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.

clippy_lints/src/casts/fn_to_numeric_cast.rs

@@ -15,7 +15,7 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>,
     }
 
     match cast_from.kind() {
-        ty::FnDef(..) | ty::FnPtr(_) => {
+        ty::FnDef(..) | ty::FnPtr(..) => {
             let mut applicability = Applicability::MaybeIncorrect;
             let from_snippet = snippet_with_applicability(cx, cast_expr.span, "x", &mut applicability);
             let to_nbits = utils::int_ty_to_nbits(cast_to, cx.tcx);

clippy_lints/src/casts/fn_to_numeric_cast_any.rs

@@ -14,7 +14,7 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>,
         _ => { /* continue to checks */ },
     }
 
-    if let ty::FnDef(..) | ty::FnPtr(_) = cast_from.kind() {
+    if let ty::FnDef(..) | ty::FnPtr(..) = cast_from.kind() {
         let mut applicability = Applicability::MaybeIncorrect;
         let from_snippet = snippet_with_applicability(cx, cast_expr.span, "..", &mut applicability);
 

clippy_lints/src/casts/fn_to_numeric_cast_with_truncation.rs

@@ -14,7 +14,7 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>,
         _ => return,
     }
     match cast_from.kind() {
-        ty::FnDef(..) | ty::FnPtr(_) => {
+        ty::FnDef(..) | ty::FnPtr(..) => {
             let mut applicability = Applicability::MaybeIncorrect;
             let from_snippet = snippet_with_applicability(cx, cast_expr.span, "x", &mut applicability);
 

clippy_lints/src/default_numeric_fallback.rs

@@ -236,7 +236,7 @@ fn fn_sig_opt<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId) -> Option<PolyFnSig<'
     // We can't use `Ty::fn_sig` because it automatically performs args, this may result in FNs.
     match node_ty.kind() {
         ty::FnDef(def_id, _) => Some(cx.tcx.fn_sig(*def_id).instantiate_identity()),
-        ty::FnPtr(fn_sig) => Some(*fn_sig),
+        ty::FnPtr(sig_tys, hdr) => Some(sig_tys.with(*hdr)),
         _ => None,
     }
 }

clippy_lints/src/dereference.rs

@@ -872,7 +872,7 @@ impl TyCoercionStability {
                 | ty::Pat(..)
                 | ty::Float(_)
                 | ty::RawPtr(..)
-                | ty::FnPtr(_)
+                | ty::FnPtr(..)
                 | ty::Str
                 | ty::Slice(..)
                 | ty::Adt(..)

clippy_lints/src/eta_reduction.rs

@@ -158,7 +158,7 @@ fn check_clousure<'tcx>(cx: &LateContext<'tcx>, outer_receiver: Option<&Expr<'tc
 
                     cx.tcx.fn_sig(def).skip_binder().skip_binder()
                 },
-                ty::FnPtr(sig) => sig.skip_binder(),
+                ty::FnPtr(sig_tys, hdr) => sig_tys.with(*hdr).skip_binder(),
                 ty::Closure(_, subs) => cx
                     .tcx
                     .signature_unclosure(subs.as_closure().sig(), Safety::Safe)

clippy_lints/src/methods/map_flatten.rs

@@ -58,7 +58,7 @@ fn try_get_caller_ty_name_and_method_name(
 fn is_map_to_option(cx: &LateContext<'_>, map_arg: &Expr<'_>) -> bool {
     let map_closure_ty = cx.typeck_results().expr_ty(map_arg);
     match map_closure_ty.kind() {
-        ty::Closure(_, _) | ty::FnDef(_, _) | ty::FnPtr(_) => {
+        ty::Closure(_, _) | ty::FnDef(_, _) | ty::FnPtr(..) => {
             let map_closure_sig = match map_closure_ty.kind() {
                 ty::Closure(_, args) => args.as_closure().sig(),
                 _ => map_closure_ty.fn_sig(cx.tcx),

clippy_lints/src/mixed_read_write_in_expression.rs

@@ -166,7 +166,7 @@ impl<'a, 'tcx> Visitor<'tcx> for DivergenceVisitor<'a, 'tcx> {
             ExprKind::Call(func, _) => {
                 let typ = self.cx.typeck_results().expr_ty(func);
                 match typ.kind() {
-                    ty::FnDef(..) | ty::FnPtr(_) => {
+                    ty::FnDef(..) | ty::FnPtr(..) => {
                         let sig = typ.fn_sig(self.cx.tcx);
                         if self.cx.tcx.instantiate_bound_regions_with_erased(sig).output().kind() == &ty::Never {
                             self.report_diverging_sub_expr(e);

clippy_lints/src/multiple_unsafe_ops_per_block.rs

@@ -130,7 +130,7 @@ fn collect_unsafe_exprs<'tcx>(
             ExprKind::Call(path_expr, _) => {
                 let sig = match *cx.typeck_results().expr_ty(path_expr).kind() {
                     ty::FnDef(id, _) => cx.tcx.fn_sig(id).skip_binder(),
-                    ty::FnPtr(sig) => sig,
+                    ty::FnPtr(sig_tys, hdr) => sig_tys.with(hdr),
                     _ => return Continue(Descend::Yes),
                 };
                 if sig.safety() == Safety::Unsafe {

clippy_lints/src/mut_reference.rs

@@ -79,7 +79,7 @@ fn check_arguments<'tcx>(
     fn_kind: &str,
 ) {
     match type_definition.kind() {
-        ty::FnDef(..) | ty::FnPtr(_) => {
+        ty::FnDef(..) | ty::FnPtr(..) => {
             let parameters = type_definition.fn_sig(cx.tcx).skip_binder().inputs();
             for (argument, parameter) in iter::zip(arguments, parameters) {
                 match parameter.kind() {

clippy_utils/src/ty.rs

@@ -541,7 +541,7 @@ pub fn peel_mid_ty_refs_is_mutable(ty: Ty<'_>) -> (Ty<'_>, usize, Mutability) {
 /// Returns `true` if the given type is an `unsafe` function.
 pub fn type_is_unsafe_function<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
     match ty.kind() {
-        ty::FnDef(..) | ty::FnPtr(_) => ty.fn_sig(cx.tcx).safety() == Safety::Unsafe,
+        ty::FnDef(..) | ty::FnPtr(..) => ty.fn_sig(cx.tcx).safety() == Safety::Unsafe,
         _ => false,
     }
 }
@@ -721,7 +721,7 @@ pub fn ty_sig<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> Option<ExprFnSig<'t
             cx.tcx.item_super_predicates(def_id).iter_instantiated(cx.tcx, args),
             cx.tcx.opt_parent(def_id),
         ),
-        ty::FnPtr(sig) => Some(ExprFnSig::Sig(sig, None)),
+        ty::FnPtr(sig_tys, hdr) => Some(ExprFnSig::Sig(sig_tys.with(hdr), None)),
         ty::Dynamic(bounds, _, _) => {
             let lang_items = cx.tcx.lang_items();
             match bounds.principal() {

clippy_utils/src/visitors.rs

@@ -441,7 +441,7 @@ pub fn is_expr_unsafe<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) -> bool {
                     ty::FnDef(id, _) if self.cx.tcx.fn_sig(id).skip_binder().safety() == Safety::Unsafe => {
                         self.is_unsafe = true;
                     },
-                    ty::FnPtr(sig) if sig.safety() == Safety::Unsafe => self.is_unsafe = true,
+                    ty::FnPtr(_, hdr) if hdr.safety == Safety::Unsafe => self.is_unsafe = true,
                     _ => walk_expr(self, e),
                 },
                 ExprKind::Path(ref p)