Update bazel build modes

.bazelignore

@@ -0,0 +1,4 @@
+bazel-bazel
+bazel-bin
+bazel-out
+bazel-testlogs

.bazelrc

@@ -0,0 +1,44 @@
+# Bazel configuration file
+# https://docs.bazel.build/versions/main/guide.html#bazelrc
+# --compilation_mode [-c] (fastbuild, dbg or opt; default: "fastbuild")
+# --[no]subcommands [-s] (true, pretty_print or false; default: "false")
+# bazel clean --expunge
+
+# Enable Bzlmod for every Bazel command
+common --enable_bzlmod
+
+# Build options ###############################################################
+build --color=yes
+
+# Disk cache setup ############################################################
+build --disk_cache=/tmp/bazel_disk_cache
+
+# Alias definition ############################################################
+# Allow  to specify the build mode and precision with the following flags:
+# --mode=[serial|parallel]
+# --precision=[single|double]
+build --flag_alias=mode=//:mode
+build --flag_alias=precision=//:precision
+
+# Compilation options #########################################################
+build --copt=-g
+build --copt=-Wall
+# build --copt=-Wextra
+# build --copt=-Werror
+build --copt=-std=c++17
+build --cxxopt=-Wno-unused-parameter
+build --strip=never
+
+# build:macos --copt=-Werror
+# build:macos --cxxopt=-Wno-error=deprecated-declarations
+# build:macos --cxxopt=-Wno-error=deprecated-enum-enum-conversion
+# build:macos --cxxopt=-Wno-error=deprecated-anon-enum-enum-conversion
+# build:macos --cxxopt=-Wno-error=inconsistent-missing-override
+# build:macos --cxxopt=-Wno-error=overloaded-virtual
+
+# Prevent Bazel from detecting the system's C++ toolchain.
+# build --action_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
+# build --incompatible_strict_action_env=true
+
+# Enable the CC toolchain resolution based on platforms.
+# build --incompatible_enable_cc_toolchain_resolution

.gitignore

@@ -19,6 +19,9 @@ CMakeFiles/
 # Clangd server cache
 *.cache*
 
+# VSCode files
+.vscode/
+
 # Backup files
 *~
 
@@ -51,6 +54,11 @@ doc/warnings.log
 *.dSYM
 .DS_Store
 
+# Bazel specific
+bazel-*
+*.bazel_cache
+MODULE.bazel.lock
+
 # Example and miniapp binaries and outputs
 
 examples/ex[0-9]

BUILD

@@ -0,0 +1,468 @@
+### String Flags ##############################################################
+load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
+
+### Load rules ################################################################
+load("@rules_cc//cc:defs.bzl", "cc_library")
+
+### Load config/bazel/config ##################################################
+load("//config/bazel:config.bzl", "mfem_serial_examples",
+                                  "mfem_parallel_examples", "mfem_use")
+
+### Load config/bazel/settings ################################################
+### https://bazel.build/docs/configurable-attributes
+load("//config/bazel:settings.bzl", "mode", "precision", "print_mode", "print_precision")
+
+# FLAG: Serial/Parallel MODE ##################################################
+string_flag(
+    name = "mode",
+    values = ["serial", "parallel"],
+    build_setting_default = "serial",
+)
+
+mode(name = "serial")
+
+mode(name = "parallel")
+
+config_setting(
+    name = "serial_mode",
+    flag_values = {":mode": "serial"},
+)
+
+config_setting(
+    name = "parallel_mode",
+    flag_values = {":mode": "parallel"},
+)
+
+print_mode(
+    name = "print_mode",
+    mode = select({
+        ":serial_mode": "serial",
+        ":parallel_mode": "parallel",
+    }),
+)
+
+# FLAG: Double/Single PRECISION ###############################################
+string_flag(
+    name = "precision",
+    values = ["single", "double"],
+    build_setting_default = "double",
+)
+
+precision(name = "double")
+
+precision(name = "single")
+
+config_setting(
+    name = "single_precision",
+    flag_values = {":precision": "single"},
+)
+
+config_setting(
+    name = "double_precision",
+    flag_values = {":precision": "double"},
+)
+
+print_precision(
+    name = "print_precision",
+    precision = select({
+        ":single_precision": "single",
+        ":double_precision": "double",
+    }),
+)
+
+# MFEM_USE_* definitions ######################################################
+mfem_use(
+    name = "mfem_not_mpi",
+    define = "MFEM_USE_MPI",
+    use = False,
+)
+
+mfem_use(
+    name = "mfem_use_mpi",
+    define = "MFEM_USE_MPI",
+    use = True,
+)
+
+mfem_use(
+    name = "mfem_not_metis",
+    define = "MFEM_USE_METIS",
+    use = False,
+)
+
+mfem_use(
+    name = "mfem_use_metis",
+    define = "MFEM_USE_METIS",
+    use = True,
+)
+
+mfem_use(
+    name = "mfem_not_metis_5",
+    define = "MFEM_USE_METIS_5",
+    use = False,
+)
+
+mfem_use(
+    name = "mfem_use_metis_5",
+    define = "MFEM_USE_METIS_5",
+    use = True,
+)
+
+mfem_use(
+    name = "no_mfem_hypre_version",
+    define = "MFEM_HYPRE_VERSION",
+    use = False,
+)
+
+mfem_use(
+    name = "mfem_hypre_version",
+    define = "MFEM_HYPRE_VERSION",
+    use = True,
+)
+
+mfem_use(
+    name = "mfem_not_double",
+    define = "MFEM_USE_DOUBLE",
+    use = False,
+)
+
+mfem_use(
+    name = "mfem_use_double",
+    define = "MFEM_USE_DOUBLE",
+    use = True,
+)
+
+mfem_use(
+    name = "mfem_not_single",
+    define = "MFEM_USE_SINGLE",
+    use = False,
+)
+
+mfem_use(
+    name = "mfem_use_single",
+    define = "MFEM_USE_SINGLE",
+    use = True,
+)
+
+### https://bazel.build/reference/be/general#genrule
+genrule(
+    name = "genrule_config_bazel",
+    srcs = ["BUILD"],
+    outs = ["config/bazel.hpp"],
+    cmd = """cat <<EOF > $@
+// Copyright (c) 2010-2025, Lawrence Livermore National Security, LLC. Produced
+// at the Lawrence Livermore National Laboratory. All Rights reserved. See files
+// LICENSE and NOTICE for details. LLNL-CODE-806117.
+//
+// This file is part of the MFEM library. For more information and source code
+// availability visit https://mfem.org.
+//
+// MFEM is free software; you can redistribute it and/or modify it under the
+// terms of the BSD-3 license. We welcome feedback and contributions, see file
+// CONTRIBUTING.md for details.
+
+#ifndef MFEM_CONFIG_HEADER
+#define MFEM_CONFIG_HEADER
+
+// MFEM version: integer of the form: (major*100 + minor)*100 + patch.
+#define MFEM_VERSION 40701
+
+// MFEM version string of the form "3.3" or "3.3.1".
+#define MFEM_VERSION_STRING "4.7.1"
+
+// MFEM version type, see the MFEM_VERSION_TYPE_* constants below.
+#define MFEM_VERSION_TYPE ((MFEM_VERSION) % 2)
+
+// MFEM version type constants.
+#define MFEM_VERSION_TYPE_RELEASE 0
+#define MFEM_VERSION_TYPE_DEVELOPMENT 1
+
+// Separate MFEM version numbers for major, minor, and patch.
+#define MFEM_VERSION_MAJOR ((MFEM_VERSION) / 10000)
+#define MFEM_VERSION_MINOR (((MFEM_VERSION) / 100) % 100)
+#define MFEM_VERSION_PATCH ((MFEM_VERSION) % 100)
+
+// The absolute path of the MFEM source prefix.
+#define MFEM_SOURCE_DIR "$$(realpath $$(realpath BUILD)/..)"
+
+// The absolute path of the MFEM installation prefix.
+#define MFEM_INSTALL_DIR "$$(realpath $(BINDIR))"
+
+// Description of the git commit used to build MFEM.
+#define MFEM_GIT_STRING "heads/bazel-git-..."
+
+// Build the parallel MFEM library.
+// Requires an MPI compiler, and the libraries HYPRE and METIS.
+$(MFEM_USE_MPI)
+
+// Enable MFEM features that use the METIS library (parallel MFEM).
+$(MFEM_USE_METIS)
+
+// Enable this option if linking with METIS version 5 (parallel MFEM).
+$(MFEM_USE_METIS_5)
+
+// Version of HYPRE used for building MFEM.
+// macOS: 23200, ubuntu: 21821
+$(MFEM_HYPRE_VERSION) 23200
+
+// Use single/double-precision floating point type
+$(MFEM_USE_DOUBLE)
+$(MFEM_USE_SINGLE)
+
+// Internal MFEM option: enable group/batch allocation for some small objects.
+#define MFEM_USE_MEMALLOC
+
+// Which library functions to use in class StopWatch for measuring time.
+// For a list of the available options, see INSTALL.
+// If not defined, an option is selected automatically.
+// 0/1/2/3/4/5/6/NO
+#define MFEM_TIMER_TYPE 0
+
+#endif // MFEM_CONFIG_HEADER
+EOF""",
+    local = False,
+    message = "Generating config bazel.hpp file",
+    toolchains = select({
+        ":serial_mode": [
+            ":mfem_not_mpi",
+            ":mfem_not_metis",
+            ":mfem_not_metis_5",
+            ":no_mfem_hypre_version",
+        ],
+        ":parallel_mode": [
+            ":mfem_use_mpi",
+            ":mfem_use_metis",
+            ":mfem_use_metis_5",
+            ":mfem_hypre_version",
+        ],
+        "//conditions:default": [
+            ":mfem_not_mpi",
+            ":mfem_not_metis",
+            ":mfem_not_metis_5",
+            ":mfem_hypre_version",
+            ":mfem_use_double",
+            ":mfem_not_single",
+        ],
+    }) + select({
+        ":double_precision": [
+            ":mfem_use_double",
+            ":mfem_not_single",
+        ],
+        ":single_precision": [
+            ":mfem_not_double",
+            ":mfem_use_single",
+        ],
+    }),
+)
+
+cc_library(
+    name = "config_bazel_hpp",
+    srcs = ["config/bazel.hpp"],
+    includes = ["config"],
+)
+
+### MFEM Examples #############################################################
+mfem_serial_examples()
+mfem_parallel_examples()
+
+### MFEM library ##############################################################
+
+cc_library(
+    name = "mfem",
+    deps = [
+        "fem",
+        "general",
+        "linalg",
+        "mesh",
+        "@config",
+    ] + select({
+        ":parallel_mode": ["@mpi"],
+        "//conditions:default": [],
+    }),
+)
+
+### Sources ###################################################################
+
+cc_library(
+    name = "fem",
+    srcs = glob([
+        "fem/*.cpp",
+        "fem/ceed/**/*.cpp",
+        "fem/fe/*.cpp",
+        "fem/integ/*.cpp",
+        "fem/lor/*.cpp",
+        # skip moonolith
+        "fem/qinterp/*.cpp",
+        "fem/tmop/*.cpp",
+    ]),
+    deps = [
+        "config_hpp",
+        "fem_hpp",
+        "general_hpp",
+        "linalg_hpp",
+        "mesh_hpp",
+    ] + select({
+        ":parallel_mode": ["@mpi"],
+        "//conditions:default": [],
+    }),
+)
+
+cc_library(
+    name = "general",
+    srcs = glob(["general/*.cpp"]),
+    deps = [
+        "config_hpp",
+        "general_hpp",
+        "linalg_hpp",
+    ] + select({
+        ":parallel_mode": ["@mpi"],
+        "//conditions:default": [],
+    }),
+)
+
+cc_library(
+    name = "linalg",
+    srcs = glob(["linalg/**/*.cpp"]),
+    deps = [
+        "config_hpp",
+        "fem_hpp",
+        "general_hpp",
+        "linalg_hpp",
+        "mesh_hpp",
+    ] + select({
+        ":parallel_mode": ["@mpi"],
+        "//conditions:default": [],
+    }),
+)
+
+cc_library(
+    name = "mesh",
+    srcs = glob([
+        "mesh/*.cpp",
+        "mesh/submesh/*.cpp",
+    ]),
+    deps = [
+        "config_hpp",
+        "fem_hpp",
+        "general_hpp",
+        "linalg_hpp",
+        "mesh_hpp",
+    ] + select({
+        ":parallel_mode": ["@mpi"],
+        "//conditions:default": [],
+    }),
+)
+
+### Headers ###################################################################
+
+cc_library(
+    name = "examples_hpp",
+    srcs = glob(["examples/*.hpp"]),
+)
+
+cc_library(
+    name = "general_hpp",
+    srcs = glob(
+        [
+            "general/*.hpp",
+            "general/*.h",
+        ],
+    ),
+)
+
+cc_library(
+    name = "mfem_hpp",
+    srcs = ["mfem.hpp"],
+    deps = ["config_hpp"],
+)
+
+cc_library(
+    name = "config_hpp",
+    srcs = glob(["config/*.hpp"]),
+    deps = [
+        ":config_bazel_hpp",
+        "@config",
+    ],
+)
+
+cc_library(
+    name = "fem_hpp",
+    srcs = glob([
+        "fem/*.hpp",
+        "fem/*.h",
+    ]),
+    deps = [
+        "config_hpp",
+        "fem_ceed_hpp",
+        "fem_fe_hpp",
+        "fem_integ_hpp",
+        "fem_lor_hpp",
+        # "fem_moonolith_hpp",
+        "fem_qinterp_hpp",
+        "fem_tmop_hpp",
+    ],
+)
+
+cc_library(
+    name = "fem_ceed_hpp",
+    srcs = glob(["fem/ceed/**/*.hpp"]),
+)
+
+cc_library(
+    name = "fem_moonolith_hpp",
+    srcs = glob(["fem/moonolith/**/*.hpp"]),
+)
+
+cc_library(
+    name = "fem_fe_hpp",
+    srcs = glob(["fem/fe/*.hpp"]),
+)
+
+cc_library(
+    name = "fem_integ_hpp",
+    srcs = glob(["fem/integ/*.hpp"]),
+)
+
+cc_library(
+    name = "fem_lor_hpp",
+    srcs = glob(["fem/lor/*.hpp"]),
+)
+
+cc_library(
+    name = "fem_qinterp_hpp",
+    srcs = glob(["fem/qinterp/*.hpp"]),
+)
+
+cc_library(
+    name = "fem_tmop_hpp",
+    srcs = glob(["fem/tmop/*.hpp"]),
+)
+
+cc_library(
+    name = "linalg_hpp",
+    srcs = glob(["linalg/*.hpp"]),
+    deps = [
+        "linalg_batched_hpp",
+        "linalg_simd_hpp",
+    ],
+)
+
+cc_library(
+    name = "linalg_batched_hpp",
+    srcs = glob(["linalg/batched/*.hpp"]),
+)
+
+cc_library(
+    name = "linalg_simd_hpp",
+    srcs = glob(["linalg/simd/*.hpp"]),
+)
+
+cc_library(
+    name = "mesh_hpp",
+    srcs = glob(["mesh/*.hpp"]),
+    deps = ["submesh_hpp"],
+)
+
+cc_library(
+    name = "submesh_hpp",
+    srcs = glob(["mesh/submesh/*.hpp"]),
+)

MODULE.bazel

@@ -0,0 +1,36 @@
+"""
+Bazel Central Registry: https://registry.bazel.build
+"""
+module(name = "mfem", version = "4.7")
+
+# https://github.com/bazelbuild/rules_cc
+# https://registry.bazel.build/modules/rules_cc
+bazel_dep(name = "rules_cc", version = "0.1.1")
+
+# https://github.com/bazel-contrib/toolchains_llvm/releases
+# https://github.com/bazel-contrib/toolchains_llvm/blob/master/toolchain/internal/llvm_distributions.bzl
+bazel_dep(name = "toolchains_llvm", version = "1.3.0")
+
+# Configure and register the toolchain.
+llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")
+# llvm.toolchain(llvm_version = "16.0.0")
+llvm.toolchain(llvm_version = "18.1.8")
+# x86_64-linux-gnu-ubuntu-20.04     13.0.0
+# x86_64-linux-gnu-ubuntu-22.04     17.0.6
+# macOS                             17.0.6
+# llvm.toolchain(llvm_version = "17.0.6", stdlib = {"linux-x86_64": "stdc++"}) # libc++ / stdc++
+
+use_repo(llvm, "llvm_toolchain")
+register_toolchains("@llvm_toolchain//:all")
+
+# https://github.com/bazelbuild/bazel-skylib/releases
+bazel_dep(name = "bazel_skylib", version = "1.7.1")
+
+# External dependencies
+external = use_extension("//config/bazel:external.bzl", "external")
+use_repo(external, "mpi")
+use_repo(external, "hypre")
+use_repo(external, "metis")
+use_repo(external, "config")
+
+

config/bazel/config.bzl

@@ -0,0 +1,49 @@
+"""
+Bazel configuration helper functions
+"""
+
+load("@rules_cc//cc:defs.bzl", "cc_binary")
+
+### Examples ##################################################################
+def mfem_serial_examples():
+    for n in range(1, 40):
+        example = "ex" + str(n)
+        cc_binary(
+            name = example,
+            srcs = ["examples/" + example + ".cpp"],
+            deps = [
+                "mfem",
+                "mfem_hpp",
+                "examples_hpp",
+            ],
+        )
+
+def mfem_parallel_examples():
+    for n in range(1, 40):
+        example = "ex" + str(n) + "p"
+        cc_binary(
+            name = example,
+            srcs = ["examples/" + example + ".cpp"],
+            deps = [
+                "mfem",
+                "mfem_hpp",
+                "examples_hpp",
+                "@mpi",
+            ],
+        )
+
+### MFEM_USE_* ################################################################
+def _mfem_use(ctx):
+    value = "//" if not ctx.attr.use else ""
+    value += "#define " + ctx.attr.define
+    return [
+        platform_common.TemplateVariableInfo({ctx.attr.define: value}),
+    ]
+
+mfem_use = rule(
+    implementation = _mfem_use,
+    attrs = {
+        "define": attr.string(),
+        "use": attr.bool(),
+    },
+)

config/bazel/external.bzl

@@ -0,0 +1,57 @@
+"""Modules for dependencies not included in the Bazel Central Registry"""
+# https://bazel.build/rules/lib/repo/local
+
+load("@bazel_tools//tools/build_defs/repo:local.bzl", "new_local_repository")
+
+def _external(_):
+    # MFEM configuration file
+    new_local_repository(
+        name = "config",
+        build_file_content = """
+cc_library(
+    name = "config",
+    defines = ["MFEM_CONFIG_FILE=\\\\\\"config/bazel.hpp\\\\\\""],
+    visibility = ["//visibility:public"],
+)""",
+        path = "",
+    )
+    # MPI implementation
+    new_local_repository(
+        name = "mpi",
+        build_file_content = """
+cc_library(
+    name = "mpi",
+    srcs = ["lib/libmpi.dylib"],
+    hdrs = glob(["include/**/*.h"]),
+    includes = ["include"],
+    visibility = ["//visibility:public"],
+    deps = ["@hypre", "@metis"],
+)""",
+        path = "/opt/homebrew/opt/open-mpi",
+    )
+    new_local_repository(
+        name = "hypre",
+        build_file_content = """
+cc_library(
+    name = "hypre",
+    srcs = ["lib/libHYPRE.a"],
+    hdrs = glob(["include/*.h"]),
+    includes = ["include"],
+    visibility = ["//visibility:public"],
+)""",
+        path = "/opt/homebrew/opt/hypre",
+    )
+    new_local_repository(
+        name = "metis",
+        build_file_content = """
+cc_library(
+    name = "metis",
+    srcs = ["lib/libmetis.dylib"],
+    hdrs = glob(["include/metis.h"]),
+    includes = ["include"],
+    visibility = ["//visibility:public"],
+)""",
+        path = "/opt/homebrew/opt/metis",
+    )
+
+external = module_extension(implementation = _external)

config/bazel/external_linux.bzl

@@ -0,0 +1,61 @@
+"""Modules for dependencies not included in the Bazel Central Registry"""
+# https://bazel.build/rules/lib/repo/local
+
+load("@bazel_tools//tools/build_defs/repo:local.bzl", "new_local_repository")
+
+def _external(_):
+    new_local_repository(
+        name = "config",
+        build_file_content = """
+cc_library(
+    name = "config",
+    defines = ["MFEM_CONFIG_FILE=\\\\\\"config/bazel.hpp\\\\\\""],
+    visibility = ["//visibility:public"],
+)""",
+        path = "",
+    )
+    new_local_repository(
+        name = "mpi",
+        build_file_content = """
+cc_library(
+    name = "mpi",
+    # srcs = ["lib/libmpi.dylib"],
+    srcs = ["lib/libmpi.so"],
+    hdrs = glob(["include/**/*.h"]),
+    includes = ["include"],
+    visibility = ["//visibility:public"],
+    deps = ["@hypre", "@metis"],
+)""",
+        path = "/usr/lib/x86_64-linux-gnu/openmpi"
+    )
+    new_local_repository(
+        name = "hypre",
+        build_file_content = """
+cc_library(
+    name = "hypre",
+    # srcs = ["lib/libHYPRE.a"],
+    srcs = ["lib/x86_64-linux-gnu/libHYPRE.so",
+            "lib/x86_64-linux-gnu/libHYPRE_core.so"],
+    hdrs = glob(["include/hypre/*.h"]),
+    # includes = ["include"],
+    includes = ["include/hypre"],
+    visibility = ["//visibility:public"],
+)""",
+        path = "/usr"
+    )
+    new_local_repository(
+        name = "metis",
+        build_file_content = """
+cc_library(
+    name = "metis",
+    # srcs = ["lib/libmetis.dylib"],
+    srcs = ["lib/x86_64-linux-gnu/libmetis.so.5"],
+    hdrs = glob(["include/metis.h"]),
+    # includes = ["include"],
+    visibility = ["//visibility:public"],
+)""",
+        # path = "/opt/homebrew/Cellar/metis/5.1.0",
+        path = "/usr",
+    )
+
+external = module_extension(implementation = _external)

config/bazel/external_macos.bzl

@@ -0,0 +1,55 @@
+"""Modules for dependencies not included in the Bazel Central Registry"""
+# https://bazel.build/rules/lib/repo/local
+
+load("@bazel_tools//tools/build_defs/repo:local.bzl", "new_local_repository")
+
+def _external(_):
+    new_local_repository(
+        name = "config",
+        build_file_content = """
+cc_library(
+    name = "config",
+    defines = ["MFEM_CONFIG_FILE=\\\\\\"config/bazel.hpp\\\\\\""],
+    visibility = ["//visibility:public"],
+)""",
+        path = "",
+    )
+    new_local_repository(
+        name = "mpi",
+        build_file_content = """
+cc_library(
+    name = "mpi",
+    srcs = ["lib/libmpi.dylib"],
+    hdrs = glob(["include/**/*.h"]),
+    includes = ["include"],
+    visibility = ["//visibility:public"],
+    deps = ["@hypre", "@metis"],
+)""",
+        path = "/opt/homebrew/opt/open-mpi",
+    )
+    new_local_repository(
+        name = "hypre",
+        build_file_content = """
+cc_library(
+    name = "hypre",
+    srcs = ["lib/libHYPRE.a"],
+    hdrs = glob(["include/*.h"]),
+    includes = ["include"],
+    visibility = ["//visibility:public"],
+)""",
+        path = "/opt/homebrew/opt/hypre",
+    )
+    new_local_repository(
+        name = "metis",
+        build_file_content = """
+cc_library(
+    name = "metis",
+    srcs = ["lib/libmetis.dylib"],
+    hdrs = glob(["include/metis.h"]),
+    includes = ["include"],
+    visibility = ["//visibility:public"],
+)""",
+        path = "/opt/homebrew/opt/metis",
+    )
+
+external = module_extension(implementation = _external)

config/bazel/settings.bzl

@@ -0,0 +1,21 @@
+"""
+This module defines build settings: 'mode' and 'precision'.
+"""
+
+# Mode ########################################################################
+ModeInfo = provider(doc = "serial or parallel", fields = ["type"])
+def PrintMode(ctx):
+    ctx.actions.write(output = ctx.outputs.log, 
+                      content = "Compiling in " + ctx.attr.mode[ModeInfo].type + "!")
+print_mode = rule(implementation = PrintMode, attrs = {"mode": attr.label()})
+def Mode(ctx): return ModeInfo(type = ctx.label.name)
+mode = rule(implementation = Mode)
+
+# Precision ###################################################################
+PrecisionInfo = provider(doc = "single or double", fields = ["type"])
+def PrintPrecision(ctx):
+    ctx.actions.write(output = ctx.outputs.log, 
+                      content = "Compiling in " + ctx.attr.precision[PrecisionInfo].type + "!")
+print_precision = rule(implementation = PrintPrecision, attrs = {"precision": attr.label()})
+def Precision(ctx): return PrecisionInfo(type = ctx.label.name)
+precision = rule(implementation = Precision)