Indicates unfinished code.

This can be useful if you are prototyping and just want a placeholder to let your code pass type analysis.

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 todo! is just a shorthand for panic! with a fixed, specific message.

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

Examples

Here's an example of some in-progress code. We have a trait Foo:

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

We want to implement Foo on one of our types, but we also want to work on just bar() first. In order for our code to compile, we need to implement baz() and qux(), so we can use todo!:

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

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

    fn baz(&self) {
        // Let's not worry about implementing baz() for now
        todo!();
    }

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

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

    // We aren't even using baz() or qux(), so this is fine.
}