Auto merge of #111029 - Nilstrieb:when-the-errs-are-too-big, r=petrochenkov

Shrink `SelectionError` a lot

`SelectionError` used to be 80 bytes (on 64 bit). That's quite big. Especially because the selection cache contained `Result<_, SelectionError>. The Ok type is only 32 bytes, so the 80 bytes significantly inflate the size of the cache.

Most variants of the `SelectionError` seem to be hard errors, only `Unimplemented` shows up in practice (for cranelift-codegen, it occupies 23.4% of all cache entries). We can just box away the biggest variant, `OutputTypeParameterMismatch`, to get the size down to 16 bytes, well within the size of the Ok type inside the cache.

2 files changed, 19 insertions, 7 deletions

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 c9e2ed092d1..98365223923 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -28,6 +28,7 @@ use rustc_hir::{GenericParam, Item, Node};
 use rustc_infer::infer::error_reporting::TypeErrCtxt;
 use rustc_infer::infer::{InferOk, TypeTrace};
 use rustc_middle::traits::select::OverflowError;
+use rustc_middle::traits::SelectionOutputTypeParameterMismatch;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
@@ -1087,17 +1088,21 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 }
             }
 
-            OutputTypeParameterMismatch(
+            OutputTypeParameterMismatch(box SelectionOutputTypeParameterMismatch {
                 found_trait_ref,
                 expected_trait_ref,
-                terr @ TypeError::CyclicTy(_),
-            ) => self.report_type_parameter_mismatch_cyclic_type_error(
+                terr: terr @ TypeError::CyclicTy(_),
+            }) => self.report_type_parameter_mismatch_cyclic_type_error(
                 &obligation,
                 found_trait_ref,
                 expected_trait_ref,
                 terr,
             ),
-            OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
+            OutputTypeParameterMismatch(box SelectionOutputTypeParameterMismatch {
+                found_trait_ref,
+                expected_trait_ref,
+                terr: _,
+            }) => {
                 match self.report_type_parameter_mismatch_error(
                     &obligation,
                     span,
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 616187b69dd..4dc84e0ad10 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -10,6 +10,7 @@ use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
 use rustc_infer::infer::{DefineOpaqueTypes, InferOk};
+use rustc_middle::traits::SelectionOutputTypeParameterMismatch;
 use rustc_middle::ty::{
     self, Binder, GenericParamDefKind, InternalSubsts, SubstsRef, ToPolyTraitRef, ToPredicate,
     TraitRef, Ty, TyCtxt, TypeVisitableExt,
@@ -811,7 +812,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn confirm_poly_trait_refs(
         &mut self,
         obligation: &TraitObligation<'tcx>,
-        expected_trait_ref: ty::PolyTraitRef<'tcx>,
+        self_ty_trait_ref: ty::PolyTraitRef<'tcx>,
     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
         let obligation_trait_ref = obligation.predicate.to_poly_trait_ref();
         // Normalize the obligation and expected trait refs together, because why not
@@ -822,7 +823,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     obligation.param_env,
                     obligation.cause.clone(),
                     obligation.recursion_depth + 1,
-                    (obligation_trait_ref, expected_trait_ref),
+                    (obligation_trait_ref, self_ty_trait_ref),
                 )
             });
 
@@ -834,7 +835,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 obligations.extend(nested);
                 obligations
             })
-            .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
+            .map_err(|terr| {
+                OutputTypeParameterMismatch(Box::new(SelectionOutputTypeParameterMismatch {
+                    expected_trait_ref: obligation_trait_ref,
+                    found_trait_ref: expected_trait_ref,
+                    terr,
+                }))
+            })
     }
 
     fn confirm_trait_upcasting_unsize_candidate(