explain why __rust_start_panic does not take a Box

diff --git a/src/libstd/panicking.rs b/src/libstd/panicking.rs
index 8f9a4c05b5702fb43283cea0daafedcdb9096464..e0d980fc306b550ea05728106fda0d10f47a6765 100644 (file)
--- a/src/libstd/panicking.rs
+++ b/src/libstd/panicking.rs
@@ -47,6 +47,8 @@ fn __rust_maybe_catch_panic(f: fn(*mut u8),
                                 vtable_ptr: *mut usize) -> u32;
 
     /// `payload` is actually a `*mut &mut dyn BoxMeUp` but that would cause FFI warnings.
+    /// It cannot be `Box<dyn BoxMeUp>` because the other end of this call does not depend
+    /// on liballoc, and thus cannot use `Box`.
     #[unwind(allowed)]
     fn __rust_start_panic(payload: usize) -> u32;
 }