incompatible_enable_cc_toolchain_resolution: Turn on toolchain resolution for cc rules

[katre]

Flag: --incompatible_enable_cc_toolchain_resolution
Available since: 0.23
Will be flipped in: 7.0.0
Tracking issue: #6516

Current status

C++ rules are to our best knowledge ready to be migrated to platforms. Fat APK binary of Android rules and Multiarch binary for Apple rules are still using the legacy C++ toolchain selection. We expect those to be fixed in Bazel 0.26.

Motivation
Currently, cc rules access the cc_toolchain by a combination of the --crosstool_top, --cpu, and --compiler flags. This system is complex, lacks flexibility, and will be removed in favor of using the toolchain resolution system added to Bazel.

When this flag is enabled, the legacy system for accessing the cc toolchain will be disabled, and the cc rules will instead use the toolchain resolution system. Eventually, the legacy code will be removed.

Migration

If you're not using C++, or custom --crosstool_top, --cpu, or --compiler,
you don't use select on these options, you can stop reading now, there is
nothing to migrate for you.

Build configured by the current C++ toolchain

You will eventually have to migrate selects and conditional logic in Starlark
rules to toolchain/platforms. There is no generally applicable migration tool.

But to make incremental migration possible, support for PLATFORM_MAPPING file
was added to Bazel. See #6426 for up
to date infomation and documentation.

C++ Toolchain owners

For users who are using --crosstool_top to define custom cc_toolchain
targets, it will be necessary to add the corresponding toolchain targets (see
the very rudimentary example in Bazel's autoconfigured toolchain), and to
register those toolchains (Bazel's autoconfigured toolchain example).

For users who are cross-compiling by using --cpu, it will be necessary to add
appropriate platform targets and to use the --platforms flag to ensure targets
are built correctly. Until the --cpuflag can be fully deprecated or a migration
path can be in place, it will be necessary to set both --cpu and --platforms.

Please use constraint_settings and constraint_values from
the canonical Bazel Platforms
Repository (don't hesitate to propose
missing targets!). If there is a need for C++ specific constraints, feel free to
upload a PR to the rules_cc
repository. It is extremely important that the whole ecosystem uses the same
constraints, we can only reuse libraries and toolchains when we speak the same
language.

Starlark rules owners

toolchain_type

For Starlark rules owners who depend on C++ toolchain it will be necessary to
declare dependency on C++ toolchain type.

Before:

foo = rule(
    implementation = _foo_impl,
    attrs = {
        "_cc_toolchain": attr.label(default = Label("@bazel_tools//tools/cpp:current_cc_toolchain")),
    },
)

After:

foo = rule(
    implementation = _foo_impl,
    attrs = {
        "_cc_toolchain": attr.label(default = Label("@bazel_tools//tools/cpp:current_cc_toolchain")),
    },
    toolchains = ["@bazel_tools//tools/cpp:toolchain_type"],
  )

In case you're already using Bazel 0.27, save yourself a migration and depend
on rules_cc:

    toolchains = ["@rules_cc//cc:toolchain_type"],

find_cpp_toolchain

See the docs and use @rules_cc//cc:find_cc_toolchain.bzl
(if using Bazel >= 0.27) or @bazel_tools//tools/cpp:toolchain_utils.bzl to locate current C++ toolchain (otherwise). Also see examples for general usage.

[katre]

Note that this flag will supercede and replace the previous --enabled_toolchain_types flag, which was introduced before the incompatible change policy was formulated, and which was only ever used for the cc rules, anyway.

[katre]

People who are interested: @hlopko, @lberki, @nlopezgi.

[gregestren]

Will native transitions like

bazel/src/main/java/com/google/devtools/build/lib/rules/android/AndroidRuleClasses.java

Line 252 in 4b00ab1

splitOptions.get(BuildConfiguration.Options.class).cpu = androidOptions.cpu;

also need attention?

[katre]

Yes, @aragos and I have been discussing a plan. We will probably not be able to flip this flag until we have a solution, so I am removing the "Will be flipped in" tag.

[dslomov]

Please do not assign issues to more than one team

[aaron-michaux]

It looks like target_settings works in certain contexts and not others. Modifying the above example:

This just doesn't work! It doesn't matter what flavor is set to, you always get the "gcc" toolchain.

But if I use "compiler" in my config_settings, then it works.

!@#$%

string_flag(
    name = "flavor",
    build_setting_default = "gcc",
    values = [
        "gcc",
        "clang",
    ],
)

config_setting(
    name = "use_gcc",
    flag_values = {":flavor": "gcc"},
)

config_setting(
    name = "use_clang",
    flag_values = {":flavor": "clang"},
)

toolchain(
    name = "clang_toolchain",
    exec_compatible_with = ...,
    target_compatible_with = ....,
    target_settings = [":use_clang"],
    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
    toolchain = ":actual_clang_cc_toolchain",
)

toolchain(
    name = "clang_toolchain",
    exec_compatible_with = ...,
    target_compatible_with = ....,
    target_settings = [":use_gcc"],
    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
    toolchain = ":actual_gcc_cc_toolchain",
)

[keertk]

For tracking:
cc @meteorcloudy

Failures: https://buildkite.com/bazel/bazelisk-plus-incompatible-flags/builds/1339

To fix:

• Bazel Examples: Flag --incompatible_enable_cc_toolchain_resolution will break Bazel Examples in Bazel 7.0 examples#271
• Google Logging: Flag --incompatible_enable_cc_toolchain_resolution will break Google Logging in Bazel 7.0 google/glog#876
• rules_android_ndk: Support cc toolchain resolution rules_android_ndk#21
• rules_apple: Flag --incompatible_enable_cc_toolchain_resolution will break rules_apple in Bazel 7.0 rules_apple#1765
• rules_foreign_cc: Flag --incompatible_enable_cc_toolchain_resolution will break rules_foreign_cc in Bazel 7.0 rules_foreign_cc#984 (blocked by rules_android_ndk)
• rules_swift: Flag --incompatible_enable_cc_toolchain_resolution will break rules_swift in Bazel 7.0 rules_swift#991

[jsharpe]

The rules_foreign_cc breakage is due to the rules_android_ndk breakage as it is just one of the examples that use rules_android_ndk that is broken.

[keertk]

Hi all, confirming if we still plan to flip this flag in 7.0?