Any magic to find out whether type X implements some triats T at compile time?

[MoZhonghua]

When I write a derive proc-macro, I need to generates a method for each field of struct, which returns true if the type of the field implements a trait T, else returns false. Is any way to do this?

I treid some ways, but found that I can't specialize code based on whether T impl or not impl trait.

pub trait T {}
pub struct X;  // implements T
pub struct Y;  // doesn't implement T
impl X for T { }

type Z = X;
#[derive(MySillyDerive)]
struct Foo {
   val1: Z,   // users can define type aliases, so we can't rely on the name of type
   val2: Y,
}

// Want to generates something like this
impl Foo {
  fn is_val1_implement_t() -> bool {  // TODO: always return true
  }
  fn is_val2_implement_t() -> bool {  // TODO: always return false
  }
}

[riking]

Unfortunately, that step happens after macro expansion.

[quinedot]

You can make those methods at compile time and hope they optimize away, but I don't know of any stable way to make them const, i.e., callable at compile time. I can't tell if that's sufficient for your use case or not.

Example.

[MoZhonghua]

Wonderful solution !!!!

The basic idea is using same method name, but one is provided by type itself, while another is provided by a extension trait.

One more question, for ImplType<X>, both methods should be available, is there some document to clarify the rules to tell us which method will be chosen?

[H2CO3]

Inherent methods are preferred over trait methods, if that's what you are asking.

[MoZhonghua]

Thanks!