How to implement traits for a type containing a function type?

[vsajip]

I have an example of a type which contains a configurable function with a default implementation. This compiles and runs fine as long as the [derive(PartialEq)] is commented out. If it's not commented out, I get errors:

type StringMunger = fn(&str) -> String;

fn default_string_munger(s: &str) -> String {
    format!("Hello, {}!", s)
}

#[derive(PartialEq)]
struct Thing {
    string_munger: StringMunger,
}

impl Thing {
    fn new() -> Self {
        Self{string_munger: default_string_munger}
    }
    
    fn munge(&mut self, s: &str) -> String {
        (self.string_munger)(s)
    }
}

fn main() {
    let mut thing = Thing::new();
    
    println!("{}", thing.munge("world"));
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0277]: `for<'r> fn(&'r str) -> std::string::String` doesn't implement `std::fmt::Debug`
 --> src/main.rs:9:5
  |
9 |     string_munger: StringMunger,
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ `for<'r> fn(&'r str) -> std::string::String` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
  |
  = help: the trait `std::fmt::Debug` is not implemented for `for<'r> fn(&'r str) -> std::string::String`
  = note: required because of the requirements on the impl of `std::fmt::Debug` for `&for<'r> fn(&'r str) -> std::string::String`
  = note: required for the cast to the object type `dyn std::fmt::Debug`

error[E0369]: binary operation `==` cannot be applied to type `for<'r> fn(&'r str) -> std::string::String`
 --> src/main.rs:9:5
  |
9 |     string_munger: StringMunger,
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: an implementation of `std::cmp::PartialEq` might be missing for `for<'r> fn(&'r str) -> std::string::String`

error[E0369]: binary operation `!=` cannot be applied to type `for<'r> fn(&'r str) -> std::string::String`
 --> src/main.rs:9:5
  |
9 |     string_munger: StringMunger,
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: an implementation of `std::cmp::PartialEq` might be missing for `for<'r> fn(&'r str) -> std::string::String`

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0277, E0369.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

If I naïvely try to add

impl PartialEq for StringMunger {
    fn eq(&self, other: &Self) -> bool {
        false
    }
}

then I get a different error:

error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
  --> src/main.rs:12:1
   |
12 | impl PartialEq for StringMunger {
   | ^^^^^---------^^^^^------------
   | |    |             |
   | |    |             `for<'r> fn(&'r str) -> std::string::String` is not defined in the current crate
   | |    `for<'r> fn(&'r str) -> std::string::String` is not defined in the current crate
   | impl doesn't use only types from inside the current crate
   |
   = note: define and implement a trait or new type instead

Of course, I want to implement PartialEq, Debug etc. for Thing, but I can't derive them automagically because of the use of the function type. How do I achieve the ability to implement those traits for Thing as easily as possible?

[Riateche]

Function pointer types are not defined in your crate, so you can't implement foreign traits for them. If you make a newtype struct, you can implement any traits for it:

struct StringMunger(fn(&str) -> String);

Or you can just implement the traits you want on Thing instead of deriving them. That should also work.

[vsajip]

Neat - thanks. So that's a tuple struct - the function pointer would be accessed as .0 of a struct instance.