Rollup merge of rust-lang#130758 - compiler-errors:ctype-recursion-limit, r=jieyouxu

Revert "Add recursion limit to FFI safety lint"

It's not necessarily clear if warning when we hit the recursion limit is the right thing to do, first of all.

**More importantly**, this PR was implemented incorrectly in the first place; it was not decrementing the recursion limit when stepping out of a type, so it would trigger when a ctype has more than RECURSION_LIMIT fields *anywhere* in the type's set of recursively reachable fields.

Reverts rust-lang#130598
Reopens rust-lang#130310
Fixes rust-lang#130757

Author: compiler-errors

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 =
-            CTypesVisitorState { cache: FxHashSet::default(), base_ty: ty, recursion_depth: 0 };
+        let mut acc = CTypesVisitorState { cache: FxHashSet::default(), base_ty: ty };
         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

@@ -0,0 +1,32 @@
+//@ check-pass
+
+#![recursion_limit = "5"]
+#![allow(unused)]
+#![deny(improper_ctypes)]
+
+#[repr(C)]
+struct F1(*const ());
+#[repr(C)]
+struct F2(*const ());
+#[repr(C)]
+struct F3(*const ());
+#[repr(C)]
+struct F4(*const ());
+#[repr(C)]
+struct F5(*const ());
+#[repr(C)]
+struct F6(*const ());
+
+#[repr(C)]
+struct B {
+    f1: F1,
+    f2: F2,
+    f3: F3,
+    f4: F4,
+    f5: F5,
+    f6: F6,
+}
+
+extern "C" fn foo(_: B) {}
+
+fn main() {}