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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: How to have a private function in a public trait

I have a trait that needs to be public so that it can be used as a constraint for a public function. The implementation of the trait is not meant to be public. This seems to be related to sealed traits ?

Here is an example of what I am trying to accomplish:

use std::convert::From;
//
// PrivateStruct
// This structure is private and not part of the API
struct PrivateStruct { }
//
// PublicTrait
// This trait is public because it is a constraint in check_le_max
pub trait PublicTrait {
    #[doc(hidden)]
    fn max(_ : Self, _ : PrivateStruct) -> Self;
}
impl PublicTrait for u8 {
    // This function is not part of the API
    #[doc(hidden)]
    fn max(_ : Self , _ : PrivateStruct) -> Self
    {   return Self::MAX }
}
//
// check_le_max
// This function is public because it is part of the API
pub fn check_le_max<T>(u : usize, _ : T) -> bool
where
    T     : From<u8> + PublicTrait ,
    usize : From<T> ,
{   let z_t  : T             = From::from(0u8);
    let z_ps : PrivateStruct = PrivateStruct{ };
    let max  = PublicTrait::max(z_t, z_ps );
    u <= From::from(max)
}
//
fn main() {
    println!( "( {} <= u8 max) = {}", 255, check_le_max(255, 0u8) );
    println!( "( {} <= u8 max) = {}", 256, check_le_max(256, 0u8) );
}

Here is the error message I am getting:

  --> src/main.rs:11:5
   |
11 |     fn max(_ : Self, _ : PrivateStruct) -> Self;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ associated function `PublicTrait::max` is reachable at visibility `pub`
   |
note: but type `PrivateStruct` is only usable at visibility `pub(crate)`
  --> src/main.rs:5:1
   |
5  | struct PrivateStruct { }
   | ^^^^^^^^^^^^^^^^^^^^
   = note: `#[warn(private_interfaces)]` on by default

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