Document catch_fn in r#try cannot unwind

library/core/src/intrinsics.rs

@@ -2276,6 +2276,8 @@ extern "rust-intrinsic" {
     /// takes the data pointer and a pointer to the target-specific exception
     /// object that was caught. For more information see the compiler's
     /// source as well as std's catch implementation.
+    ///
+    /// `catch_fn` must not unwind.
     #[rustc_nounwind]
     pub fn r#try(try_fn: fn(*mut u8), data: *mut u8, catch_fn: fn(*mut u8, *mut u8)) -> i32;
 

library/std/src/panicking.rs

@@ -498,6 +498,7 @@ pub unsafe fn r#try<R, F: FnOnce() -> R>(f: F) -> Result<R, Box<dyn Any + Send>>
     // This function cannot be marked as `unsafe` because `intrinsics::r#try`
     // expects normal function pointers.
     #[inline]
+    #[rustc_nounwind] // `intrinsic::r#try` requires catch fn to be nounwind
     fn do_catch<F: FnOnce() -> R, R>(data: *mut u8, payload: *mut u8) {
         // SAFETY: this is the responsibility of the caller, see above.
         //