Commits on Apr 18, 2024

1. Substitute ${SDKROOT} and ${DEVELOPER_DIR} by their environment v… …

   …ariables on darwin.
   
   This PR attempts to fix an issue with bindgen when used on darwin with clang
   compiled from source (i.e. from `@llvm-project`).
   
   When using clang compiled from source on darwin, libclang (used through `clang-sys`) failed
   to find system header files (such as `stdarg.h` or `stdbool.h`) because these
   files are actually living under the SDK path (under `<SDK_PATH>/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/` and `<SDK_PATH>/usr/include`).
   
   Similar to what Bazel did in their [`xcrunwrapper.sh`](https://github.com/bazelbuild/bazel/blob/d8c27bfcd37a74dfbf1bdb9a1e3df13af8360a01/tools/objc/xcrunwrapper.sh#L17), we must have a way to tell bindgen's libclang
   where to find these system headers.
   
   In this commit, I add the following two substitutes variables:
     - `${SDKROOT}`: replaced by `std::env::var("SDKROOT")` if found.
     - `${DEVELOPER_DIR}`: replaced by `std::env::var("DEVELOPER")` if found.
   
   This is similar to what it is currently done for `${pwd}`.
   
   The following rule should now work on darwin:
   
   ```c
   // mylib.h
   #include <stdarg.h>
   ```
   
   ```starlark
   # BUILD
   cc_library(
       name = "mylib",
       srcs = ["mylib.c"],
       hdrs = ["mylib.h"],
   )
   
   rust_bindgen_library(
       name = "mylib_bindgen",
       cc_lib = ":mylib",
       clang_flags = select({
           "@platforms//os:macos": [
               "--sysroot=${SDKROOT}",
               "-isystem${SDKROOT}/usr/include",
               "-isystem${SDKROOT}/System/Library/Frameworks/Kernel.framework/Versions/A/Headers/",
           ],
           "//conditions:default": [],
       }),
       crate_name = "mylib",
       header = "mylib.h",
   )
   ```
   
   It may also fix bazelbuild#1834.

   

   thb-sb committed Apr 18, 2024
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