[swift-evolution] [Pitch] #warning

Harlan Haskins harlan at harlanhaskins.com
Sat May 28 18:58:11 CDT 2016

Hey everyone,

I’m working on a draft for #warning in Swift. I’ve implemented the draft as it stands, and it’s pretty nice to work with.

I’ve pasted it below, and I’d love some feedback! Thanks!

— Harlan Haskins


Proposal: SE-NNNN <https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md>
Author: Harlan Haskins <https://github.com/harlanhaskins>
Status: Awaiting review <https://gist.github.com/harlanhaskins/bfe2d56d7655c5bfdb1855eaf00addb4#rationale>
Review manager: TBD

It's really common for developers to add TODO/FIXME comments in their source code, but there currently isn't a supported facility to make these visible. People have implemented special workarounds <https://bendodson.com/weblog/2014/10/02/showing-todo-as-warning-in-swift-xcode-project/> to coax Xcode into emitting TODOs and FIXMEs as warnings, but there isn't an accessible way to provide arbitrary warnings, and does not work in a non-Xcode environment.


A #warning is for something you intend to fix before submitting your code or for writing future tasks that you or your teammates intend to complete later. Because this is such a common programming pattern, Swift should have a similar facility.

 <https://gist.github.com/harlanhaskins/bfe2d56d7655c5bfdb1855eaf00addb4#proposed-solution>Proposed solution

Add #warning(_:) as a new compiler directive that emits a warning diagnostic with the contents, pointing to the start of the message.

func configPath() -> String {
  #warning("TODO: load this more safely") // expected-warning {{TODO: load this more safely}}
  return Bundle.main().path(forResource: "Config", ofType: "plist")!
 <https://gist.github.com/harlanhaskins/bfe2d56d7655c5bfdb1855eaf00addb4#detailed-design>Detailed design

This will add two new productions to the Swift grammar:

compiler-control-statement → warning-directive
warning-directive → #warning( static-string-literal )
Upon parsing this statement, the Swift compiler will immediately emit a warning and discard the statement.

If a #warning exists inside a branch of a #if statement that is not taken, then no warning is emitted.

#if false
#warning(“This won’t exist”)
 <https://gist.github.com/harlanhaskins/bfe2d56d7655c5bfdb1855eaf00addb4#impact-on-existing-code>Impact on existing code

This change is purely additive; no migration will be required.

 <https://gist.github.com/harlanhaskins/bfe2d56d7655c5bfdb1855eaf00addb4#alternatives-considered>Alternatives considered

We could do some kind of comment-parsing based approach to surface TODOs and FIXMEs, but #warning serves as a general-purpose facility for reporting at compile time. Plus, not all TODO or FIXME comments should surface as warnings in the source.
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