Rollup merge of rust-lang#121551 - nbdd0121:ffi_unwind, r=RalfJung

Forbid use of `extern "C-unwind"` inside standard library

Those libraries are build with `-C panic=unwind` and is expected to be linkable to `-C panic=abort` library. To ensure unsoundness compiler needs to prevent a `C-unwind` call to exist, as doing so may leak foreign exceptions into `-C panic=abort`.

r? ``@RalfJung``

Author: matthiaskrgr

library/alloc/src/lib.rs

@@ -92,6 +92,7 @@
 #![warn(multiple_supertrait_upcastable)]
 #![allow(internal_features)]
 #![allow(rustdoc::redundant_explicit_links)]
+#![deny(ffi_unwind_calls)]
 //
 // Library features:
 // tidy-alphabetical-start

library/core/src/lib.rs

@@ -106,6 +106,7 @@
 #![allow(incomplete_features)]
 #![warn(multiple_supertrait_upcastable)]
 #![allow(internal_features)]
+#![deny(ffi_unwind_calls)]
 // Do not check link redundancy on bootstraping phase
 #![allow(rustdoc::redundant_explicit_links)]
 //

library/proc_macro/src/lib.rs

@@ -36,6 +36,7 @@
 #![feature(strict_provenance)]
 #![recursion_limit = "256"]
 #![allow(internal_features)]
+#![deny(ffi_unwind_calls)]
 
 #[unstable(feature = "proc_macro_internals", issue = "27812")]
 #[doc(hidden)]