build: Only replace cl.exe with clang-cl for ARM64 Windows builds

When cross-compiling to Windows from Linux or similar, it's common to
use the `clang-cl` driver from the LLVM toolchain which supports parsing
`cl.exe`-like arguments.

Ring however doesn't seem to compile for ARM64 Windows using `cl.exe`,
and contains a `// FIXME`-style workaround to use `clang` to compile its
C files instead.

The command-line interface for `clang` isn't always compatible with that
of `cl.exe` and `clang-cl`.  There didn't seem to be any trouble with
this yet, but when cross-compiling from Linux it's common to explicitly
provide "sysroots" via `-vctoolsdir` and `-winsdkdir` in `CFLAGS`.
In such a setup this workaround in `ring` would pass those arguments
to `clang`, resulting in "unknown argument" errors.

`cc-rs` can tell us exactly what compiler it found, and we can use this
information to decide how to fill in this workaround.  If the user was
already compiling their code with `clang`, nothing has to be replaced.
In the end, all this entails is changing the workaround to not compile
with `clang`, but with `clang-cl` instead.

BUILDING.md

@@ -22,9 +22,11 @@ edition of Visual Studio, like Community, Standard, or Enterprise). The
 “Desktop development with C++” workflow must be installed. Visual Studio
 2022 Version 17.5 is supported; earlier versions of Visual Studio may work.
 
+### (Cross-)compiling to Windows ARM64
+
 For Windows ARM64 targets (aarch64-pc-windows-msvc), the Visual Studio Build
 Tools “VS 2022 C++ ARM64 build tools” and "clang" components must be installed.
-Add Microsoft's provided version of `clang` to `%PATH%`, which will allow the
+Add Microsoft's provided version of `clang-cl` to `%PATH%`, which will allow the
 build to work in GitHub Actions without installing anything:
 ```
 $env:Path += ";C:\Program Files (x86)\Microsoft Visual Studio\2022\Enterprise\VC\Tools\Llvm\x64\bin"

Cargo.toml

@@ -227,3 +227,8 @@ default-members = [
     ".",
     "cavp"
 ]
+
+[patch.crates-io]
+# https://github.com/rust-lang/cc-rs/pull/1357
+# https://github.com/rust-lang/cc-rs/pull/1358
+cc = { git = "https://github.com/rust-lang/cc-rs", rev = "e551e8b" }

build.rs

@@ -147,6 +147,7 @@ fn cpp_flags(compiler: &cc::Tool) -> &'static [&'static str] {
         NON_MSVC_FLAGS
     } else {
         static MSVC_FLAGS: &[&str] = &[
+            "/std:c11",
             "/Gy", // Enable function-level linking.
             "/Zc:wchar_t",
             "/Zc:forScope",
@@ -547,10 +548,18 @@ fn obj_path(out_dir: &Path, src: &Path) -> PathBuf {
 
 fn configure_cc(c: &mut cc::Build, target: &Target, c_root_dir: &Path, include_dir: &Path) {
     let compiler = c.get_compiler();
-    // FIXME: On Windows AArch64 we currently must use Clang to compile C code
-    let compiler = if target.os == WINDOWS && target.arch == AARCH64 && !compiler.is_like_clang() {
-        let _ = c.compiler("clang");
-        c.get_compiler()
+    // FIXME: On Windows AArch64 we currently must use LLVM (clang-cl) to compile C code
+    let compiler = if target.os == WINDOWS && target.arch == AARCH64 {
+        match compiler.family() {
+            cc::ToolFamily::Clang { .. } | cc::ToolFamily::Msvc { clang_cl: true, .. } => compiler,
+            cc::ToolFamily::Msvc {
+                clang_cl: false, ..
+            } => {
+                c.compiler("clang-cl");
+                c.get_compiler()
+            }
+            t => panic!("Unsupported tool `{t:?}` for ARM64 Windows compilation"),
+        }
     } else {
         compiler
     };