Worked on expanding the generator so it works with protocol buffers. Does not work yet.

backends.py

@@ -535,6 +535,8 @@ class NinjaBackend(Backend):
                 outfilename = os.path.join(self.get_target_private_dir(target), outfilelist[i])
                 args = [x.replace("@INPUT@", infilename).replace('@OUTPUT@', outfilename)\
                         for x in base_args]
+                args = [x.replace("@SOURCE_DIR@", self.environment.get_source_dir()).replace("@BUILD_DIR@", self.environment.get_build_dir())
+                        for x in args]
                 cmdlist = [exe_file] + args
                 elem = NinjaBuildElement(outfilename, 'CUSTOM_COMMAND', infilename)
                 elem.add_item('DESC', 'Generating $out')

test cases/frameworks/5 protocol buffers/defs.proto

@@ -0,0 +1,3 @@
+message Dummy {
+  required string text = 1;
+}

test cases/frameworks/5 protocol buffers/main.cpp

@@ -0,0 +1,6 @@
+#include "defs.pb.h"
+
+int main(int argc, char **argv) {
+    Dummy d;
+    return 0;
+}

test cases/frameworks/5 protocol buffers/meson.build

@@ -0,0 +1,14 @@
+project('protocol buffer test', 'cxx')
+
+protoc = find_program('protoc', required : true)
+dep = find_dep('protobuf', required : true)
+
+gen = generator(protoc, \
+  output_name  : '@BASENAME@.pb.cc',
+  arguments : ['-I=@SOURCE_DIR@', '--cpp_out=@BUILD_DIR@', '@INPUT@'])
+
+generated = gen.process('defs.proto')
+e = executable('prog', 'main.cpp', generated,
+ include_dirs: include_directories('.'),
+ dep : dep)
+add_test('prototest', e)