Macro unimplemented

macro_rules! unimplemented {
    () => { ... };
    ($($arg:tt)+) => { ... };
}

Indicates unimplemented code by panicking with a message of “not implemented”.

This allows your code to type-check, which is useful if you are prototyping or implementing a trait that requires multiple methods which you don’t plan to use all of.

The difference between unimplemented! and todo! is that while todo! conveys an intent of implementing the functionality later and the message is “not yet implemented”, unimplemented! makes no such claims. Its message is “not implemented”.

Also, some IDEs will mark todo!s.

Panics

This will always panic! because unimplemented! is just a shorthand for panic! with a fixed, specific message.

Like panic!, this macro has a second form for displaying custom values.

Examples

Say we have a trait Foo:

trait Foo {
    fn bar(&self) -> u8;
    fn baz(&self);
    fn qux(&self) -> Result<u64, ()>;
}

We want to implement Foo for ‘MyStruct’, but for some reason it only makes sense to implement the bar() function. baz() and qux() will still need to be defined in our implementation of Foo, but we can use unimplemented! in their definitions to allow our code to compile.

We still want to have our program stop running if the unimplemented methods are reached.

struct MyStruct;

impl Foo for MyStruct {
    fn bar(&self) -> u8 {
        1 + 1
    }

    fn baz(&self) {
        // It makes no sense to `baz` a `MyStruct`, so we have no logic here
        // at all.
        // This will display "thread 'main' panicked at 'not implemented'".
        unimplemented!();
    }

    fn qux(&self) -> Result<u64, ()> {
        // We have some logic here,
        // We can add a message to unimplemented! to display our omission.
        // This will display:
        // "thread 'main' panicked at 'not implemented: MyStruct isn't quxable'".
        unimplemented!("MyStruct isn't quxable");
    }
}

fn main() {
    let s = MyStruct;
    s.bar();
}