Commits on Oct 24, 2020

1. Add basic incompatible target skipping support …

   This patch aims to implement a basic version of incompatible target
   skipping outlined here:
   https://docs.google.com/document/d/12n5QNHmFSkuh5yAbdEex64ot4hRgR-moL1zRimU7wHQ/edit?usp=sharing
   
   The implementation in this patch supports target skipping based
   on the target platform. In a `BUILD` file you can now add constraints
   that the target platform must satisfy in order for the target to be
   built and/or tested. For example, use the following snippet to declare
   a target to be compatible with Windows platforms only:
   
     cc_binary(
         name = "bin",
         srcs = ["bin.cc"],
         target_compatible_with = [
             "@platforms//os:windows",
         ],
     )
   
   Builds triggered with `:all` or `...` on a non-Windows platform will
   simply skip the incompatible target. An appropriate note is shown on
   the command line if the `--show_result` threshold is high enough.
   Targets that transitively depend on incompatible targets are
   themselves considered incompatible and will also be skipped.
   
   Explicitly requesting an incompatible target on the command line is an
   error and will cause the build to fail. Bazel will print out an
   appropriate error message and inform the user what constraint could
   not be satisfied.
   
   See the new documentation in this patch for more information. In
   particular, https://docs.bazel.build/versions/master/platforms.html
   should be a good bit more informative.
   
   This implementation does not make any effort to support expressing
   compatibility with toolchains. It is possible that using `select()`
   (commented on below) already makes this possible, but it's not
   validated or explicitly supported in this patch.
   
   During implementation we noticed that `select()` can be quite powerful
   in combination with `target_compatible_with`. A basic summary of this
   is also documented on the Platforms page.
   
   It may be useful to create helper functions in, say, skylib to help
   make complex `select()` statements more readable. For example, we
   could replace the following:
   
     target_compatible_with = select({
         "@platforms//os:linux": [],
         "@platforms//os:macos": [],
         "//conditions:default": [":not_compatible"],
     })
   
   with something like:
   
     target_compatible_with = constraints.any_of([
         "@platforms//os:linux",
         "@platforms//os:macos",
     ])
   
   That, however, is work for follow-up patches.
   
   Many thanks to Austin Schuh (@AustinSchuh) and Greg Estren
   (@gregestren) for working on the proposal and helping a ton on this
   patch itself. Also thanks to many others who provided feedback on the
   implementation.
   
   RELNOTES: Bazel skips incompatible targets based on target platform
   and `target_compatible_with` contents. See
   https://docs.bazel.build/versions/master/platforms.html for more
   details.

   

   philsc authored and AustinSchuh committed Oct 24, 2020
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