Under certain conditions, the CMAKE environment variable is not included in TemplateVariableInfo

[ghost]

Hello,

I've noticed that under some certain conditions, the CMAKE environment variable is not included in the TemplateVariableInfo object owned by the current_cmake_toolchain.

For instance, if CMake is built from source, then the variable is set:

https://github.com/bazelbuild/rules_foreign_cc/blob/c2e097455d2bbf92b2ae71611d1261ba79eb8aa8/toolchains/private/BUILD.bazel#L62-L65

However, when it comes to prebuilt CMake bundle, the variable isn't set.

https://github.com/bazelbuild/rules_foreign_cc/blob/c2e097455d2bbf92b2ae71611d1261ba79eb8aa8/toolchains/prebuilt_toolchains.py#L333-L342

Note that this issue does not affect prebuilt Ninja bundles:

https://github.com/bazelbuild/rules_foreign_cc/blob/c2e097455d2bbf92b2ae71611d1261ba79eb8aa8/toolchains/prebuilt_toolchains.py#L449

I'm trying to compile a rust crate which uses the cmake crate in its build.rs build script. I can include the CMake toolchain in the build script processing using the build_script_toolchains annotation, but since the CMAKE environment variable isn't necessary set, the build script fails to find the CMake binary.

Do you think this is a real issue?

I've written a quick-but-unsound fix locally, but before trying to open a PR to fix it I wanted to make sure that it's a real issue.

[jsharpe]

I think this is just an oversight rather than intentional design - feel free to put together a PR to make this consistent!