Ensure ffi_unwind_calls lint is gated behind c_unwind

src/test/ui/unwind-abis/feature-gate-c-unwind.rs

@@ -1,6 +1,10 @@
 // Test that the "C-unwind" ABI is feature-gated, and cannot be used when the
 // `c_unwind` feature gate is not used.
 
+#![allow(ffi_unwind_calls)]
+//~^ WARNING unknown lint: `ffi_unwind_calls`
+//~| WARNING unknown lint: `ffi_unwind_calls`
+
 extern "C-unwind" fn f() {}
 //~^ ERROR C-unwind ABI is experimental and subject to change [E0658]
 

src/test/ui/unwind-abis/feature-gate-c-unwind.stderr

@@ -1,5 +1,16 @@
+warning: unknown lint: `ffi_unwind_calls`
+  --> $DIR/feature-gate-c-unwind.rs:4:1
+   |
+LL | #![allow(ffi_unwind_calls)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(unknown_lints)]` on by default
+   = note: the `ffi_unwind_calls` lint is unstable
+   = note: see issue #74990 <https://github.com/rust-lang/rust/issues/74990> for more information
+   = help: add `#![feature(c_unwind)]` to the crate attributes to enable
+
 error[E0658]: C-unwind ABI is experimental and subject to change
-  --> $DIR/feature-gate-c-unwind.rs:4:8
+  --> $DIR/feature-gate-c-unwind.rs:8:8
    |
 LL | extern "C-unwind" fn f() {}
    |        ^^^^^^^^^^
@@ -7,6 +18,16 @@ LL | extern "C-unwind" fn f() {}
    = note: see issue #74990 <https://github.com/rust-lang/rust/issues/74990> for more information
    = help: add `#![feature(c_unwind)]` to the crate attributes to enable
 
-error: aborting due to previous error
+warning: unknown lint: `ffi_unwind_calls`
+  --> $DIR/feature-gate-c-unwind.rs:4:1
+   |
+LL | #![allow(ffi_unwind_calls)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `ffi_unwind_calls` lint is unstable
+   = note: see issue #74990 <https://github.com/rust-lang/rust/issues/74990> for more information
+   = help: add `#![feature(c_unwind)]` to the crate attributes to enable
+
+error: aborting due to previous error; 2 warnings emitted
 
 For more information about this error, try `rustc --explain E0658`.