Don't use "VCINSTALLDIR" if its target differs from the Rust target

Author: ChrisDenton

src/windows_registry.rs

@@ -67,8 +67,8 @@ pub fn find_tool(target: &str, tool: &str) -> Option<Tool> {
     }
 
     // If VCINSTALLDIR is set, then someone's probably already run vcvars and we
-    // should just find whatever that indicates.
-    if env::var_os("VCINSTALLDIR").is_some() {
+    // should just find whatever that indicates, unless its target arch differs.
+    if env::var_os("VCINSTALLDIR").is_some() && is_vscmd_target(target) {
         return env::var_os("PATH")
             .and_then(|path| {
                 env::split_paths(&path)
@@ -91,6 +91,26 @@ pub fn find_tool(target: &str, tool: &str) -> Option<Tool> {
         .or_else(|| impl_::find_msvc_11(tool, target));
 }
 
+/// Checks to see if the `VSCMD_ARG_TGT_ARCH` environment variable matches the
+/// given target's arch. Returns false otherwise.
+/// Fixes: https://github.com/rust-lang/rust/issues/43468
+#[cfg(windows)]
+fn is_vscmd_target(target: &str) -> bool {
+    // Convert the Rust target arch to its VS arch equivalent.
+	let arch = match target.split("-").next() {
+		Some("x86_64") => "x64",
+		Some("aarch64") => "arm64",
+		Some("i686") | Some("i586") => "x86",
+		Some("thumbv7a") => "arm",
+		_ => return false,
+	};
+	if let Ok(vscmd_target) = std::env::var("VSCMD_ARG_TGT_ARCH") {
+		vscmd_target == arch
+	} else {
+		false
+	}
+}
+
 /// A version of Visual Studio
 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 pub enum VsVers {