fix(loader): allow parallel compilation on windows

Author: amaanq

.github/workflows/build.yml

@@ -74,10 +74,7 @@ jobs:
       run: |
         printf 'EMSCRIPTEN_VERSION=%s\n' "$(<crates/loader/emscripten-version)" >> $GITHUB_ENV
 
-        if [[ '${{ matrix.platform }}' =~ ^windows ]]; then
-          # Prevent race condition (see #2041)
-          printf 'RUST_TEST_THREADS=1\n' >> $GITHUB_ENV
-        elif [[ '${{ matrix.cross }}' == true ]]; then
+        if [[ '${{ matrix.cross }}' == true ]]; then
           for target in armv7-unknown-linux-gnueabihf i686-unknown-linux-gnu powerpc64-unknown-linux-gnu; do
             camel_target=${target//-/_}; target_cc=${target/-unknown/}
             printf 'CC_%s=%s\n' "$camel_target" "${target_cc/v7/}-gcc"

crates/loader/src/loader.rs

@@ -904,12 +904,27 @@ impl Loader {
 
         let output_path = config.output_path.as_ref().unwrap();
 
-        if compiler.is_like_msvc() {
+        let temp_dir = if compiler.is_like_msvc() {
             let out = format!("-out:{}", output_path.to_str().unwrap());
             command.arg(if self.debug_build { "-LDd" } else { "-LD" });
             command.arg("-utf-8");
+
+            // Windows creates intermediate files when compiling (.exp, .lib, .obj), which causes
+            // issues when multiple processes are compiling in the same directory. This creates a
+            // temporary directory for those files to go into, which is deleted after compilation.
+            let temp_dir = output_path.parent().unwrap().join(format!(
+                "tmp_{}_{:?}",
+                std::process::id(),
+                std::thread::current().id()
+            ));
+            std::fs::create_dir_all(&temp_dir).unwrap();
+
+            command.arg(format!("/Fo{}\\", temp_dir.display()));
             command.args(cc_config.get_files());
             command.arg("-link").arg(out);
+            command.arg(format!("/IMPLIB:{}.lib", temp_dir.join("temp").display()));
+
+            Some(temp_dir)
         } else {
             command.arg("-Werror=implicit-function-declaration");
             if cfg!(any(target_os = "macos", target_os = "ios")) {
@@ -921,12 +936,18 @@ impl Loader {
             }
             command.args(cc_config.get_files());
             command.arg("-o").arg(output_path);
-        }
+
+            None
+        };
 
         let output = command.output().with_context(|| {
             format!("Failed to execute the C compiler with the following command:\n{command:?}")
         })?;
 
+        if let Some(temp_dir) = temp_dir {
+            let _ = fs::remove_dir_all(temp_dir);
+        }
+
         FileExt::unlock(lock_file)?;
         fs::remove_file(lock_path)?;
 

crates/xtask/src/test.rs

@@ -73,9 +73,6 @@ pub fn run(args: &Test) -> Result<()> {
             .arg("--no-run")
             .arg("--message-format=json");
 
-        #[cfg(target_os = "windows")]
-        cargo_cmd.arg("--").arg("--test-threads=1");
-
         let cargo_cmd = cargo_cmd.stdout(Stdio::piped()).spawn()?;
 
         let jq_cmd = Command::new("jq")
@@ -103,9 +100,6 @@ pub fn run(args: &Test) -> Result<()> {
         }
         cargo_cmd.args(&args.args);
 
-        #[cfg(target_os = "windows")]
-        cargo_cmd.arg("--").arg("--test-threads=1");
-
         if args.nocapture {
             #[cfg(not(target_os = "windows"))]
             cargo_cmd.arg("--");