[swift-evolution] [draft] Target environment platform condition

[Graydon Hoare]

> On Oct 27, 2017, at 5:06 AM, Karl Wagner via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Yeah, but by extension, every since compile-time flag is also an “axis of configuration”. What makes these few worthy of special language integration and compiler support?

The "environment" field is an optional 4th component of the (not-quite-accurately-named) target-triple. This is not new, it's a thing lots of toolchains have some more-or-less standard support for (different toolchains use it for different purposes: simulator-ness, ABI, libc variation, etc.)

Swift's grammar for compilation conditions[1] provides three different types of leaf node:

  - Boolean literals
  - Identifiers, representing user-provided ("-D") command-line flags
  - Platform conditions, representing language version and 3 existing target-triple components

There's currently no way to write a compilation condition that depends on the 4th field (environment) of the target-triple. This proposal is to surface that 4th field in the same place the other 3 fields are surfaces: as a platform condition.

Conflated with this, the conversation in this thread seems to have ventured into asking -- independently of the question of how to express the environment field as a compilation condition -- whether it's right to express simulator-ness as a target-triple component at all, rather than as a preprocessor symbol. This ship has, however, already sailed: a patch expressing this choice entered LLVM a week ago[2], on the justification that the simulators are literally separate SDKs: from the perspective of the compiler, they're about as different as targeting different OSs.

-Graydon

[1] https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/compilation-condition <https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Statements.html#//apple_ref/swift/grammar/compilation-condition>
[2] https://reviews.llvm.org/D39143 <https://reviews.llvm.org/D39143>

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