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使用集合让一切井井有条 根据您的偏好保存内容并对其进行分类。

bar_binary = rule(
    implementation = _bar_binary_impl,
    attrs = {
        "srcs": attr.label_list(allow_files = True),
        ...
        "_compiler": attr.label(
            default = "//bar_tools:barc_linux",  # the compiler running on linux
            providers = [BarcInfo],
        ),
    },
)

BarcInfo = provider(
    doc = "Information about how to invoke the barc compiler.",
    # In the real world, compiler_path and system_lib might hold File objects,
    # but for simplicity they are strings for this example. arch_flags is a list
    # of strings.
    fields = ["compiler_path", "system_lib", "arch_flags"],
)

def _bar_binary_impl(ctx):
    ...
    info = ctx.attr._compiler[BarcInfo]
    command = "%s -l %s %s" % (
        info.compiler_path,
        info.system_lib,
        " ".join(info.arch_flags),
    )
    ...

bar_binary(
    name = "myprog_on_linux",
    srcs = ["mysrc.bar"],
    compiler = "//bar_tools:barc_linux",
)

bar_binary(
    name = "myprog_on_windows",
    srcs = ["mysrc.bar"],
    compiler = "//bar_tools:barc_windows",
)

config_setting(
    name = "on_linux",
    constraint_values = [
        "@platforms//os:linux",
    ],
)

config_setting(
    name = "on_windows",
    constraint_values = [
        "@platforms//os:windows",
    ],
)

bar_binary(
    name = "myprog",
    srcs = ["mysrc.bar"],
    compiler = select({
        ":on_linux": "//bar_tools:barc_linux",
        ":on_windows": "//bar_tools:barc_windows",
    }),
)

# By convention, toolchain_type targets are named "toolchain_type" and
# distinguished by their package path. So the full path for this would be
# //bar_tools:toolchain_type.
toolchain_type(name = "toolchain_type")

bar_binary = rule(
    implementation = _bar_binary_impl,
    attrs = {
        "srcs": attr.label_list(allow_files = True),
        ...
        # No `_compiler` attribute anymore.
    },
    toolchains = ["//bar_tools:toolchain_type"],
)

def _bar_binary_impl(ctx):
    ...
    info = ctx.toolchains["//bar_tools:toolchain_type"].barcinfo
    # The rest is unchanged.
    command = "%s -l %s %s" % (
        info.compiler_path,
        info.system_lib,
        " ".join(info.arch_flags),
    )
    ...

bar_binary = rule(
    ...
    toolchains = [
        config_common.toolchain_type("//bar_tools:toolchain_type", mandatory = False),
    ],
)

bar_binary = rule(
    ...
    toolchains = [
        "//foo_tools:toolchain_type",
        config_common.toolchain_type("//bar_tools:toolchain_type", mandatory = False),
    ],
)

bar_aspect = aspect(
    implementation = _bar_aspect_impl,
    attrs = {},
    toolchains = ['//bar_tools:toolchain_type'],
)

def _bar_aspect_impl(target, ctx):
  toolchain = ctx.toolchains['//bar_tools:toolchain_type']
  # Use the toolchain provider like in a rule.
  return []

def _bar_toolchain_impl(ctx):
    toolchain_info = platform_common.ToolchainInfo(
        barcinfo = BarcInfo(
            compiler_path = ctx.attr.compiler_path,
            system_lib = ctx.attr.system_lib,
            arch_flags = ctx.attr.arch_flags,
        ),
    )
    return [toolchain_info]

bar_toolchain = rule(
    implementation = _bar_toolchain_impl,
    attrs = {
        "compiler_path": attr.string(),
        "system_lib": attr.string(),
        "arch_flags": attr.string_list(),
    },
)

bar_toolchain(
    name = "barc_linux",
    arch_flags = [
        "--arch=Linux",
        "--debug_everything",
    ],
    compiler_path = "/path/to/barc/on/linux",
    system_lib = "/usr/lib/libbarc.so",
)

bar_toolchain(
    name = "barc_windows",
    arch_flags = [
        "--arch=Windows",
        # Different flags, no debug support on windows.
    ],
    compiler_path = "C:\\path\\on\\windows\\barc.exe",
    system_lib = "C:\\path\\on\\windows\\barclib.dll",
)

toolchain(
    name = "barc_linux_toolchain",
    exec_compatible_with = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
    target_compatible_with = [
        "@platforms//os:linux",
        "@platforms//cpu:x86_64",
    ],
    toolchain = ":barc_linux",
    toolchain_type = ":toolchain_type",
)

toolchain(
    name = "barc_windows_toolchain",
    exec_compatible_with = [
        "@platforms//os:windows",
        "@platforms//cpu:x86_64",
    ],
    target_compatible_with = [
        "@platforms//os:windows",
        "@platforms//cpu:x86_64",
    ],
    toolchain = ":barc_windows",
    toolchain_type = ":toolchain_type",
)

def _bar_toolchain_impl(ctx):
    # The implementation is mostly the same as above, so skipping.
    pass

bar_toolchain = rule(
    implementation = _bar_toolchain_impl,
    attrs = {
        "compiler": attr.label(
            executable = True,
            mandatory = True,
            cfg = "exec",
        ),
        "system_lib": attr.label(
            mandatory = True,
            cfg = "target",
        ),
        "arch_flags": attr.string_list(),
    },
)

register_toolchains(
    "//bar_tools:barc_linux_toolchain",
    "//bar_tools:barc_windows_toolchain",
    # Target patterns are also permitted, so you could have also written:
    # "//bar_tools:all",
    # or even
    # "//bar_tools/...",
)

# my_pkg/BUILD

platform(
    name = "my_target_platform",
    constraint_values = [
        "@platforms//os:linux",
    ],
)

bar_binary(
    name = "my_bar_binary",
    ...
)

bazel build //my_pkg:my_bar_binary --platforms=//my_pkg:my_target_platform

# Find all direct dependencies of //cc:my_cc_lib. This includes explicitly
# declared dependencies, implicit dependencies, and toolchain dependencies.
$ bazel cquery 'deps(//cc:my_cc_lib, 1)'
//cc:my_cc_lib (96d6638)
@bazel_tools//tools/cpp:toolchain (96d6638)
@bazel_tools//tools/def_parser:def_parser (HOST)
//cc:my_cc_dep (96d6638)
@local_config_platform//:host (96d6638)
@bazel_tools//tools/cpp:toolchain_type (96d6638)
//:default_host_platform (96d6638)
@local_config_cc//:cc-compiler-k8 (HOST)
//cc:my_cc_lib.cc (null)
@bazel_tools//tools/cpp:grep-includes (HOST)

# Which of these are from toolchain resolution?
$ bazel cquery 'deps(//cc:my_cc_lib, 1)' --transitions=lite | grep "toolchain dependency"
  [toolchain dependency]#@local_config_cc//:cc-compiler-k8#HostTransition -> b6df211