Historically, the bindgen configuration has been using a compiler that is not the build compiler (gcc or msvc), and does its own detection for clang. However, now that clang/clang-cl is the default, it can end up using a different version as shown in bug 1526497. The situation should be improved.

The current setup for bindgen relies on either finding clang/libclang
from the output of llvm-config, or from the paths given via the
configure flags --with-clang-path/--with-libclang-path.

One very common problem is that the llvm-config we end up using does
not correspond to the clang used for compilation, which has some
undesirable side effect, like failing to build.

So instead of relying on llvm-config, we do the following:

• when the compiler is clang, we just use that
• when the compiler is clang-cl, we use clang from the same directory
• otherwise, we either try to find clang in PATH, or rely on
  --with-clang-path.

Once clang is found, we try to deduce the location of the corresponding
libclang via the output of clang -print-search-dirs, or rely on
--with-libclang-path.

Can we get this reviewed?

https://hg.mozilla.org/mozilla-central/rev/ccd1356fc8f1

I can't build anymore. Somehow this causes
ERROR: Could not find libclang to generate rust bindings for C/C++. Please install the necessary packages, run mach bootstrap, or use --with-libclang-path to give the path containing it.

And running ./mach bootstrap does not help.

This make by mac build succeed, thanks!