pw_protobuf_compiler: Remove pw_protobuf_GENERATORS build arg

This updates pw_protobuf_compiler to always create a target for each
supported protobuf generator, instead of having a build arg to select
which ones to use, simplifying the use of pw_proto_library.

Change-Id: I2699c9c5045e8671a061f8c8bc11a94244d008f1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/22721
Reviewed-by: Armando Montanez <amontanez@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Alexei Frolov <frolv@google.com>
diff --git a/docs/build_system.rst b/docs/build_system.rst
index 69f753e..6938d19 100644
--- a/docs/build_system.rst
+++ b/docs/build_system.rst
@@ -213,7 +213,6 @@
 
   # Build arguments set across all Pigweed targets.
   default_args = {
-    pw_protobuf_GENERATORS = [ "nanopb_rpc" ]
     dir_pw_third_party_nanopb = "//third_party/nanopb-0.4.2"
   }
 
diff --git a/docs/targets.rst b/docs/targets.rst
index 8ada893..21f4ccc 100644
--- a/docs/targets.rst
+++ b/docs/targets.rst
@@ -98,7 +98,6 @@
 
       # Extend with custom build arguments for the target.
       pw_log_BACKEND = dir_pw_log_tokenized
-      pw_protobuf_GENERATORS = [ "nanopb" ]
     }
   }
 
diff --git a/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py b/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py
index 6396dc0..6638cad 100755
--- a/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py
+++ b/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py
@@ -102,8 +102,7 @@
     build.gn_gen(ctx.root,
                  ctx.output_dir,
                  dir_pw_third_party_nanopb='"{}"'.format(ctx.package_root /
-                                                         'nanopb'),
-                 pw_protobuf_GENERATORS='["nanopb", "nanopb_rpc", "pwpb"]')
+                                                         'nanopb'))
     build.ninja(
         ctx.output_dir,
         *_at_all_optimization_levels('stm32f429i'),
diff --git a/pw_protobuf_compiler/BUILD.gn b/pw_protobuf_compiler/BUILD.gn
index 76549c3..58dce83 100644
--- a/pw_protobuf_compiler/BUILD.gn
+++ b/pw_protobuf_compiler/BUILD.gn
@@ -23,27 +23,16 @@
   sources = [ "docs.rst" ]
 }
 
-_compiling_for_nanopb = pw_protobuf_GENERATORS + [ "nanopb" ] - [ "nanopb" ] !=
-                        pw_protobuf_GENERATORS
-
 pw_test_group("tests") {
-  tests = []
-  if (_compiling_for_nanopb) {
-    tests += [ ":nanopb_test" ]
-  }
+  tests = [ ":nanopb_test" ]
 }
 
-if (_compiling_for_nanopb) {
-  pw_test("nanopb_test") {
-    deps = [ ":nanopb_test_protos.nanopb" ]
-    sources = [ "nanopb_test.cc" ]
-  }
+pw_test("nanopb_test") {
+  deps = [ ":nanopb_test_protos.nanopb" ]
+  sources = [ "nanopb_test.cc" ]
+  enable_if = dir_pw_third_party_nanopb != ""
+}
 
-  pw_proto_library("nanopb_test_protos") {
-    sources = [ "pw_protobuf_compiler_protos/nanopb_test.proto" ]
-  }
-} else {
-  pw_input_group("not_needed") {
-    inputs = [ "nanopb_test.cc" ]
-  }
+pw_proto_library("nanopb_test_protos") {
+  sources = [ "pw_protobuf_compiler_protos/nanopb_test.proto" ]
 }
diff --git a/pw_protobuf_compiler/docs.rst b/pw_protobuf_compiler/docs.rst
index ae966ef..3e63881 100644
--- a/pw_protobuf_compiler/docs.rst
+++ b/pw_protobuf_compiler/docs.rst
@@ -3,13 +3,11 @@
 --------------------
 pw_protobuf_compiler
 --------------------
-
 The Protobuf compiler module provides build system integration and wrapper
 scripts for generating source code for Protobuf definitions.
 
 Generator support
 =================
-
 Protobuf code generation is currently supported for the following generators:
 
 +-------------+----------------+-----------------------------------------------+
@@ -30,30 +28,29 @@
 | Raw RPC     | ``raw_rpc``    | Compiles raw binary pw_rpc service code.      |
 +-------------+----------------+-----------------------------------------------+
 
-The build variable ``pw_protobuf_GENERATORS`` tells the module the generators
-for which it should compile code. It is defined as a list of generator codes.
-
 GN template
 ===========
-
 The ``pw_proto_library`` GN template is provided by the module.
 
-It tells the build system to compile a set of source proto files to a library in
-each chosen generator. A different target is created for each generator, with
-the generator's code appended as a suffix to the template's target name.
+It defines a collection of protobuf files that should be compiled together. The
+template creates a sub-target for each supported generator, named
+``<target_name>.<generator>``. These sub-targets generate their respective
+protobuf code, and expose it to the build system appropriately (e.g. a
+``pw_source_set`` for C/C++).
 
-For example, given the definitions:
+For example, given the following target:
 
-.. code::
-
-  pw_protobuf_GENERATORS = [ "pwpb", "go" ]
+.. code-block::
 
   pw_proto_library("test_protos") {
     sources = [ "test.proto" ]
   }
 
-Two targets are created, named ``test_protos.pwpb`` and ``test_protos.go``,
-containing the generated code from their respective generators.
+``test_protos.pwpb`` compiles code for pw_protobuf, and ``test_protos.nanopb``
+compiles using Nanopb (if it's installed).
+
+Protobuf code is only generated when a generator sub-target is listed as a
+dependency of another GN target.
 
 **Arguments**
 
diff --git a/pw_protobuf_compiler/proto.gni b/pw_protobuf_compiler/proto.gni
index 6878156..ad9f175 100644
--- a/pw_protobuf_compiler/proto.gni
+++ b/pw_protobuf_compiler/proto.gni
@@ -14,24 +14,12 @@
 
 import("//build_overrides/pigweed.gni")
 
+import("$dir_pw_build/error.gni")
 import("$dir_pw_build/input_group.gni")
 import("$dir_pw_build/python_action.gni")
 import("$dir_pw_build/target_types.gni")
 import("$dir_pw_third_party/nanopb/nanopb.gni")
 
-declare_args() {
-  # Generators with which to compile protobuf code. These are used by the
-  # pw_proto_library template to determine which build targets to create.
-  #
-  # Supported generators:
-  #   "pwpb", "nanopb", "nanopb_rpc", "raw_rpc", "go"
-  pw_protobuf_GENERATORS = [
-    "pwpb",
-    "raw_rpc",
-    "go",
-  ]
-}
-
 # Python script that invokes protoc.
 _gen_script_path =
     "$dir_pw_protobuf_compiler/py/pw_protobuf_compiler/generate_protos.py"
@@ -81,8 +69,7 @@
   }
 
   # For C++ proto files, the generated proto directory is added as an include
-  # path for the code. This requires using "all_dependent_configs" to force the
-  # include on any code that transitively depends on the generated protos.
+  # path for the code.
   _include_config_target = "${target_name}_includes"
   config(_include_config_target) {
     include_dirs = [ "$_proto_gen_dir" ]
@@ -90,7 +77,7 @@
 
   # Create a library with the generated source files.
   pw_source_set(target_name) {
-    all_dependent_configs = [ ":$_include_config_target" ]
+    public_configs = [ ":$_include_config_target" ]
     deps = [ ":$_gen_target" ]
     public_deps = [ dir_pw_protobuf ] + invoker.gen_deps
     sources = get_target_outputs(":$_gen_target")
@@ -106,9 +93,6 @@
 #  protos: List of input .proto files.
 #
 template("_pw_nanopb_rpc_proto_library") {
-  assert(defined(dir_pw_third_party_nanopb) && dir_pw_third_party_nanopb != "",
-         "\$dir_pw_third_party_nanopb must be set to compile nanopb protobufs")
-
   _proto_gen_dir = "$root_gen_dir/protos"
   _module_path = get_path_info(".", "abspath")
   _relative_proto_paths = rebase_path(invoker.protos, _module_path)
@@ -147,8 +131,7 @@
   }
 
   # For C++ proto files, the generated proto directory is added as an include
-  # path for the code. This requires using "all_dependent_configs" to force the
-  # include on any code that transitively depends on the generated protos.
+  # path for the code.
   _include_root = rebase_path(get_path_info(".", "abspath"), "//")
   _include_config_target = "${target_name}_includes"
   config(_include_config_target) {
@@ -160,7 +143,7 @@
 
   # Create a library with the generated source files.
   pw_source_set(target_name) {
-    all_dependent_configs = [ ":$_include_config_target" ]
+    public_configs = [ ":$_include_config_target" ]
     deps = [ ":$_gen_target" ]
     public_deps = [
                     "$dir_pw_rpc:server",
@@ -178,9 +161,6 @@
 # Args:
 #  protos: List of input .proto files.
 template("_pw_nanopb_proto_library") {
-  assert(defined(dir_pw_third_party_nanopb) && dir_pw_third_party_nanopb != "",
-         "\$dir_pw_third_party_nanopb must be set to compile nanopb protobufs")
-
   _proto_gen_dir = "$root_gen_dir/protos"
   _module_path = get_path_info(".", "abspath")
   _relative_proto_paths = rebase_path(invoker.protos, _module_path)
@@ -231,8 +211,7 @@
   }
 
   # For C++ proto files, the generated proto directory is added as an include
-  # path for the code. This requires using "all_dependent_configs" to force the
-  # include on any code that transitively depends on the generated protos.
+  # path for the code.
   _include_root = rebase_path(get_path_info(".", "abspath"), "//")
   _include_config_target = "${target_name}_includes"
   config(_include_config_target) {
@@ -247,7 +226,7 @@
 
   # Create a library with the generated source files.
   pw_source_set(target_name) {
-    all_dependent_configs = [ ":$_include_config_target" ]
+    public_configs = [ ":$_include_config_target" ]
     deps = [ ":$_gen_target" ]
     public_deps = [ "$dir_pw_third_party/nanopb" ] + invoker.gen_deps
     sources = get_target_outputs(":$_gen_target")
@@ -358,29 +337,15 @@
 }
 
 # Generates protobuf code from .proto definitions for various languages.
-#
-# The generators to use are defined in the pw_protobuf_GENERATORS build
-# variable. Each listed generator creates a generated code target called
+# For each supported generator, creates a sub-target named:
 #
 #   <target_name>.<generator>
 #
-# For example, with the following definitions:
-#
-#   pw_protobuf_GENERATORS = [ "pwpb", "py" ]
-#
-#   pw_proto_library("my_protos") {
-#     sources = [ "foo.proto" ]
-#   }
-#
-# Two build targets will be created for the declared "my_protos" target.
-#
-#   "my_protos.pwpb"  <-- C++ source_set containing generated proto code
-#   "my_protos.py"    <-- Python module containing generated proto code
-#
 # Args:
 #  sources: List of input .proto files.
 #  deps: List of other pw_proto_library dependencies.
 #  inputs: Other files on which the protos depend (e.g. nanopb .options files).
+#
 template("pw_proto_library") {
   assert(defined(invoker.sources) && invoker.sources != [],
          "pw_proto_library requires .proto source files")
@@ -416,89 +381,72 @@
     _deps += [ ":$_input_target_name" ]
   }
 
-  # If the nanopb_rpc generator is selected, make sure that nanopb is also
-  # selected.
-  has_nanopb_rpc = pw_protobuf_GENERATORS + [ "nanopb_rpc" ] -
-                   [ "nanopb_rpc" ] != pw_protobuf_GENERATORS
-  if (has_nanopb_rpc) {
-    _generators =
-        pw_protobuf_GENERATORS + [ "nanopb" ] - [ "nanopb" ] + [ "nanopb" ]
+  _base_target = target_name
+
+  if (defined(invoker.deps)) {
+    _invoker_deps = invoker.deps
   } else {
-    _generators = pw_protobuf_GENERATORS
+    _invoker_deps = []
   }
 
-  foreach(_gen, _generators) {
-    _lang_target = "${target_name}.${_gen}"
-    _gen_deps = []
+  # Enumerate all of the protobuf generator targets.
 
-    if (_gen == "nanopb_rpc") {
-      # Generated RPC code depends on the library's core protos.
-      _gen_deps += [ ":${target_name}.nanopb" ]
+  _pw_pwpb_proto_library("${_base_target}.pwpb") {
+    forward_variables_from(invoker, _forwarded_vars)
+    protos = invoker.sources
+    deps = _deps
+    include_file = _include_metadata_file
+    gen_deps = process_file_template(_invoker_deps, "{{source}}.pwpb")
+    protoc_deps = [ "$dir_pw_protobuf/py" ]
+  }
+
+  if (dir_pw_third_party_nanopb != "") {
+    _pw_nanopb_rpc_proto_library("${_base_target}.nanopb_rpc") {
+      forward_variables_from(invoker, _forwarded_vars)
+      protos = invoker.sources
+      deps = _deps
+      include_file = _include_metadata_file
+      gen_deps = process_file_template(_invoker_deps, "{{source}}.nanopb") +
+                 [ ":${_base_target}.nanopb" ]
+      protoc_deps = [ "$dir_pw_rpc/py" ]
     }
 
-    if (defined(invoker.deps)) {
-      _gen_deps += process_file_template(invoker.deps, "{{source}}.${_gen}")
-
-      if (_gen == "nanopb_rpc") {
-        # RPC dependencies also depend on their core generated protos.
-        _gen_deps += process_file_template(invoker.deps, "{{source}}.nanopb")
-      }
+    _pw_nanopb_proto_library("${target_name}.nanopb") {
+      forward_variables_from(invoker, _forwarded_vars)
+      protos = invoker.sources
+      deps = _deps
+      include_file = _include_metadata_file
+      gen_deps = process_file_template(_invoker_deps, "{{source}}.nanopb")
+    }
+  } else {
+    pw_error("${_base_target}.nanopb_rpc") {
+      message =
+          "\$dir_pw_third_party_nanopb must be set to generate nanopb RPC code."
     }
 
-    if (_gen == "pwpb") {
-      _pw_pwpb_proto_library(_lang_target) {
-        forward_variables_from(invoker, _forwarded_vars)
-        protos = invoker.sources
-        deps = _deps
-        include_file = _include_metadata_file
-        gen_deps = _gen_deps
-        protoc_deps = [ "$dir_pw_protobuf/py" ]
-      }
-    } else if (_gen == "nanopb_rpc") {
-      _pw_nanopb_rpc_proto_library(_lang_target) {
-        forward_variables_from(invoker, _forwarded_vars)
-        protos = invoker.sources
-        deps = _deps
-        include_file = _include_metadata_file
-        gen_deps = _gen_deps
-        protoc_deps = [ "$dir_pw_rpc/py" ]
-      }
-    } else if (_gen == "nanopb") {
-      _pw_nanopb_proto_library(_lang_target) {
-        forward_variables_from(invoker, _forwarded_vars)
-        protos = invoker.sources
-        deps = _deps
-        include_file = _include_metadata_file
-        gen_deps = _gen_deps
-      }
-    } else if (_gen == "raw_rpc") {
-      _pw_raw_rpc_proto_library(_lang_target) {
-        forward_variables_from(invoker, _forwarded_vars)
-        protos = invoker.sources
-        deps = _deps
-        include_file = _include_metadata_file
-        gen_deps = _gen_deps
-        protoc_deps = [ "$dir_pw_rpc/py" ]
-      }
-    } else if (_gen == "go") {
-      _pw_go_proto_library(_lang_target) {
-        forward_variables_from(invoker, _forwarded_vars)
-        protos = invoker.sources
-        deps = _deps
-        include_file = _include_metadata_file
-        gen_deps = _gen_deps
-      }
-    } else {
-      assert(false,
-             string_join(
-                 " ",
-                 [
-                   "pw_proto_library doesn't know how to generate code for",
-                   "generator '$_gen'. Please add support if you require it.",
-                 ]))
+    pw_error("${_base_target}.nanopb") {
+      message =
+          "\$dir_pw_third_party_nanopb must be set to compile nanopb protobufs."
     }
   }
 
+  _pw_raw_rpc_proto_library("${target_name}.raw_rpc") {
+    forward_variables_from(invoker, _forwarded_vars)
+    protos = invoker.sources
+    deps = _deps
+    include_file = _include_metadata_file
+    gen_deps = []
+    protoc_deps = [ "$dir_pw_rpc/py" ]
+  }
+
+  _pw_go_proto_library("${target_name}.go") {
+    forward_variables_from(invoker, _forwarded_vars)
+    protos = invoker.sources
+    deps = _deps
+    include_file = _include_metadata_file
+    gen_deps = process_file_template(_invoker_deps, "{{source}}.go")
+  }
+
   # All supported pw_protobuf generators.
   _protobuf_generators = [
     "pwpb",
@@ -508,28 +456,6 @@
     "go",
   ]
 
-  # Create stub versions of the proto library for other protobuf generators.
-  foreach(_gen, _protobuf_generators - _generators) {
-    pw_python_action("${target_name}.${_gen}") {
-      forward_variables_from(invoker, _forwarded_vars)
-      script = string_join("/",
-                           [
-                             dir_pw_protobuf_compiler,
-                             "py",
-                             "pw_protobuf_compiler",
-                             "generator_not_selected.py",
-                           ])
-      args = [
-        "--library",
-        target_name,
-        "--generator",
-        _gen,
-      ]
-      inputs = invoker.sources
-      stamp = true
-    }
-  }
-
   # If the user attempts to use the target directly instead of one of the
   # generator targets, run a script which prints a nice error message.
   pw_python_action(target_name) {
@@ -547,7 +473,7 @@
              get_path_info(".", "abspath"),
              "--root",
              "//",
-           ] + pw_protobuf_GENERATORS
+           ] + _protobuf_generators
     stamp = true
   }
 }
diff --git a/pw_protobuf_compiler/py/BUILD.gn b/pw_protobuf_compiler/py/BUILD.gn
index 6927bb1..3c5f663 100644
--- a/pw_protobuf_compiler/py/BUILD.gn
+++ b/pw_protobuf_compiler/py/BUILD.gn
@@ -21,7 +21,6 @@
   sources = [
     "pw_protobuf_compiler/__init__.py",
     "pw_protobuf_compiler/generate_protos.py",
-    "pw_protobuf_compiler/generator_not_selected.py",
     "pw_protobuf_compiler/proto_target_invalid.py",
     "pw_protobuf_compiler/python_protos.py",
   ]
diff --git a/pw_protobuf_compiler/py/pw_protobuf_compiler/generator_not_selected.py b/pw_protobuf_compiler/py/pw_protobuf_compiler/generator_not_selected.py
deleted file mode 100644
index e089cdb..0000000
--- a/pw_protobuf_compiler/py/pw_protobuf_compiler/generator_not_selected.py
+++ /dev/null
@@ -1,40 +0,0 @@
-# Copyright 2020 The Pigweed Authors
-#
-# Licensed under the Apache License, Version 2.0 (the "License"); you may not
-# use this file except in compliance with the License. You may obtain a copy of
-# the License at
-#
-#     https://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
-# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
-# License for the specific language governing permissions and limitations under
-# the License.
-"""Emits an error when using a protobuf library that is not generated"""
-
-import argparse
-import sys
-
-
-def parse_args():
-    parser = argparse.ArgumentParser(description=__doc__)
-    parser.add_argument('--library',
-                        required=True,
-                        help='The protobuf library being built')
-    parser.add_argument('--generator',
-                        required=True,
-                        help='The protobuf generator requested')
-    return parser.parse_args()
-
-
-def main(library: str, generator: str):
-    print(f'ERROR: Attempting to build protobuf library {library}, but the '
-          f'{generator} protobuf generator is not in use.')
-    print(f'To use {generator} protobufs, list "{generator}" in '
-          'pw_protobuf_GENERATORS.')
-    sys.exit(1)
-
-
-if __name__ == '__main__':
-    main(**vars(parse_args()))
diff --git a/pw_rpc/nanopb/docs.rst b/pw_rpc/nanopb/docs.rst
index 702ae4a..5e3b0d2 100644
--- a/pw_rpc/nanopb/docs.rst
+++ b/pw_rpc/nanopb/docs.rst
@@ -8,23 +8,12 @@
 
 Usage
 =====
-To enable nanopb code generation, add ``nanopb_rpc`` as a generator to your
-Pigweed target's ``pw_protobuf_GENERATORS`` list. Refer to
-:ref:`module-pw_protobuf_compiler` for additional information.
-
-.. code::
-
-  # my_target/target_toolchains.gni
-
-  defaults = {
-    pw_protobuf_GENERATORS = [
-      "pwpb",
-      "nanopb_rpc",  # Enable RPC codegen
-    ]
-  }
+To enable nanopb code generation, the build argument
+``dir_pw_third_party_nanopb`` must be set to point to a local nanopb
+installation.
 
 Define a ``pw_proto_library`` containing the .proto file defining your service
-(and optionally other related protos), then depend on the ``_nanopb_rpc``
+(and optionally other related protos), then depend on the ``nanopb_rpc``
 version of that library in the code implementing the service.
 
 .. code::
@@ -44,7 +33,7 @@
       "chat_service.cc",
       "chat_service.h",
     ]
-    public_deps = [ ":chat_protos_nanopb_rpc" ]
+    public_deps = [ ":chat_protos.nanopb_rpc" ]
   }
 
 A C++ header file is generated for each input .proto file, with the ``.proto``
diff --git a/targets/docs/BUILD.gn b/targets/docs/BUILD.gn
index c82345f..73b1606 100644
--- a/targets/docs/BUILD.gn
+++ b/targets/docs/BUILD.gn
@@ -34,12 +34,6 @@
   defaults = {
     forward_variables_from(_base_toolchain.defaults, "*")
 
-    _has_nanopb_rpc = pw_protobuf_GENERATORS + [ "nanopb_rpc" ] -
-                      [ "nanopb_rpc" ] != pw_protobuf_GENERATORS
-    if (dir_pw_third_party_nanopb != "" && !_has_nanopb_rpc) {
-      pw_protobuf_GENERATORS += [ "nanopb_rpc" ]
-    }
-
     # This is the docs target.
     pw_docgen_BUILD_DOCS = true
   }
diff --git a/targets/host/target_toolchains.gni b/targets/host/target_toolchains.gni
index 462cb02..51be296 100644
--- a/targets/host/target_toolchains.gni
+++ b/targets/host/target_toolchains.gni
@@ -38,13 +38,6 @@
   # Tokenizer trace time.
   pw_trace_tokenizer_time = "$dir_pw_trace_tokenized:host_trace_time"
 
-  # Allow nanopb to be toggled via a build arg on host for easy testing.
-  _has_nanopb = pw_protobuf_GENERATORS + [ "nanopb" ] - [ "nanopb" ] !=
-                pw_protobuf_GENERATORS
-  if (dir_pw_third_party_nanopb != "" && !_has_nanopb) {
-    pw_protobuf_GENERATORS += [ "nanopb" ]
-  }
-
   # Specify builtin GN variables.
   current_os = host_os
   current_cpu = host_cpu