fix: search for the sysroot in compile args

[lukasoyen]

Wit the new cc_rules based toolchains, the sysroot on the toolchain is not set. But we can infer it from the command line args a normal compilation with that toolchain would get.

[lukasoyen]

I am quite sure that this is the wrong place and should be moved into _impl of cuda_toolchain_config. Currently this get_memory_inefficient_command_line() is called for every? cuda_compile() action.

Moving that would require plumbing through a new sysroot attribute and possibly the sysroot feature in cuda_toolchain_config has never been tested. So I hope I can leave that improvement for someone else.

[cloudhan]

@lukasoyen Do you know anyway to query the info from cc_toolchain directly. This creates an unused args on each action run. And is quite wasteful... Better way should fuse it into the toolchain configuration.

[cloudhan]

Seems to be designed to be accessible from cc_toolchain directly

https://github.com/bazelbuild/rules_cc/blob/6be85c266b1df3a5bd38290c814d83ee643185dd/cc/private/rules_impl/cc_flags_supplier_lib.bzl#L57-L62

Then we can attach it to our cuda_info then add a corresponding feature for it.

[lukasoyen]

Seems to be designed to be accessible from cc_toolchain directly

I read through that code while figuring things out. I think that is different and not meant for the rule based toolchains.

• This is the corresponding code for the rule based toolchains. It (and the old variant) use CC_FLAGS_MAKE_VARIABLE_ACTION_NAME
• CC_FLAGS_MAKE_VARIABLE_ACTION_NAME is not used in cc_sysroot
• cc_toolchain_config does not set builtin_sysroot

I think the rule based toolchains are designed in a way to make the builtin_sysroot obsolete.

[lukasoyen]

Do you know anyway to query the info from cc_toolchain directly.

As said in #313 (comment) I agree this is wasteful and slow. I don't see a reason to put the detection into the toolchain config. The CC toolchain is an attribute already at the detection already queries the CC toolchain for the compiler binary.

I am just not sure how all that plumbing would turn out. But I am up to try if you prefer it that way.