cpp-early-specialization.rs

// --- original code ---

cpp_inst! { SomeTrait for (T,U,V) in (i32,u32,u64), (i32,String,bool), ... {
  fn function1<A,B,C>(...) 
    where (A,B,C): SomeTrait<T=A,U=B,V=C> {
    ...
    cpp!( (...) { ... }; // callsite 1
    ...
    cpp!( (...) { ... }; // callsite 2
    ...
  }
  fn function2<X,Y>(...)
    where (X,Y,u64): SomeTrait<T=A,U=B,V=u64> {
    ...
    cpp!( (...) { ... }; // callsite 3
    ...  
  }
}}

// --- after macro expansion --- 

trait SomeTrait {
  type T;
  type U;
  type V;
  fn cpp_callsite1(x:Self::T, y:Self::U) -> Self::V;
  fn cpp_callsite2(x:Self::U) -> Self::T;
  fn cpp_callsite3() -> Self::V;
  // and so on ...
}
// impl for combo 1
impl Sometrait for (i32,u32,u64) {
  type T = i32;
  type U = u32;
  type V = u64;
  fn cpp_callsite1(x:i32, y:u32) -> u64 { ffi_callsite1_combo1(x,y) }
  fn cpp_callsite2(x:u32) -> i32 { ffi_callsite2_combo1(x) }
  fn cpp_callsite3() -> u64 { ffi_callsite3_combo1() }
}
extern {
  fn ffi_callsite1_combo1(x:i32, y:u32) -> u64;
  fn ffi_callsite2_combo1(x:u32) -> i32;
  fn ffi_callsite3_combo1() -> u64;
}
// impl for combo 2
impl Sometrait for (i32,String,bool) {
  ...
}
extern {
  fn ffi_callsite1_combo2(x:i32, y:String) -> bool;
  fn ffi_callsite2_combo2(x:String) -> i32;
  fn ffi_callsite3_combo2() -> bool;
}
// impls for all other type combinations
...

fn function1<A,B,C>(...) 
  where (A,B,C): SomeTrait<T=A,U=B,V=C> {
  ...
  <(A,B,C) as Sometrait>::cpp_callsite1(...);
  ...
  <(A,B,C) as Sometrait>::cpp_callsite2(...);
  ...
}
fn function2<X,Y>(...)
  where (X,Y,u64): SomeTrait<T=X,U=Y,V=u64> {
  ...
  <(X,Y,u64) as Sometrait>::cpp_callsite3(...);
  ...  
}