Better document why we use env config instead of just cmd.env

src/main.rs

@@ -384,16 +384,20 @@ fn cargo_careful(args: env::Args) -> Result<()> {
 
     // Enable Main Thread Checker on Apple platforms, as documented here:
     // <https://developer.apple.com/documentation/xcode/diagnosing-memory-thread-and-crash-issues-early#Detect-improper-UI-updates-on-background-threads>
+    //
+    // `DYLD_INSERT_LIBRARIES` is not inherited by child processes, so we need
+    // to tell Cargo to set this environment variable for the processes it
+    // launches (instead of just setting it using `cmd.env`).
+    //
+    // FIXME: We only do this if the host is running on macOS, since cargo
+    // doesn't currently have a good way of passing environment variables to
+    // only target binaries (the `env` config makes `rustc` also run under the
+    // main thread checker, which will probably fail on other platforms).
+    // See <https://github.com/rust-lang/cargo/issues/4505>.\
     let apple_target = target.contains("-darwin")
         || target.contains("-ios")
         || target.contains("-tvos")
         || target.contains("-watchos");
-    // FIXME: We only do this if the host is running on macOS, since cargo
-    // doesn't currently have a good way of passing environment variables only
-    // to target binaries when using `cargo run` or `cargo test` (and this
-    // means that `rustc` is also run under the main thread checker, which will
-    // probably fail on other platforms).
-    // See <https://github.com/rust-lang/cargo/issues/4505>.
     if apple_target && cfg!(target_os = "macos") {
         if let Some(path) = main_thread_checker_path()? {
             cmd.arg("--config");