Switch everything to platforms

This patch switches the codebase over from using the "cpu"
mechanism to using bazel platforms. See
https://docs.bazel.build/versions/master/platforms.html for some more
information.

Most of the substantial changes are in //tools. Instead of using
`cc_toolchain_suite` rules, we now use regular `toolchain` rules that
are registered in the WORKSPACE. That also means that bazel now uses
the target platform to select the compiler.

All --cpu=* arguments should now be --config=* arguments. For example,
`--cpu=roborio` should now be `--config=roborio`. The CI script and
all documentation has been updated to reflect that.

The remainder of the changes revolve around tagging all targets with
`target_compatible_with`.  The old mechanism allowed us to specify
repo-wide defaults. The new mechanism does not. That means every
target that didn't have any compatibility specified, now requires
compatibility with `@platforms//os:linux`.

I used buildozer for the vast majority of `target_compatible_with`
changes. buildozer automatically buildifies any BUILD files it
touches. That means this patch also contains a few non-functional
changes that I was too lazy to remove.

Change-Id: I66d6e6ad9161520ee397597cdb492585820a3acd
diff --git a/tools/BUILD b/tools/BUILD
index 50c9141..3acd443 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -7,44 +7,47 @@
     name = "compiler_clang",
     flag_values = {
         "@bazel_tools//tools/cpp:compiler": "clang",
-    }
+    },
 )
 
 config_setting(
     name = "compiler_gcc",
     flag_values = {
         "@bazel_tools//tools/cpp:compiler": "gcc",
-    }
+    },
 )
 
+# TODO(phil): Get rid of these and just use the constraint values directly
+# everywhere.
 config_setting(
     name = "cpu_k8",
-    values = {"cpu": "k8"},
+    constraint_values = ["@platforms//cpu:x86_64"],
 )
 
 config_setting(
     name = "cpu_roborio",
-    values = {"cpu": "roborio"},
+    constraint_values = ["@//tools/platforms/hardware:roborio"],
 )
 
 config_setting(
     name = "cpu_cortex_m4f",
-    values = {"cpu": "cortex-m4f"},
+    constraint_values = ["@//tools/platforms/hardware:cortex_m4f"],
 )
 
-config_setting(
-    name = "cpu_cortex_m4f_k22",
-    values = {"cpu": "cortex-m4f-k22"},
-)
+# TODO(phil): Re-enable this with proper support.
+#config_setting(
+#    name = "cpu_cortex_m4f_k22",
+#    constraint_values = ["@platforms//cpu:?"],
+#)
 
 config_setting(
     name = "cpu_armhf",
-    values = {"cpu": "armhf-debian"},
+    constraint_values = ["@//tools/platforms/hardware:raspberry_pi"],
 )
 
 config_setting(
     name = "cpu_aarch64",
-    values = {"cpu": "aarch64"},
+    constraint_values = ["@platforms//cpu:arm64"],
 )
 
 config_setting(
@@ -61,33 +64,3 @@
     name = "has_ubsan",
     values = {"define": "have_ubsan=true"},
 )
-
-environment(name = "k8")
-
-environment(name = "armhf-debian")
-
-environment(name = "roborio")
-
-environment(name = "cortex-m4f")
-
-environment(name = "cortex-m4f-k22")
-
-environment(name = "aarch64")
-
-environment_group(
-    name = "cpus",
-    defaults = [
-        ":k8",
-        ":roborio",
-        ":aarch64",
-        ":armhf-debian",
-    ],
-    environments = [
-        ":k8",
-        ":roborio",
-        ":armhf-debian",
-        ":cortex-m4f",
-        ":cortex-m4f-k22",
-        ":aarch64",
-    ],
-)
diff --git a/tools/actions/BUILD b/tools/actions/BUILD
index 46c9541..7c69bfb 100644
--- a/tools/actions/BUILD
+++ b/tools/actions/BUILD
@@ -3,6 +3,7 @@
     srcs = [
         "generate_compile_command.py",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     deps = [
         "//third_party/bazel:extra_actions_proto_py",
     ],
@@ -14,6 +15,7 @@
     mnemonics = [
         "CppCompile",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )
 
@@ -24,6 +26,7 @@
     out_templates = [
         "$(ACTION_ID)_compile_command",
     ],
+    target_compatible_with = ["@platforms//os:linux"],
     tools = [
         ":generate_compile_command",
     ],
diff --git a/tools/build_rules/BUILD b/tools/build_rules/BUILD
index 8ad7018..cf27ca0 100644
--- a/tools/build_rules/BUILD
+++ b/tools/build_rules/BUILD
@@ -1,12 +1,14 @@
 sh_binary(
     name = "quiet_success",
     srcs = ["quiet_success.sh"],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
 )
 
 py_binary(
     name = "jinja2_generator",
     srcs = ["jinja2_generator.py"],
+    target_compatible_with = ["@platforms//os:linux"],
     visibility = ["//visibility:public"],
     deps = ["@python_jinja2"],
 )
diff --git a/tools/build_rules/fortran.bzl b/tools/build_rules/fortran.bzl
index 9e6e789..d55f1c1 100644
--- a/tools/build_rules/fortran.bzl
+++ b/tools/build_rules/fortran.bzl
@@ -1,109 +1,5 @@
 load("@//tools/build_rules:select.bzl", "compiler_select")
 
-def _single_fortran_object_impl(ctx):
-    toolchain_cflags = (ctx.fragments.cpp.compiler_options([]) +
-                        ctx.fragments.cpp.c_options +
-                        ctx.fragments.cpp.unfiltered_compiler_options([]) +
-                        [
-                            "-fPIC",
-                            "-Wno-maybe-uninitialized",
-                            "-Wno-unused-dummy-argument",
-                            "-Wno-conversion",
-                            "-Wno-unused-variable",
-                            "-Wno-character-truncation",
-                        ])
-
-    cmd = toolchain_cflags + ["-c", ctx.file.src.path, "-o", ctx.outputs.pic_o.path]
-    filtered_cmd = []
-
-    # Strip out the C/C++/Clang specific flags.
-    exclude_flags = [
-        "-fcolor-diagnostics",
-        "-Wswitch-enum",
-        "-Wpointer-arith",
-        "-Wcast-qual",
-        "-Wwrite-strings",
-        "-Wsign-compare",
-        "-Wformat=2",
-        "-Werror",
-        "-Wextra",
-        "-Wno-builtin-macro-redefined",
-        "-Wunused-local-typedefs",
-        "-D__has_feature(x)=0",
-        "-fmacro-backtrace-limit=0",
-    ]
-
-    for flag in cmd:
-        if flag not in exclude_flags and not (flag.startswith("-fsanitize") or
-                                              flag.startswith("-fno-sanitize")):
-            filtered_cmd.append(flag)
-
-    ctx.action(
-        inputs = [ctx.file.src] + ctx.files._cc_toolchain,
-        outputs = [ctx.outputs.pic_o],
-        mnemonic = "Fortran",
-        executable = ctx.fragments.cpp.compiler_executable,
-        arguments = filtered_cmd,
-        progress_message = "Building %s" % ctx.outputs.pic_o.short_path,
-    )
-
-def _define_fortran_output(src):
-    if not src.name.endswith(".f"):
-        fail("Fortran files must end in '.f'", "src")
-
-    fortran_file_base = src.name[:-2]
-    return {
-        "pic_o": fortran_file_base + ".pic.o",
-    }
-
-_single_fortran_object = rule(
-    attrs = {
-        "src": attr.label(
-            allow_single_file = [".f"],
-        ),
-        "cc_libs": attr.label_list(providers = ["cc"]),
-        # TODO(Brian): Replace this with something more fine-grained from the
-        # configuration fragment or something.
-        "_cc_toolchain": attr.label(
-            default = Label("@//tools/cpp:toolchain"),
-        ),
-    },
-    fragments = [
-        "cpp",
-    ],
-    outputs = _define_fortran_output,
-    implementation = _single_fortran_object_impl,
-)
-
-def fortran_library(name, srcs, deps = [], visibility = None):
-    """Builds a shared library from a set of fortran files.
-
-    Args:
-      srcs: list of fortran files ending in .f
-      deps: cc_library or fortran_library dependencies.
-    """
-    pic_o_files = []
-    for src in srcs:
-        pic_o_file = src[:-2] + ".pic.o"
-        _single_fortran_object(
-            name = name + "_" + pic_o_file,
-            src = src,
-            visibility = ["//visibility:private"],
-            restricted_to = ["@//tools:k8"],
-        )
-        pic_o_files.append(pic_o_file)
-
-    native.cc_library(
-        name = name,
-        deps = deps,
-        srcs = pic_o_files,
-        linkopts = [
-            "-lgfortran",
-        ],
-        visibility = visibility,
-        restricted_to = ["@//tools:k8"],
-    )
-
 f2c_copts = compiler_select({
     "clang": [
         "-Wno-incompatible-pointer-types-discards-qualifiers",
diff --git a/tools/build_rules/pandoc.bzl b/tools/build_rules/pandoc.bzl
index bc63bac..ad47ccc 100644
--- a/tools/build_rules/pandoc.bzl
+++ b/tools/build_rules/pandoc.bzl
@@ -6,7 +6,7 @@
   src: Markdown file to convert. Only one file can be specified.
 """
 
-def pandoc_html(name, src):
+def pandoc_html(name, src, target_compatible_with = None):
     output = name + ".html"
     native.genrule(
         name = name,
@@ -15,4 +15,5 @@
         cmd = "$(location @pandoc//:pandoc_wrapper) -s $< -o $@",
         tools = ["@pandoc//:all_files", "@pandoc//:pandoc_wrapper"],
         executable = True,
+        target_compatible_with = target_compatible_with,
     )
diff --git a/tools/build_rules/select.bzl b/tools/build_rules/select.bzl
index 1c48eb7..23addf5 100644
--- a/tools/build_rules/select.bzl
+++ b/tools/build_rules/select.bzl
@@ -46,7 +46,8 @@
         "@//tools:cpu_roborio": values["roborio"],
         "@//tools:cpu_armhf": values["armhf"],
         "@//tools:cpu_cortex_m4f": values["cortex-m"],
-        "@//tools:cpu_cortex_m4f_k22": values["cortex-m"],
+        # TODO(phil): Support this properly.
+        #"@//tools:cpu_cortex_m4f_k22": values["cortex-m"],
     })
 
 """A select wrapper for address space sizes.
@@ -66,7 +67,8 @@
         "@//tools:cpu_roborio": values["32"],
         "@//tools:cpu_armhf": values["32"],
         "@//tools:cpu_cortex_m4f": values["32"],
-        "@//tools:cpu_cortex_m4f_k22": values["32"],
+        # TODO(phil): Support this properly.
+        #"@//tools:cpu_cortex_m4f_k22": values["32"],
     })
 
 """A select wrapper for compilers.
diff --git a/tools/ci/run-tests.sh b/tools/ci/run-tests.sh
index fd92ea3..ef4cff8 100755
--- a/tools/ci/run-tests.sh
+++ b/tools/ci/run-tests.sh
@@ -16,21 +16,21 @@
 # bugs with Eigen.
 tools/bazel --output_base=../k8_output_base test \
     ${COMMON} \
-    --cpu=k8 \
+    --config=k8 \
     --config=eigen \
     ${TARGETS}
 
 tools/bazel --output_base=../roborio_output_base build \
     ${COMMON} \
-    --cpu=roborio \
+    --config=roborio \
     ${TARGETS}
 
 tools/bazel --output_base=../armhf-debian_output_base build \
     ${COMMON} \
-    --cpu=armhf-debian \
+    --config=armhf-debian \
     ${TARGETS}
 
 tools/bazel --output_base=../cortex-m4f_output_base build \
     ${COMMON} \
-    --cpu=cortex-m4f \
+    --config=cortex-m4f \
     ${M4F_TARGETS}
diff --git a/tools/cpp/BUILD b/tools/cpp/BUILD
index d370750..4f0e6d9 100644
--- a/tools/cpp/BUILD
+++ b/tools/cpp/BUILD
@@ -2,16 +2,25 @@
 
 package(default_visibility = ["//visibility:public"])
 
-[cc_toolchain_config(
-    name = "{}_toolchain_config".format(cpu),
-    cpu = cpu,
-) for cpu in
-    ["armeabi-v7a", "armhf-debian", "cortex-m4f", "cortex-m4f-k22", "k8", "roborio"]
+[
+    cc_toolchain_config(
+        name = "{}_toolchain_config".format(cpu),
+        cpu = cpu,
+    )
+    for cpu in [
+        "armeabi-v7a",
+        "armhf-debian",
+        "cortex-m4f",
+        "cortex-m4f-k22",
+        "k8",
+        "roborio",
+    ]
 ]
 
 cc_library(
     name = "empty_main",
     srcs = ["empty_main.c"],
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 cc_library(
@@ -20,13 +29,15 @@
         "//tools:has_asan": [],
         "//tools:has_tsan": [],
         "//tools:cpu_cortex_m4f": [],
-        "//tools:cpu_cortex_m4f_k22": [],
+        # TODO(phil): Support this properly.
+        #"//tools:cpu_cortex_m4f_k22": [],
         "//conditions:default": ["//third_party/gperftools:tcmalloc"],
     }),
 )
 
 cc_library(
     name = "stl",
+    target_compatible_with = ["@platforms//os:linux"],
 )
 
 filegroup(
@@ -34,18 +45,6 @@
     srcs = [],
 )
 
-# This is the entry point for --crosstool_top.
-cc_toolchain_suite(
-    name = "toolchain",
-    toolchains = {
-        "k8": ":cc-compiler-k8",
-        "roborio": ":cc-compiler-roborio",
-        "armhf-debian": "cc-compiler-armhf-debian",
-        "cortex-m4f": "cc-compiler-cortex-m4f",
-        "cortex-m4f-k22": "cc-compiler-cortex-m4f-k22",
-    },
-)
-
 # Compiler inputs given by --copt etc in //tools:bazel.rc.
 filegroup(
     name = "flags_compiler_inputs",
@@ -111,16 +110,30 @@
 cc_toolchain(
     name = "cc-compiler-k8",
     all_files = ":clang_6p0_all_files",
+    ar_files = ":clang_6p0_ar_files",
+    as_files = ":clang_6p0_compiler_files",
     compiler_files = ":clang_6p0_compiler_files",
     dwp_files = ":empty",
     linker_files = ":clang_6p0_linker_files",
-    ar_files = ":clang_6p0_ar_files",
-    as_files = ":clang_6p0_compiler_files",
     objcopy_files = "//tools/cpp/clang_6p0:objcopy",
     strip_files = ":clang_6p0_strip_files",
     supports_param_files = 1,
-    toolchain_identifier = "k8_linux",
     toolchain_config = ":k8_toolchain_config",
+    toolchain_identifier = "k8_linux",
+)
+
+toolchain(
+    name = "cc-toolchain-k8",
+    exec_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+    target_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+    toolchain = ":cc-compiler-k8",
+    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
 )
 
 filegroup(
@@ -173,16 +186,30 @@
 cc_toolchain(
     name = "cc-compiler-roborio",
     all_files = ":roborio-compiler-files",
+    ar_files = ":roborio_ar_files",
+    as_files = ":roborio_compiler_files",
     compiler_files = ":roborio_compiler_files",
     dwp_files = ":empty",
     linker_files = ":roborio_linker_files",
-    ar_files = ":roborio_ar_files",
-    as_files = ":roborio_compiler_files",
     objcopy_files = "//tools/cpp/arm-frc-linux-gnueabi:objcopy",
     strip_files = ":roborio_strip_files",
     supports_param_files = 1,
-    toolchain_identifier = "roborio_linux",
     toolchain_config = ":roborio_toolchain_config",
+    toolchain_identifier = "roborio_linux",
+)
+
+toolchain(
+    name = "cc-toolchain-roborio",
+    exec_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+    target_compatible_with = [
+        "@platforms//os:linux",
+        "//tools/platforms/hardware:roborio",
+    ],
+    toolchain = ":cc-compiler-roborio",
+    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
 )
 
 filegroup(
@@ -240,16 +267,30 @@
 cc_toolchain(
     name = "cc-compiler-armhf-debian",
     all_files = ":linaro-gcc-files",
+    ar_files = "linaro_linux_ar_files",
+    as_files = "linaro_linux_compiler_files",
     compiler_files = ":linaro_linux_compiler_files",
     dwp_files = ":empty",
     linker_files = ":linaro_linux_linker_files",
-    ar_files = "linaro_linux_ar_files",
-    as_files = "linaro_linux_compiler_files",
     objcopy_files = "//tools/cpp/linaro_linux_gcc:objcopy",
     strip_files = ":linaro_linux_strip_files",
     supports_param_files = 1,
-    toolchain_identifier = "clang_linux_armhf",
     toolchain_config = ":armhf-debian_toolchain_config",
+    toolchain_identifier = "clang_linux_armhf",
+)
+
+toolchain(
+    name = "cc-toolchain-armhf-debian",
+    exec_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+    target_compatible_with = [
+        "@platforms//os:linux",
+        "//tools/platforms/hardware:raspberry_pi",
+    ],
+    toolchain = ":cc-compiler-armhf-debian",
+    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
 )
 
 filegroup(
@@ -292,15 +333,29 @@
 cc_toolchain(
     name = "cc-compiler-cortex-m4f",
     all_files = ":gcc_arm_none_eabi_none_files",
+    ar_files = ":gcc_arm_none_eabi_ar_files",
     compiler_files = ":gcc_arm_none_eabi_compiler_files",
     dwp_files = ":empty",
     linker_files = ":gcc_arm_none_eabi_linker_files",
-    ar_files = ":gcc_arm_none_eabi_ar_files",
     objcopy_files = "//tools/cpp/gcc_arm_none_eabi:objcopy",
     strip_files = "//tools/cpp/gcc_arm_none_eabi:strip",
     supports_param_files = 1,
-    toolchain_identifier = "cortex-m4f",
     toolchain_config = ":cortex-m4f_toolchain_config",
+    toolchain_identifier = "cortex-m4f",
+)
+
+toolchain(
+    name = "cc-toolchain-cortex-m4f",
+    exec_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+    target_compatible_with = [
+        "@platforms//os:none",
+        "//tools/platforms/hardware:cortex_m4f",
+    ],
+    toolchain = ":cc-compiler-cortex-m4f",
+    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
 )
 
 cc_toolchain(
@@ -312,6 +367,20 @@
     objcopy_files = ":empty",
     strip_files = ":empty",
     supports_param_files = 1,
-    toolchain_identifier = "cortex-m4f-k22",
     toolchain_config = ":cortex-m4f-k22_toolchain_config",
+    toolchain_identifier = "cortex-m4f-k22",
+)
+
+toolchain(
+    name = "cc-toolchain-cortex-m4f-k22",
+    exec_compatible_with = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+    target_compatible_with = [
+        "@platforms//os:none",
+        "//tools/platforms/hardware:cortex_m4f",
+    ],
+    toolchain = ":cc-compiler-cortex-m4f-k22",
+    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
 )
diff --git a/tools/environments.bzl b/tools/environments.bzl
deleted file mode 100644
index c82a01b..0000000
--- a/tools/environments.bzl
+++ /dev/null
@@ -1,6 +0,0 @@
-# Contains some helpers for working with environments.
-
-mcu_cpus = [
-    "@//tools:cortex-m4f",
-    "@//tools:cortex-m4f-k22",
-]
diff --git a/tools/platforms.bzl b/tools/platforms.bzl
new file mode 100644
index 0000000..fb2f69d
--- /dev/null
+++ b/tools/platforms.bzl
@@ -0,0 +1,11 @@
+load("@bazel_skylib//lib:selects.bzl", "selects")
+
+def _any_of(constraint_values):
+    return selects.with_or({
+        tuple(constraint_values): [],
+        "//conditions:default": ["@platforms//:incompatible"],
+    })
+
+platforms = struct(
+    any_of = _any_of,
+)
diff --git a/tools/platforms/BUILD b/tools/platforms/BUILD
new file mode 100644
index 0000000..65563bf
--- /dev/null
+++ b/tools/platforms/BUILD
@@ -0,0 +1,45 @@
+package(default_visibility = ["//visibility:public"])
+
+platform(
+    name = "linux_x86",
+    constraint_values = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+)
+
+platform(
+    name = "linux_armhf",
+    constraint_values = [
+        "@platforms//os:linux",
+        "@platforms//cpu:armv7",
+        "//tools/platforms/hardware:raspberry_pi",
+    ],
+)
+
+platform(
+    name = "linux_arm64",
+    constraint_values = [
+        "@platforms//os:linux",
+        "@platforms//cpu:arm64",
+    ],
+)
+
+platform(
+    name = "linux_roborio",
+    constraint_values = [
+        "@platforms//os:linux",
+        "@platforms//cpu:armv7",
+        "//tools/platforms/hardware:roborio",
+    ],
+)
+
+platform(
+    name = "cortex_m4f",
+    constraint_values = [
+        "@platforms//os:none",
+        "//tools/platforms/hardware:cortex_m4f",
+    ],
+)
+
+# TODO(phil): Create something for "cortex-m4f-k22" builds.
diff --git a/tools/platforms/hardware/BUILD b/tools/platforms/hardware/BUILD
new file mode 100644
index 0000000..521b243
--- /dev/null
+++ b/tools/platforms/hardware/BUILD
@@ -0,0 +1,21 @@
+# Additional constraint values not available in @platforms. Use these in things
+# like select() statements or target_compatible_with.
+
+package(default_visibility = ["//visibility:public"])
+
+constraint_setting(name = "hardware")
+
+constraint_value(
+    name = "raspberry_pi",
+    constraint_setting = ":hardware",
+)
+
+constraint_value(
+    name = "roborio",
+    constraint_setting = ":hardware",
+)
+
+constraint_value(
+    name = "cortex_m4f",
+    constraint_setting = ":hardware",
+)