Revert "Add recursion limit to FFI safety lint"

This reverts commit 716044751b.

compiler/rustc_lint/messages.ftl

@@ -395,8 +395,6 @@ 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_recursion_limit_reached = type is infinitely recursive
 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

@@ -592,8 +592,6 @@ struct CTypesVisitorState<'tcx> {
     /// The original type being checked, before we recursed
     /// to any other types it contains.
     base_ty: Ty<'tcx>,
-    /// Number of times we recursed while checking the type
-    recursion_depth: usize,
 }
 
 enum FfiResult<'tcx> {
@@ -899,23 +897,12 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
         // Protect against infinite recursion, for example
         // `struct S(*mut S);`.
+        // FIXME: A recursion limit is necessary as well, for irregular
+        // recursive types.
         if !acc.cache.insert(ty) {
             return FfiSafe;
         }
 
-        // Additional recursion check for more complex types like
-        // `struct A<T> { v: *const A<A<T>>, ... }` for which the
-        // cache check above won't be enough (fixes #130310)
-        if !tcx.recursion_limit().value_within_limit(acc.recursion_depth) {
-            return FfiUnsafe {
-                ty: acc.base_ty,
-                reason: fluent::lint_improper_ctypes_recursion_limit_reached,
-                help: None,
-            };
-        }
-
-        acc.recursion_depth += 1;
-
         match *ty.kind() {
             ty::Adt(def, args) => {
                 if let Some(boxed) = ty.boxed_ty()
@@ -1261,8 +1248,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
             return;
         }
 
-        let mut acc =
+        let mut acc = CTypesVisitorState { cache: FxHashSet::default(), base_ty: ty };
-            CTypesVisitorState { cache: FxHashSet::default(), base_ty: ty, recursion_depth: 0 };
         match self.check_type_for_ffi(&mut acc, ty) {
             FfiResult::FfiSafe => {}
             FfiResult::FfiPhantom(ty) => {

tests/crashes/130310.rs

@@ -0,0 +1,15 @@
+//@ known-bug: rust-lang/rust#130310
+
+use std::marker::PhantomData;
+
+#[repr(C)]
+struct A<T> {
+    a: *const A<A<T>>,
+    p: PhantomData<T>,
+}
+
+extern "C" {
+    fn f(a: *const A<()>);
+}
+
+fn main() {}

tests/ui/lint/improper-types-stack-overflow-130310.rs

@@ -1,20 +0,0 @@
-// Regression test for #130310
-// Tests that we do not fall into infinite
-// recursion while checking FFI safety of
-// recursive types like `A<T>` below
-
-//@ build-pass
-use std::marker::PhantomData;
-
-#[repr(C)]
-struct A<T> {
-    a: *const A<A<T>>, // Recursive because of this field
-    p: PhantomData<T>,
-}
-
-extern "C" {
-    fn f(a: *const A<()>);
-    //~^ WARN `extern` block uses type `*const A<()>`, which is not FFI-safe
-}
-
-fn main() {}

tests/ui/lint/improper-types-stack-overflow-130310.stderr

@@ -1,11 +0,0 @@
-warning: `extern` block uses type `*const A<()>`, which is not FFI-safe
-  --> $DIR/improper-types-stack-overflow-130310.rs:16:13
-   |
-LL |     fn f(a: *const A<()>);
-   |             ^^^^^^^^^^^^ not FFI-safe
-   |
-   = note: type is infinitely recursive
-   = note: `#[warn(improper_ctypes)]` on by default
-
-warning: 1 warning emitted
-