[tialaramex]

Some languages (Rust and Swift for example) have expressions which are placeholders but won't actually work.

  let x = 5 + todo!("Complicated arithmetic expression");

... is valid Rust, it typechecks, your syntax highlighter should be fine with it, it'll compile, but if that line ever executes, you panic immediately.

[layer8]

Personally I think execution time is too late.

[LegibleCrimson]

I disagree. In Rust it's extremely useful to be able to tell if your types are correct and the code will compile with the signatures given, and with multiple placeholders, you can iteratively replace them and test them progressively.

I use them heavily during development. Most of my new functions in progress end with an unimplemented!

I'm not sure what advantage at all could be gained by preventing it from compiling, other than to save people who copy code without even looking at it some minor inconvenience.

Edit: I suppose if you really wanted an existing macro to cause compilation to fail, you could use https://doc.rust-lang.org/std/macro.compile_error.html

But I think that's a strange use of it.