I'm setting up a C/C++ VS Code project using CMake Tools that I want to share, and that makes use of a custom toolchain that the user is required to install. Because the toolchain is specified using the toolchainFile setting in either cmake-kits.json or CMakePresents.json and there's no standardized place for this toolchain to go, the project needs the user the specify its path in a way that can be referenced in various workspace configuration files like c_cpp_properties.json, cmake-kits.json, CMakePresents.json, or tasks.json.
Because this setting applies per-system, it ought to go in a file that's not going to be tracked in git. Ideally it would only be defined within the scope of this one project.
I'm aware that VS Code allows looking up the value of environment variables using ${env:VARIABLE_NAME} syntax, but I'd like to avoid environment variables if I can, because these settings are only needed for this one project, and I don't particularly like cluttering up the system-wide environment with variables that are used for one isolated purpose. (Unless, that is, there's some way to define environment variables that would only apply to this project.)
What are some options for doing this?
I'd use CMakeUserPresets.json. Use the toolchainFile property in a configure preset. Cache variables can be specified in the cacheVariables property of a configure preset.
For what it's worth, presets also support reading from environment variables. I make use of this in a project with an EMSDK environment variable for Emscripten. But again, anyone can define a CMakeUserPresets.json file and put whatever they want in it as long as it's been properly gitignored.
there's no standardized place for this toolchain to go, the project needs the user the specify its path in a way that can be referenced in various workspace configuration files like c_cpp_properties.json, cmake-kits.json, CMakePresents.json, or tasks.json.
The claim that presets "aren't standardized" is just false. If you're using CMake Tools, and you use CMake Presets, then CMake Tools will pick up the presets and use them. You wouldn't need to use c_cpp_properties.json, cmake-kits.json, or tasks.json. CMake presets and CMake kits are mutually exclusive. Presets are defined by CMake itself, and kits are specific to CMake Tools, and I'm pretty sure were created before presets. You don't really need c_cpp_properties.json once you've defined CMake Tools as the configuration provider, and CMake Tools provides commands for configure and build which use the currently selected configure and build presets, so you don't need to manually define VS Code tasks.