incompatible_override_toolchain_transition: Migrate rules to use the toolchain transition

[katre]

This flag unconditionally makes all rule uses the new toolchain transition. This means that any rule that uses toolchain resolution to fetch tools will need to migrate so that tools are in the correct configuration.

Flag: --incompatible_override_toolchain_transition

Migration:

First, wait for the new feature to be released (presumably in the release after 3.3.0).

See toolchain transition migration for details, but the process is fairly simple:

1. For rules that create toolchains (i.e., cc_toolchain, go_toolchain, java_toolchain), add the proper cfg attribute.
   1. If the attribute is a tool used in builds (a compiler, a linker, anything that is executed), add cfg = 'exec' so that it will be built for the correct execution platform.
   2. If the attribute is intended to be part of the final target (a library, included source, anything else), add cfg = 'target', so that it will be built in the correct target configuration.
   3. At this point, there will be no change, because toolchains still use the host transition, and that overrides anything set on the toolchain rule itself.
2. For each rule that uses a toolchain (i.e., cc_library, go_binary, java_test), add the incompatible_use_toolchain_transition = True parameter to the rule definition in Starlark.
   1. This enables the toolchain transition, instead of the host transition, when this rule type depends on a toolchain.
   2. Every toolchain type this rule uses should have been migrated in step 1 above.
3. Once every rule has been migrated, wait for Bazel to flip the value of the --incompatible_override_toolchain_transition flag. This will take several releases for all rules to finish steps 1. and 2.
4. Once the flag has flipped, remove the incompatible_use_toolchain_transition parameter from the rule declaration.
5. Bazel will eventually deprecate the flag and rule parameter and remove them.

[konste]

@katre I have a question: we use custom C++ and Python toolchains, but to implement those toolchains we use native cc_toolchain rule and toolchain rule, like that:

py_runtime_pair(
    name = "tab_python_runtime_pair",
    py3_runtime = "@tab_toolchains//thirdparty/python:tab_python_runtime",
)

toolchain(
    name = "tab_python_toolchain",
    toolchain = ":tab_python_runtime_pair",
    toolchain_type = "@bazel_tools//tools/python:toolchain_type",
)

As far as I understand toolchain transition migration is to be done by the maintainers of the cc_toolchain rule and toolchain rule, right? The changes needed are in the implementation of those native rules, not at the place where they are used. Correct?

Specific reason I need to clarify that is because recently bazel cquery started to give us warnings like this:

WARNING: Targets were missing from graph: [ConfiguredTargetKey{label=@tab_msvc_toolchain_repo_64//:tab_msvc_cc_toolchain_64, config=BuildConfigurationValue.Key[5abb50d874a0ab83869e8c0d685014657df6f34d02bb0ebbb59b2108192fcfff]}]
WARNING: Targets were missing from graph: [ConfiguredTargetKey{label=@tab_toolchains//bazel/toolchains/python:tab_python_runtime_pair, config=BuildConfigurationValue.Key[5abb50d874a0ab83869e8c0d685014657df6f34d02bb0ebbb59b2108192fcfff]}]

I cannot make sense of those warnings, but both of them point to the rule directly related to the C++ and Python toolchain. This made me think it may be relevant to toolchain transition migration.
Any advice?

[katre]

Both python rules and CC rules need to be migrated, this will be separate efforts. I'm not sure where you are using the cc toolchain: in your example, the py_runtime_pair target creates a specific python toolchain and the toolchain target allows Bazel to know it exists. I don't see an example of a cc_toolchain here. Note that toolchain and cc_toolchain are very different: cc_toolchain creates a specific type of toolchain that CC rules can use to build C and C++ files, whereas toolchain is a core configuration rule used to tell Bazel about specific toolchains that can be used.

I'm not sure what's happening with cquery: you will need to file a separate issue about that so we can route it to the appropriate experts.

[konste]

I see. I was just making sure that there is nothing for us to do if we only use cc_toolchain, but not implement it.

[katre]

There is nothing needed for users of rules.