[WIP COMMIT, EXPECT OVERWRITING] lint: change help for pointers to dyn types in FFI

Author: niacdoial

compiler/rustc_lint/messages.ftl

@@ -389,6 +389,11 @@ lint_improper_ctypes_opaque = opaque types have no C equivalent
 lint_improper_ctypes_pat_help = consider using the base type instead
 
 lint_improper_ctypes_pat_reason = pattern types have no C equivalent
+
+lint_improper_ctypes_ptr_dyn_help = to pass an opaque pointer through a FFI boundary, consider replacing `*const dyn ...` by `*const & dyn ...` (or `*mut dyn ...` by `*const &mut dyn ...`)
+
+lint_improper_ctypes_ptr_dyn_reason = pointers to trait objects have no C equivalent
+
 lint_improper_ctypes_slice_help = consider using a raw pointer instead
 
 lint_improper_ctypes_slice_reason = slices have no C equivalent

compiler/rustc_lint/src/types.rs

@@ -1057,6 +1057,12 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                 help: Some(fluent::lint_improper_ctypes_slice_help),
             },
 
+            ty::RawPtr(ty, _) if matches!(ty.kind(), ty::Dynamic(..)) => FfiUnsafe {
+                ty,
+                reason: fluent::lint_improper_ctypes_ptr_dyn_reason,
+                help: Some(fluent::lint_improper_ctypes_ptr_dyn_help),
+            },
+
             ty::Dynamic(..) => {
                 FfiUnsafe { ty, reason: fluent::lint_improper_ctypes_dyn, help: None }
             }