tools/build.rs: Only run when compiling for *-windows-msvc, and e…

…rror if trying to cross compile to `*-windows-msvc`.

tools/build.rs

@@ -1,38 +1,52 @@
+use std::env;
+
 fn main() {
+    let os = env::var("CARGO_CFG_TARGET_OS").unwrap();
+    let env = env::var("CARGO_CFG_TARGET_ENV").unwrap();
+
+    let os = os.as_str();
+    let env = env.as_str();
+
+    match (os, env) {
+        ("windows", "msvc") => {
+            if !cfg!(all(target_os = "windows", target_env = "msvc")) {
+                panic!("Cannot cross compile to *-windows-msvc");
+            }
+        }
+        #[allow(clippy::needless_return)]
+        _ => return,
+    }
+
     // NOTE: we rely on libraries that are only distributed for Windows so
     // targeting Windows/MSVC is not supported when cross compiling.
     #[cfg(all(target_os = "windows", target_env = "msvc"))]
     {
         use cc::windows_registry;
-        use std::env;
 
-        let os = env::var("CARGO_CFG_TARGET_OS").expect("missing CARGO_CFG_TARGET_OS");
-        let target = env::var("TARGET").expect("missing TARGET");
-        if os == "windows" {
-            // for sprintf, snprintf, etc.
-            println!("cargo:rustc-link-lib=static=oldnames");
-            let tool = windows_registry::find_tool(&target, "cl.exe")
-                .expect("couldn't find cl.exe; are the Visual Studio C++ tools installed?");
-            let lib_paths = &tool
-                .env()
-                .iter()
-                .find(|(key, _val)| key == "LIB")
-                .expect("LIB path not found")
-                .1;
-            for path in lib_paths.to_str().unwrap().split(';') {
-                if path != "" {
-                    println!("cargo:rustc-link-search={path}");
-                }
+        // for sprintf, snprintf, etc.
+        let target = env::var("TARGET").unwrap();
+        println!("cargo:rustc-link-lib=static=oldnames");
+        let tool = windows_registry::find_tool(&target, "cl.exe")
+            .expect("couldn't find cl.exe; are the Visual Studio C++ tools installed?");
+        let lib_paths = &tool
+            .env()
+            .iter()
+            .find(|(key, _val)| key == "LIB")
+            .expect("LIB path not found")
+            .1;
+        for path in lib_paths.to_str().unwrap().split(';') {
+            if path != "" {
+                println!("cargo:rustc-link-search={path}");
             }
-
-            let getopt = "getopt";
-            cc::Build::new()
-                .files([&"../tools/compat/getopt.c"])
-                .include("../include/compat")
-                .debug(cfg!(debug_assertions))
-                .compile(&getopt);
-            // cc automatically outputs the following line
-            // println!("cargo:rustc-link-lib=static={getopt}");
         }
+
+        let getopt = "getopt";
+        cc::Build::new()
+            .files([&"../tools/compat/getopt.c"])
+            .include("../include/compat")
+            .debug(cfg!(debug_assertions))
+            .compile(&getopt);
+        // cc automatically outputs the following line
+        // println!("cargo:rustc-link-lib=static={getopt}");
     }
 }