Cranelift: Remove ABICallee trait (#4701)

* Cranelift: Remove `ABICallee` trait

It has only one implementation: the `ABICalleeImpl` struct. By using that
directly we can avoid unnecessary layers of generics and abstractions as well as
a couple `Box`es that were previously putting the single implementation into a
`Box<dyn>`.

* Cranelift: Rename `ABICalleeImpl` to `AbiCallee`

* Fix comments as per review

* Rename `AbiCallee` to `Callee`

cranelift/codegen/src/isa/aarch64/abi.rs

@@ -21,7 +21,7 @@ use smallvec::{smallvec, SmallVec};
 // these ABIs are very similar.
 
 /// Support for the AArch64 ABI from the callee side (within a function body).
-pub(crate) type AArch64ABICallee = ABICalleeImpl<AArch64MachineDeps>;
+pub(crate) type AArch64Callee = Callee<AArch64MachineDeps>;
 
 /// Support for the AArch64 ABI from the caller side (at a callsite).
 pub(crate) type AArch64ABICaller = ABICallerImpl<AArch64MachineDeps>;
@@ -65,7 +65,7 @@ fn saved_reg_stack_size(
 
 /// AArch64-specific ABI behavior. This struct just serves as an implementation
 /// point for the trait; it is never actually instantiated.
-pub(crate) struct AArch64MachineDeps;
+pub struct AArch64MachineDeps;
 
 impl IsaFlags for aarch64_settings::Flags {}
 

cranelift/codegen/src/isa/aarch64/inst/emit.rs

@@ -630,7 +630,7 @@ pub struct EmitState {
 }
 
 impl MachInstEmitState<Inst> for EmitState {
-    fn new(abi: &dyn ABICallee<I = Inst>) -> Self {
+    fn new(abi: &Callee<AArch64MachineDeps>) -> Self {
         EmitState {
             virtual_sp_offset: 0,
             nominal_sp_to_fp: abi.frame_size() as i64,

cranelift/codegen/src/isa/aarch64/inst/mod.rs

@@ -1060,6 +1060,7 @@ fn aarch64_get_operands<F: Fn(VReg) -> VReg>(inst: &Inst, collector: &mut Operan
 // Instructions: misc functions and external interface
 
 impl MachInst for Inst {
+    type ABIMachineSpec = AArch64MachineDeps;
     type LabelUse = LabelUse;
 
     fn get_operands<F: Fn(VReg) -> VReg>(&self, collector: &mut OperandCollector<'_, F>) {

cranelift/codegen/src/isa/aarch64/mod.rs

@@ -60,7 +60,7 @@ impl AArch64Backend {
         flags: shared_settings::Flags,
     ) -> CodegenResult<(VCode<inst::Inst>, regalloc2::Output)> {
         let emit_info = EmitInfo::new(flags.clone());
-        let abi = Box::new(abi::AArch64ABICallee::new(func, self, &self.isa_flags)?);
+        let abi = abi::AArch64Callee::new(func, self, &self.isa_flags)?;
         compile::compile::<AArch64Backend>(func, self, abi, &self.machine_env, emit_info)
     }
 }