fix wording regarding UB due to unwinding linkage

Author: RalfJung

src/linkage.md

@@ -271,35 +271,39 @@ binary link:
 
 Passing `rlib`s directly into your foreign linker is currently unsupported.
 
-### Prohibited linkage and foreign unwinding
-
-r[link.foreign-code.prohibited]
-
-Undfined behavior may be caused by foreign code unwinding into a Rust crate
-with these characteristics:
-
-* The foreign unwind enters Rust via a function or function pointer declared
-  with an ABI that permits unwinding, that is, `"Rust"` (the default ABI) or
-  any `-unwind` ABI
-* The Rust crate containing the `-unwind` ABI declaration was compiled with
-  `panic=unwind`
-* The final binary is linked with [the `panic=abort` runtime][panic-runtime]
-
-> **Note**: To protect against this undefined behavior, `rustc` does not permit
-> linking the `panic=abort` runtime against any crate that was compiled with
-> `panic=unwind` if that crate also contains a call to a foreign function or
-> function pointer declared with an `-unwind` ABI. Note that this prohibition
-> applies even when linking a static or dynamic library that only includes Rust
-> code, since the resulting library may be subsequently linked against another
-> library that may unwind. However, use of the `Rust` (default) ABI does not
-> cause a link-error, since that ABI is not expected to be used as an
-> entrypoint into a static or shared library.
-
-> **Note**: the restriction can only be violated when mixing code with different
-> `-C panic` flags. This is not possible in normal use of cargo, so most users
-> need not be concerned about this.
-
-r[link.foreign-code.prohibited.lint.ffi_unwind_calls]
+> [!NOTE]
+>  Rust code compiled or linked with a different instance of the Rust runtime counts as a
+> "foreign code" for the purpose of this section.
+
+### Prohibited linkage and unwinding
+
+r[link.unwinding]
+If you are *not* using `rustc` to link Rust files, you must take care to ensure that unwinding is
+handled consistently across the entire binary. This includes using `dlopen` or similar facilities
+where linking is done by the system runtime without `rustc` being involved.
+
+r[link.unwinding.potential]
+A Rust binary or `staticlib` is called *potentially unwinding* if any of the following conditions
+is met:
+- The binary or `staticlib` is linked with [the `panic=unwind` runtime][panic-runtime].
+- The binary or `staticlib` contains a crate built with `-Cpanic=unwind` that makes a call
+  to a function using a `-unwind` ABI.
+- The binary or `staticlib` makes a `"Rust"` ABI call to code running in a separately built
+  `staticlib` (i.e., a separate instance of the Rust runtime), and that other `staticlib` is
+  potentially unwinding.
+
+> [!NOTE]
+> This definition captures whether a `"Rust"` ABI call inside a Rust binary or `staticlib` can ever
+> unwind.
+
+r[link.unwinding.prohibited]
+If a Rust binary or `staticlib` is potentially unwinding, then all its crates must be built with `-Cpanic=unwind`.
+
+> [!NOTE]
+> This restriction can only be violated when mixing code with different `-C panic` flags
+> while also using a non-`rustc` linker. Most users to not have to be concerned about this.
+
+r[link.unwinding.prohibited.lint.ffi_unwind_calls]
 To guarantee that a library will be sound (and linkable with `rustc`)
 regardless of the panic mode used at link-time, the [`ffi_unwind_calls` lint]
 may be used. The lint flags any calls to `-unwind` foreign functions or

src/panic.md

@@ -84,7 +84,7 @@ which will occur:
 * The function returns a `Result::Err` containing an opaque type.
 
 > [!NOTE]
->  Rust code compiled or linked with a different runtime counts as a
+>  Rust code compiled or linked with a different instance of the Rust runtime counts as a
 > "foreign exception" for the purpose of this guarantee. Thus, a library that
 > uses `panic!` and is linked against one version of the Rust standard library,
 > invoked from an application that uses a different version of the standard