What?

Tells you what type things are.

This crate provides the what! macro. This is functionally similar to the todo! macro, except that it also tells you type information.

fn hello() -> Result<(), Box<dyn Error>> {
    what!()
}

Just like todo!, what! passes all type-checks and makes it easy to write/build/test unfinished code. If it ever ends up in a compiled program, attempted to execute a what! will panic.

The fun part happens when you run cargo what.

$ cargo what
hole: expecting `std::result::Result<(), Box<dyn std::error::Error>>`
 --> src/hello.rs
  |
2 |     what!()
  |     ^^^^^^^

Unfortunately, custom diagnostics aren't really available to Rust libraries, requiring the extra command. cargo what can be installed with cargo:

$ cargo install cargo-what

cargo what wraps cargo build to show the type-info of any what!s you have in your code.

what! also accepts arguments and shows their types, which can be useful for reducing the "unused variable" noise.

fn hello(a: usize, b: usize) -> usize {
    let c = a..b;
    what!(a, b, c)
}

And with cargo what:

$ cargo what
hole: expecting `usize`
 --> src/hello.rs
  |
3 |     what!(a, b, c)
  |     ^^^^^^^^^^^^^^
  |
  = note: a is `usize`
  = note: b is `usize`
  = note: c is `std::ops::Range<usize>`

Emacs keybindings left as an exercise to the reader.