todo in core - Rust
Description: Indicates unfinished code.

# Macro todo

1.40.0 · Source

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

Expand description

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!`:

    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.
    }