[swift-evolution] [Pitch] #warning

[Harlan Haskins]

I can see where you're coming from, but I don't necessarily see this as "warnings which can then be ignored." In my vision, this enables developers to increase visibility of known issues, instead of letting them sit. Developers who would normally use this feature to add their own warnings are likely not the developers who will then turn around and ignore warnings.

- Harlan

> On May 28, 2016, at 8:26 PM, Jon Shier <jon at jonshier.com> wrote:
> 
> 	I appreciate the sentiment, but I think we should reserve warnings for actual compiler diagnostics. If you wish to add another, user definable diagnostic type, like official compiler support for a #todo or #fixme flag, then go ahead. But adding #warning because the IDE (read Xcode) doesn’t automatically pick up FIXMEs or TODOs isn’t a compelling enough reason. In addition, enabling developers to add their own warnings which can then just be ignored just encourages the very bad habit of ignoring warnings in general. Perhaps this could be something for libIDE, rather than the core language?
> 
> 
> 
> Jon Shier
> 
>> On May 28, 2016, at 7:58 PM, Harlan Haskins via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 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
>> 
>> 
>> 
>> #warning
>> Proposal: SE-NNNN
>> Author: Harlan Haskins
>> Status: Awaiting review
>> Review manager: TBD
>> Introduction
>> 
>> 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 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.
>> 
>> Motivation
>> 
>> 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.
>> 
>> 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")!
>> }
>> 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”)
>> #endif
>> Impact on existing code
>> 
>> This change is purely additive; no migration will be required.
>> 
>> 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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> 
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