Fix rustc wrapper to read command files for linking detection (#4481)

* Fix rustc wrapper to read command files for linking detection

* fix formatting for cargo fmt check

packages/cli/src/rustcwrapper.rs

@@ -14,7 +14,7 @@ pub const DX_RUSTC_WRAPPER_ENV_VAR: &str = "DX_RUSTC";
 /// This is primarily used to intercept cargo, enabling fast hot-patching by caching the environment
 /// cargo setups up for the user's current project.
 ///
-/// In a differenet world we could simply rely on cargo printing link args and the rustc command, but
+/// In a different world we could simply rely on cargo printing link args and the rustc command, but
 /// it doesn't seem to output that in a reliable, parseable, cross-platform format (ie using command
 /// files on windows...), so we're forced to do this interception nonsense.
 pub fn is_wrapping_rustc() -> bool {
@@ -25,19 +25,49 @@ pub fn is_wrapping_rustc() -> bool {
 pub struct RustcArgs {
     pub args: Vec<String>,
     pub envs: Vec<(String, String)>,
-    pub link_args: Vec<String>,
+    pub link_args: Vec<String>, // I don't believe this is used anymore
+}
+
+/// Check if the arguments indicate a linking step, including those in command files.
+fn has_linking_args() -> bool {
+    for arg in std::env::args() {
+        // Direct check for linker-like arguments
+        if arg.ends_with(".o") || arg == "-flavor" {
+            return true;
+        }
+
+        // Check inside command files
+        if let Some(path_str) = arg.strip_prefix('@') {
+            if let Ok(file_binary) = std::fs::read(path_str) {
+                // Handle both UTF-8 and UTF-16LE encodings for response files.
+                let content = String::from_utf8(file_binary.clone()).unwrap_or_else(|_| {
+                    let binary_u16le: Vec<u16> = file_binary
+                        .chunks_exact(2)
+                        .map(|a| u16::from_le_bytes([a[0], a[1]]))
+                        .collect();
+                    String::from_utf16_lossy(&binary_u16le)
+                });
+
+                // Check if any line in the command file contains linking indicators.
+                if content.lines().any(|line| {
+                    let trimmed_line = line.trim().trim_matches('"');
+                    trimmed_line.ends_with(".o") || trimmed_line == "-flavor"
+                }) {
+                    return true;
+                }
+            }
+        }
+    }
+
+    false
 }
 
 /// Run rustc directly, but output the result to a file.
 ///
 /// <https://doc.rust-lang.org/cargo/reference/config.html#buildrustc>
 pub async fn run_rustc() {
-    // if we happen to be both a rustc wrapper and a linker, we want to run the linker if the arguments seem linker-y
+    // If we are being asked to link, delegate to the linker action.
-    // this is a stupid hack
+    if has_linking_args() {
-    if std::env::args()
-        .take(5)
-        .any(|arg| arg.ends_with(".o") || arg == "-flavor" || arg.starts_with("@"))
-    {
         return crate::link::LinkAction::from_env()
             .expect("Linker action not found")
             .run_link()
@@ -45,11 +75,15 @@ pub async fn run_rustc() {
     }
 
     let var_file: PathBuf = std::env::var(DX_RUSTC_WRAPPER_ENV_VAR)
-        .expect("DX_RUSTC not set")
+        .expect("DX_RUSTC env var must be set")
         .into();
 
+    // Cargo invokes a wrapper like: `wrapper-name rustc [args...]`
+    // We skip our own executable name (`wrapper-name`) to get the args passed to us.
+    let captured_args = args().skip(1).collect::<Vec<_>>();
+
     let rustc_args = RustcArgs {
-        args: args().skip(1).collect::<Vec<_>>(),
+        args: captured_args.clone(),
         envs: vars().collect::<_>(),
         link_args: Default::default(),
     };
@@ -63,27 +97,31 @@ pub async fn run_rustc() {
         .nth(1)
         .is_some_and(|name| name != "___")
     {
-        std::fs::create_dir_all(var_file.parent().expect("Failed to get parent dir"))
+        let parent_dir = var_file
-            .expect("Failed to create parent dir");
+            .parent()
-        std::fs::write(
+            .expect("Args file path has no parent directory");
-            &var_file,
+        std::fs::create_dir_all(parent_dir)
-            serde_json::to_string(&rustc_args).expect("Failed to serialize rustc args"),
+            .expect("Failed to create parent directory for args file");
-        )
+
-        .expect("Failed to write rustc args to file");
+        let serialized_args =
+            serde_json::to_string(&rustc_args).expect("Failed to serialize rustc args");
+
+        std::fs::write(&var_file, serialized_args).expect("Failed to write rustc args to file");
     }
 
-    // Run the actual rustc command
+    // Run the actual rustc command.
-    // We want all stdout/stderr to be inherited, so the running process can see the output
+    // We want all stdout/stderr to be inherited, so the user sees the compiler output.
-    //
-    // Note that the args format we get from the wrapper includes the `rustc` command itself, so we
-    // need to skip that - we already skipped the first arg when we created the args struct.
     let mut cmd = std::process::Command::new("rustc");
-    cmd.args(rustc_args.args.iter().skip(1));
+
+    // The first argument in `captured_args` is "rustc", which we need to skip
+    // when passing arguments to the `rustc` command we are spawning.
+    cmd.args(captured_args.iter().skip(1));
     cmd.envs(rustc_args.envs);
     cmd.stdout(std::process::Stdio::inherit());
     cmd.stderr(std::process::Stdio::inherit());
     cmd.current_dir(std::env::current_dir().expect("Failed to get current dir"));
 
-    // Propagate the exit code
+    // Spawn the process and propagate its exit code.
-    std::process::exit(cmd.status().unwrap().code().unwrap())
+    let status = cmd.status().expect("Failed to execute rustc command");
+    std::process::exit(status.code().unwrap_or(1)); // Exit with 1 if process was killed by signal
 }