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.

compiler/rustc_hir_typeck/src/fn_ctxt/adjust_fulfillment_errors.rs

@@ -278,9 +278,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         span: Span,
     ) -> bool {
         if let traits::FulfillmentErrorCode::CodeSelectionError(
-            traits::SelectionError::OutputTypeParameterMismatch(_, expected, _),
+            traits::SelectionError::OutputTypeParameterMismatch(box traits::SelectionOutputTypeParameterMismatch{
+                expected_trait_ref, ..
+            }),
         ) = error.code
-            && let ty::Closure(def_id, _) | ty::Generator(def_id, ..) = expected.skip_binder().self_ty().kind()
+            && let ty::Closure(def_id, _) | ty::Generator(def_id, ..) = expected_trait_ref.skip_binder().self_ty().kind()
             && span.overlaps(self.tcx.def_span(*def_id))
         {
             true

compiler/rustc_hir_typeck/src/lib.rs

@@ -2,6 +2,7 @@
 #![feature(let_chains)]
 #![feature(try_blocks)]
 #![feature(never_type)]
+#![feature(box_patterns)]
 #![feature(min_specialization)]
 #![feature(control_flow_enum)]
 #![feature(drain_filter)]

compiler/rustc_middle/src/traits/mod.rs

@@ -580,11 +580,7 @@ pub enum SelectionError<'tcx> {
     /// After a closure impl has selected, its "outputs" were evaluated
     /// (which for closures includes the "input" type params) and they
     /// didn't resolve. See `confirm_poly_trait_refs` for more.
-    OutputTypeParameterMismatch(
-        ty::PolyTraitRef<'tcx>,
-        ty::PolyTraitRef<'tcx>,
-        ty::error::TypeError<'tcx>,
-    ),
+    OutputTypeParameterMismatch(Box<SelectionOutputTypeParameterMismatch<'tcx>>),
     /// The trait pointed by `DefId` is not object safe.
     TraitNotObjectSafe(DefId),
     /// A given constant couldn't be evaluated.
@@ -596,6 +592,13 @@ pub enum SelectionError<'tcx> {
     ErrorReporting,
 }
 
+#[derive(Clone, Debug, TypeVisitable, Lift)]
+pub struct SelectionOutputTypeParameterMismatch<'tcx> {
+    pub found_trait_ref: ty::PolyTraitRef<'tcx>,
+    pub expected_trait_ref: ty::PolyTraitRef<'tcx>,
+    pub terr: ty::error::TypeError<'tcx>,
+}
+
 /// When performing resolution, it is typically the case that there
 /// can be one of three outcomes:
 ///

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,

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,
@@ -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(