se-0000-pound-warning.md

#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 will not trigger a warning."
#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.

#error and #message, though discussed on the mailing list, are intentially left out of this proposal.

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Rationale

On [Date], the core team decided to (TBD) this proposal. When the core team makes a decision regarding this proposal, their rationale for the decision will be written here.