Add parameters for copts/cxxopts/conlyopts/linkops to toolchains

[sin-ack]

Problem Definition

I am currently working on an SDK which uses Bazel. The SDK exposes itself as a Bazel package to downstream users, and as part of that, provides its own toolchain. This toolchain was up until recently just using the system C++ compiler (with all the associated badness that entails), and I'm currently on a quest to make it use hermetic_cc_toolchain. However, the old toolchain was passing a bunch of compile flags in its cc_toolchain_configure, and I couldn't find any alternative in hermetic_cc_toolchain that would allow me to encapsulate them in the same way. Currently hermetic_cc_toolchain will have a bunch of pre-determined flags that work for itself (and some special-casing for Go, apparently), but it doesn't allow extending that with custom flags.

My current workaround is adding --copt flags in .bazelrc, but asking every downstream user to maintain this file manually and handle updating it is cumbersome.

Solution

It would be great if I could do this:

load("@hermetic_cc_toolchain//toolchain:defs.bzl", zig_toolchains = "toolchains")

zig_toolchains(
    copts = [
       "-Wall",
       "-Wextra",
       # yadda yadda yadda
    ],
)

This would pass the flags down to every generated toolchain repository through declare_cc_toolchains. Then I would be able to put this in a macro and just tell downstream to call it (via WORKSPACE or module extensions). Maybe in the future it would be worthwhile to be able to specify target-specific flags as well, but this would still solve the majority of the problem for me.