SE-0196 — Compiler Diagnostic Directives

[AlexanderM (Alexander Momchilov)]

Alexander Momchilov brought up the idea of using TODO and warning as functions in the standard library with special compiler magic that will warn on their uses.
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While these could be useful, I think #warning and #errorhave uses beyond just marking unfinished code that would be unwieldy or impossible with just an expression-oriented approach.

My proposal for TODO was in addition to #warning and #error. Warnings, errors, todos and fixmes are all widely adopted in the community (the Swift community, and the broader programming community), and I think that we should have first class support for them. This could allow programmers to customize how to handle todos and fixmes (e.g. ignore todos, make fixmes warnings. Or warn on todos, error on fixmes, etc.), in the same way we can get our compiler to ignore warnings, emit warnings, or promote warnings to errors.

From the proposal:

Erik Little refined that to instead use special directives warning and error in expression position, like:

let somethingSuspect = #warning("This is really the wrong function to call, but I'm being lazy", suspectFunction())

However, I think there's not much of a benefit to this syntax vs. just adding a warning above the line:

#warning "This is really the wrong function to call, but I'm being lazy"
let somethingSuspect = suspectFunction()

The first bit of code guarantees that the warning/error/todo/fix me is coupled to the default value. You can't remove the warning and forget to change the value and you can't can't the value and forget to change the warning. @anandabits Put it well:

@anandabits

However, the current proposal sacrifices one of the benefits of the custom TODO function approach that I really appreciate. The custom TODO function is an expression that can be used as a placeholder for a value of any type. This is sometimes very useful. It allows code to compile (and even run) before it is complete. This provides a very flexible workflow for writing code and interacting with the type checker. That can be especially useful when writing very generic code.