c| (p, parent, c)))
-                        .filter_map(|(p, parent, c)| match c.code {
+                        .filter_map(|(p, parent, c)| match c.code() {
                             ObligationCauseCode::ImplDerivedObligation(ref data) => {
                                 Some((data, p, parent))
                             }