Bazel & automatically generated cpp / hpp files - c++

Question

I am starting to use Bazel as my C++ project build system.
However I am stuck with the following problem:
I am in a scenario where I automatically generate the file.hpp file.cpp (literate programming).
To reproduce my problem one can simply use this minimal generator:
-- file.sh --
#!/bin/sh
echo "int foo();" >> file.hpp
echo "#include \"myLib/file.hpp\"\n\nint foo() { return 2017; }" >> file.cpp
My project repo is: (WORKSPACE is an empty file)
├── myLib
│   ├── BUILD
│   └── file.sh
└── WORKSPACE
The BUILD file is
genrule(
name = "tangle_file",
srcs = ["file.sh"],
outs = ["file.cpp","file.hpp"],
cmd = "./$(location file.sh);cp file.cpp $(#D);cp file.hpp $(#D);"
)
cc_library(
name = "file",
srcs = ["file.cpp"],
hdrs = ["file.hpp"],
# deps = [":tangle_file"],
visibility = ["//bin:__pkg__"],
)
I have two problems:
Question (A), dealing with the genrule() part:
The fact that I must use
cmd = "./$(location file.sh);cp file.cpp $(#D);cp file.hpp $(#D);"
is quite mysterious.
My first attempt was:
cmd = "./$(location file.sh)"
However in that case I get the following error:
declared output 'myLib/file.cpp' was not created by genrule. This is probably because the genrule actually didn't create this output, or because the output was a directory and the genrule was run remotely (note that only the contents of declared file outputs are copied from genrules run remotely)
Question (B), dealing with the cc_library() part
I do not know how to make Bazel aware of that the :file target depends on the :tangle_file target.
If I uncomment:
deps = [":tangle_file"],
I get the following error:
in deps attribute of cc_library rule //myLib:file: genrule rule '//myLib:tangle_file' is misplaced here (expected cc_inc_library, cc_library, objc_library, experimental_objc_library or cc_proto_library).

Answer 1

Question (A)
The error that you are seeing is because the genrule cmd is not run inside of its output directory. If you hardcoded bazel-out/local-fastbuild/genfiles/myLib/file.cpp instead of file.cpp in your file.sh script, it would work. However, the recommended approach would be for your script to takes its output directory as an argument.
For example,
genrule(
name = "tangle_file",
srcs = ["file.sh"],
outs = ["file.cpp","file.hpp"],
cmd = "./$(location file.sh) $(#D)"
)
and
#!/bin/sh
echo "int foo();" >> $1/file.hpp
echo "#include \"myLib/file.hpp\"\n\nint foo() { return 2017; }" >> $1/file.cpp
Question (B)
The fact that you have
srcs = ["file.cpp"],
hdrs = ["file.hpp"],
in your cc_library is what tells Bazel that it depends on the genrule, since the genrule creates those files. If you want to make it more explicit, you could use the label syntax, which does the same thing:
srcs = ["//myLib:file.cpp"],
hdrs = ["//myLib:file.hpp"],