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/x64/abi.rs

@@ -21,13 +21,13 @@ use std::convert::TryFrom;
 static STACK_ARG_RET_SIZE_LIMIT: u64 = 128 * 1024 * 1024;
 
 /// Support for the x64 ABI from the callee side (within a function body).
-pub(crate) type X64ABICallee = ABICalleeImpl<X64ABIMachineSpec>;
+pub(crate) type X64Callee = Callee<X64ABIMachineSpec>;
 
 /// Support for the x64 ABI from the caller side (at a callsite).
 pub(crate) type X64ABICaller = ABICallerImpl<X64ABIMachineSpec>;
 
 /// Implementation of ABI primitives for x64.
-pub(crate) struct X64ABIMachineSpec;
+pub struct X64ABIMachineSpec;
 
 impl IsaFlags for x64_settings::Flags {}
 

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -2157,6 +2157,8 @@ fn x64_get_operands<F: Fn(VReg) -> VReg>(inst: &Inst, collector: &mut OperandCol
 // Instructions: misc functions and external interface
 
 impl MachInst for Inst {
+    type ABIMachineSpec = X64ABIMachineSpec;
+
     fn get_operands<F: Fn(VReg) -> VReg>(&self, collector: &mut OperandCollector<'_, F>) {
         x64_get_operands(&self, collector)
     }
@@ -2460,7 +2462,7 @@ impl MachInstEmit for Inst {
 }
 
 impl MachInstEmitState<Inst> for EmitState {
-    fn new(abi: &dyn ABICallee<I = Inst>) -> Self {
+    fn new(abi: &Callee<X64ABIMachineSpec>) -> Self {
         EmitState {
             virtual_sp_offset: 0,
             nominal_sp_to_fp: abi.frame_size() as i64,

cranelift/codegen/src/isa/x64/mod.rs

@@ -53,7 +53,7 @@ impl X64Backend {
         // This performs lowering to VCode, register-allocates the code, computes
         // block layout and finalizes branches. The result is ready for binary emission.
         let emit_info = EmitInfo::new(flags.clone(), self.x64_flags.clone());
-        let abi = Box::new(abi::X64ABICallee::new(&func, self, &self.x64_flags)?);
+        let abi = abi::X64Callee::new(&func, self, &self.x64_flags)?;
         compile::compile::<Self>(&func, self, abi, &self.reg_env, emit_info)
     }
 }