Shrink `TyKind::FnPtr`.

By splitting the `FnSig` within `TyKind::FnPtr` into `FnSigTys` and
`FnHeader`, which can be packed more efficiently. This reduces the size
of the hot `TyKind` type from 32 bytes to 24 bytes on 64-bit platforms.
This reduces peak memory usage by a few percent on some benchmarks. It
also reduces cache misses and page faults similarly, though this doesn't
translate to clear cycles or wall-time improvements on CI.

2 files changed, 7 insertions, 2 deletions

diff --git a/compiler/rustc_smir/src/rustc_internal/internal.rs b/compiler/rustc_smir/src/rustc_internal/internal.rs
index a4577461094..f52cb010a87 100644
--- a/compiler/rustc_smir/src/rustc_internal/internal.rs
+++ b/compiler/rustc_smir/src/rustc_internal/internal.rs
@@ -131,7 +131,10 @@ impl RustcInternal for RigidTy {
             RigidTy::FnDef(def, args) => {
                 rustc_ty::TyKind::FnDef(def.0.internal(tables, tcx), args.internal(tables, tcx))
             }
-            RigidTy::FnPtr(sig) => rustc_ty::TyKind::FnPtr(sig.internal(tables, tcx)),
+            RigidTy::FnPtr(sig) => {
+                let (sig_tys, hdr) = sig.internal(tables, tcx).split();
+                rustc_ty::TyKind::FnPtr(sig_tys, hdr)
+            }
             RigidTy::Closure(def, args) => {
                 rustc_ty::TyKind::Closure(def.0.internal(tables, tcx), args.internal(tables, tcx))
             }
diff --git a/compiler/rustc_smir/src/rustc_smir/convert/ty.rs b/compiler/rustc_smir/src/rustc_smir/convert/ty.rs
index ef2eb7d52ea..332fe22d869 100644
--- a/compiler/rustc_smir/src/rustc_smir/convert/ty.rs
+++ b/compiler/rustc_smir/src/rustc_smir/convert/ty.rs
@@ -352,7 +352,9 @@ impl<'tcx> Stable<'tcx> for ty::TyKind<'tcx> {
             ty::FnDef(def_id, generic_args) => {
                 TyKind::RigidTy(RigidTy::FnDef(tables.fn_def(*def_id), generic_args.stable(tables)))
             }
-            ty::FnPtr(poly_fn_sig) => TyKind::RigidTy(RigidTy::FnPtr(poly_fn_sig.stable(tables))),
+            ty::FnPtr(sig_tys, hdr) => {
+                TyKind::RigidTy(RigidTy::FnPtr(sig_tys.with(*hdr).stable(tables)))
+            }
             ty::Dynamic(existential_predicates, region, dyn_kind) => {
                 TyKind::RigidTy(RigidTy::Dynamic(
                     existential_predicates