commit 22b4dbcaf05756d506de346728db3846da56b775
author Philipp Schrader <philipp.schrader@gmail.com> Tue Oct 27 07:17:41 2020 -0700
committer Copybara-Service <copybara-worker@google.com> Tue Oct 27 07:18:44 2020 -0700
tree 0117207eb12f17749bd88663ba3d56ee53a73ffd
parent 8dbcc8f3ac6f096c8c6ffb9e8edc65e85c639806

Add basic incompatible target skipping

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.

Closes #10945.

PiperOrigin-RevId: 339243111