Document correct reason why MTC is not set for child processes

src/main.rs

@@ -385,15 +385,18 @@ 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`).
+ // Note: The main thread checker by default removes itself from
+ // `DYLD_INSERT_LIBRARIES` upon load, see `MTC_RESET_INSERT_LIBRARIES`:
+ // <https://bryce.co/main-thread-checker-configuration/#mtc_reset_insert_libraries>
+ // This means that it is not inherited by child processes, so we have to
+ // tell Cargo to set this environment variable for the processes it
+ // launches (instead of just setting it for Cargo itself 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>.\
+ // _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")