comment on safety

Author: workingjubilee

src/backtrace/win32.rs

@@ -94,6 +94,9 @@ impl Frame {
 #[repr(C, align(16))] // required by `CONTEXT`, is a FIXME in windows metadata right now
 struct MyContext(CONTEXT);
 
+/// # Safety
+/// This function makes many calls to dbghelp, which must be called in a single-threaded fashion.
+/// Therefore, it is unsound to call this function concurrently without synchronization.
 #[inline(always)]
 pub unsafe fn trace(cb: &mut dyn FnMut(&super::Frame) -> bool) {
     // Allocate necessary structures for doing the stack walk