Consider adding user_compile_flags to cargo_build_script's get_cc_compile_args_and_env()

[bazaah]

While investigating how to build rust *-sys crates under rules_rust with custom c/cxx/ld flags (via the --copt|cxxopt|linkopt bazel build options) I discovered that:

1. The rules_rust build script wrapper does honor --linkopts (settings are reflected in LDFLAGS)
2. But seemingly ignores --copt & --cxxopt (settings are not reflected in CFLAGS/CXXFLAGS)

This confused me for a couple hours, but I eventually discovered that seemingly bazel's cc_toolchain (?) does not reflect these in the normal flow of things, and requires one to explicitly pass fragments.cpp.copts + fragments.cpp.cxxopts to cc_common.create_compile_variables() for them to show up in cc_common.get_memory_inefficient_command_line() calls.

This is the patch I have for myself:

cargo/private: add user_compile_flags to honor copt/cxxopt

diff --git a/cargo/private/cargo_build_script.bzl b/cargo/private/cargo_build_script.bzl
index be08907e..f15d5810 100644
--- a/cargo/private/cargo_build_script.bzl
+++ b/cargo/private/cargo_build_script.bzl
@@ -119,7 +119,7 @@ https://github.com/bazelbuild/bazel/issues/15486
     executable = True,
 )

-def get_cc_compile_args_and_env(cc_toolchain, feature_configuration):
+def get_cc_compile_args_and_env(ctx, cc_toolchain, feature_configuration):
     """Gather cc environment variables from the given `cc_toolchain`

     Args:
@@ -135,6 +135,7 @@ def get_cc_compile_args_and_env(cc_toolchain, feature_configuration):
     compile_variables = cc_common.create_compile_variables(
         feature_configuration = feature_configuration,
         cc_toolchain = cc_toolchain,
+        user_compile_flags = ctx.fragments.cpp.copts + ctx.fragments.cpp.cxxopts,
     )
     cc_c_args = cc_common.get_memory_inefficient_command_line(
         feature_configuration = feature_configuration,
@@ -386,7 +387,7 @@ def _cargo_build_script_impl(ctx):
     env["LDFLAGS"] = " ".join(_pwd_flags(link_args))

     # MSVC requires INCLUDE to be set
-    cc_c_args, cc_cxx_args, cc_env = get_cc_compile_args_and_env(cc_toolchain, feature_configuration)
+    cc_c_args, cc_cxx_args, cc_env = get_cc_compile_args_and_env(ctx, cc_toolchain, feature_configuration)
     include = cc_env.get("INCLUDE")
     if include:
         env["INCLUDE"] = include

---

This will have a knock on effect for anyone who's doing c/cpp and rust development in the same workspace / repo, so it may not be worth the disruption, but it strikes me as odd that rules_rust honors --linkopt (which is also a cpp.fragment), but not the other two.

I'm looking for input from a maintainer about whether this is something that should be fixed, or if not; perhaps clarified in the docs somewhere?