lint: treat binders in improper_ctypes instead of ICE

This replaces the  in the improper_ctypes
lint with a proper diagnostic. Previously, using an  binder inside
an  block caused an internal compiler error.

Now the lint emits an FFI-safe diagnostic explaining that  binders
are not yet supported in FFI.

Includes a new UI test to ensure the behavior stays stable.

Author: SATVIKsynopsis

compiler/rustc_lint/src/types/improper_ctypes.rs

@@ -669,7 +669,16 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 FfiSafe
             }
 
-            ty::UnsafeBinder(_) => todo!("FIXME(unsafe_binder)"),
+            ty::UnsafeBinder(binder) => {
+                let ty = binder.skip_binder(); // extract the inner type
+
+                FfiUnsafe {
+                    ty,
+                    reason: "types containing `unsafe` binders are not yet fully supported in FFI"
+                        .into(),
+                    help: None,
+                }
+            }
 
             ty::Param(..)
             | ty::Alias(ty::Projection | ty::Inherent | ty::Free, ..)
@@ -1016,7 +1025,6 @@ impl<'tcx> LateLintPass<'tcx> for ImproperCTypesLint {
             | hir::ItemKind::ExternCrate(..) => {}
         }
     }
-
     fn check_field_def(&mut self, cx: &LateContext<'tcx>, field: &'tcx hir::FieldDef<'tcx>) {
         self.check_type_for_external_abi_fnptr(
             cx,

tests/ui/lint/improper-ctypes/unsafe-binder-basic.rs

@@ -0,0 +1,10 @@
+#![feature(unsafe_binders)]
+#![expect(incomplete_features)]
+#![deny(improper_ctypes)]
+
+extern "C" {
+    fn exit_2(x: unsafe<'a> &'a ());
+    //~^ ERROR `extern` block uses type `&()`, which is not FFI-safe
+}
+
+fn main() {}

tests/ui/lint/improper-ctypes/unsafe-binder-basic.stderr

@@ -0,0 +1,14 @@
+error: `extern` block uses type `&()`, which is not FFI-safe
+  --> $DIR/unsafe-binder-basic.rs:6:18
+   |
+LL |     fn exit_2(x: unsafe<'a> &'a ());
+   |                  ^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = note: types containing `unsafe` binders are not yet fully supported in FFI
+note: the lint level is defined here
+  --> $DIR/unsafe-binder-basic.rs:3:9
+   |
+LL | #![deny(improper_ctypes)]
+   |         ^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error