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.

compiler/rustc_next_trait_solver/src/canonicalizer.rs

@@ -360,7 +360,7 @@ impl<D: SolverDelegate<Interner = I>, I: Interner> TypeFolder<I> for Canonicaliz
             | ty::Ref(_, _, _)
             | ty::Pat(_, _)
             | ty::FnDef(_, _)
-            | ty::FnPtr(_)
+            | ty::FnPtr(..)
             | ty::Dynamic(_, _, _)
             | ty::Closure(..)
             | ty::CoroutineClosure(..)

compiler/rustc_next_trait_solver/src/coherence.rs

@@ -334,7 +334,7 @@ where
             | ty::Str
             | ty::FnDef(..)
             | ty::Pat(..)
-            | ty::FnPtr(_)
+            | ty::FnPtr(..)
             | ty::Array(..)
             | ty::Slice(..)
             | ty::RawPtr(..)

compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs

@@ -533,7 +533,7 @@ where
             | ty::RawPtr(_, _)
             | ty::Ref(_, _, _)
             | ty::FnDef(_, _)
-            | ty::FnPtr(_)
+            | ty::FnPtr(..)
             | ty::Dynamic(..)
             | ty::Closure(..)
             | ty::CoroutineClosure(..)
@@ -620,7 +620,7 @@ where
             | ty::RawPtr(_, _)
             | ty::Ref(_, _, _)
             | ty::FnDef(_, _)
-            | ty::FnPtr(_)
+            | ty::FnPtr(..)
             | ty::Alias(..)
             | ty::Closure(..)
             | ty::CoroutineClosure(..)

compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs

@@ -31,7 +31,7 @@ where
         | ty::Bool
         | ty::Float(_)
         | ty::FnDef(..)
-        | ty::FnPtr(_)
+        | ty::FnPtr(..)
         | ty::Error(_)
         | ty::Never
         | ty::Char => Ok(vec![]),
@@ -117,7 +117,7 @@ where
         | ty::Bool
         | ty::Float(_)
         | ty::FnDef(..)
-        | ty::FnPtr(_)
+        | ty::FnPtr(..)
         | ty::RawPtr(..)
         | ty::Char
         | ty::Ref(..)
@@ -178,7 +178,7 @@ where
 {
     match ty.kind() {
         // impl Copy/Clone for FnDef, FnPtr
-        ty::FnDef(..) | ty::FnPtr(_) | ty::Error(_) => Ok(vec![]),
+        ty::FnDef(..) | ty::FnPtr(..) | ty::Error(_) => Ok(vec![]),
 
         // Implementations are provided in core
         ty::Uint(_)
@@ -269,7 +269,8 @@ pub(in crate::solve) fn extract_tupled_inputs_and_output_from_callable<I: Intern
             }
         }
         // keep this in sync with assemble_fn_pointer_candidates until the old solver is removed.
-        ty::FnPtr(sig) => {
+        ty::FnPtr(sig_tys, hdr) => {
+            let sig = sig_tys.with(hdr);
             if sig.is_fn_trait_compatible() {
                 Ok(Some(
                     sig.map_bound(|sig| (Ty::new_tup(cx, sig.inputs().as_slice()), sig.output())),

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -1145,7 +1145,7 @@ where
             | ty::RawPtr(_, _)
             | ty::Ref(_, _, _)
             | ty::FnDef(_, _)
-            | ty::FnPtr(_)
+            | ty::FnPtr(..)
             | ty::Closure(..)
             | ty::CoroutineClosure(..)
             | ty::Coroutine(_, _)