如何在具有通用参数的结构上实现非通用特征

Question

I am using the I2CDevice trait which is defined in an external crate like this:

pub trait I2CDevice {
    type Error: Error;
    // members
}

Neither it nor any of its members contain any generic arguments.

I am creating a decorator for any I2CDevice implementation which delegates to an inner/concrete I2CDevice , adding console printing for diagnostic purposes:

struct debugDeviceDecorator<'a, T: I2CDevice<Error = LinuxI2CError> + Sized + 'a> {
    device: &'a mut T,
}

impl I2CDevice__A__ for debugDeviceDecorator__B__ {
    type Error = LinuxI2CError;

    fn read(&mut self, data: &mut [u8]) -> Result<(), Self::Error> {
        println!("read: data: {:?}", data);
        self.device.read(data)
    }

    // etc.
}

I can't figure out what to put in place of __A__ and __B__ .

When I use no generic arguments whatsoever, I get compiler errors: Expected 1 lifetime parameter and Expected 1 type argument.

My best guess then is to leave __A__ blank because the definition of I2CDevice requires no generic arguments, and then to mirror the type argument used for the debugDeviceDecorator struct itself in place of __B__ like:

impl I2CDevice for debugDeviceDecorator<'a, T: I2CDevice<Error=LinuxI2CError> + Sized + 'a> { }

I get the compilation error: error: expected one of ! , ( , + , , , :: , < , or > , found : , which occurs on the colon ( : ) that begins the type constraint on the generic argument T .

Perhaps the compiler thinks that I'm trying to constrain T in a manner which diverges from the constraint in the struct definition itself, so instead I try:

impl I2CDevice for debugDeviceDecorator<'a, T> {}

Which results in the errors undeclared lifetime for the lifetime parameter, and undefined or not in scope for the type argument itself, at which point I cannot figure out how to proceed. I would assume that the lifetime and the type argument are defined by the future, as yet nonexistent code that will some day initialize a debugDeviceDecorator struct, values which cannot yet be known and seem to me as though should not prevent this existing code from compiling.

I can see the compiler needing some information about the type of self.device used within the functions of the impl, but I feel like my first guess in which I mirrored the struct's expanded type definition in my impl declaration should have provided that.

Answer 1

You should re-read The Rust Programming Language , specifically the section on how to implement traits for generic structs . A lot of hard work has gone into that documentation so that people can get started in Rust easier.

The book will show you the proper syntax for how to define a trait for a struct:

impl<'a, T> I2CDevice for DebugDeviceDecorator<'a, T>
    where T: I2CDevice<Error = LinuxI2CError> + Sized + 'a
{
    // ...
}

Note:

1. You have to declare the two generic values ( 'a and T ) before you can use them.

2. There's nothing special about the fact that the trait has no generics.

3. Types in Rust use PascalCase , not camelCase , so I've changed the name.

4. I switched to a where clause because it's too hard to read the bounds when they are crammed into the generic declaration.