warn less about non-exhaustive in ffi

Bindgen allows generating `#[non_exhaustive] #[repr(u32)]` enums.
This results in nonintuitive nonlocal `improper_ctypes` warnings,
even when the types are otherwise perfectly valid in C.

Adjust for actual tooling expectations by avoiding warning on
simple enums with only unit variants.

compiler/rustc_lint/src/types.rs

@@ -15,7 +15,7 @@ use rustc_attr as attr;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::DiagnosticMessage;
 use rustc_hir as hir;
-use rustc_hir::{is_range_literal, Expr, ExprKind, Node};
+use rustc_hir::{def::CtorKind, is_range_literal, Expr, ExprKind, Node};
 use rustc_middle::ty::layout::{IntegerExt, LayoutOf, SizeSkeleton};
 use rustc_middle::ty::GenericArgsRef;
 use rustc_middle::ty::{
@@ -1136,16 +1136,25 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                             }
                         }
 
-                        if def.is_variant_list_non_exhaustive() && !def.did().is_local() {
-                            return FfiUnsafe {
-                                ty,
-                                reason: fluent::lint_improper_ctypes_non_exhaustive,
-                                help: None,
-                            };
-                        }
+                        // non_exhaustive suggests it is possible that someone might break ABI
+                        // see: https://github.com/rust-lang/rust/issues/44109#issuecomment-537583344
+                        // so warn on complex enums being used outside their crate
+                        let nonexhaustive_nonlocal_ffi =
+                            def.is_variant_list_non_exhaustive() && !def.did().is_local();
 
                         // Check the contained variants.
                         for variant in def.variants() {
+                            // but only warn about really_tagged_union reprs,
+                            // exempt enums with unit ctors like C's (like rust-bindgen)
+                            if nonexhaustive_nonlocal_ffi
+                                && !matches!(variant.ctor_kind(), Some(CtorKind::Const))
+                            {
+                                return FfiUnsafe {
+                                    ty,
+                                    reason: fluent::lint_improper_ctypes_non_exhaustive,
+                                    help: None,
+                                };
+                            };
                             let is_non_exhaustive = variant.is_field_list_non_exhaustive();
                             if is_non_exhaustive && !variant.def_id.is_local() {
                                 return FfiUnsafe {

tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/auxiliary/types.rs

@@ -27,3 +27,14 @@ pub enum NonExhaustiveVariants {
     #[non_exhaustive] Tuple(u32),
     #[non_exhaustive] Struct { field: u32 }
 }
+
+// Note the absence of repr(C): it's not necessary, and recent C code can now use repr hints too.
+#[repr(u32)]
+#[non_exhaustive]
+pub enum NonExhaustiveCLikeEnum {
+    One = 1,
+    Two = 2,
+    Three = 3,
+    Four = 4,
+    Five = 5,
+}

tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/extern_crate_improper.rs

@@ -6,7 +6,7 @@ extern crate types;
 // This test checks that non-exhaustive types with `#[repr(C)]` from an extern crate are considered
 // improper.
 
-use types::{NonExhaustiveEnum, NonExhaustiveVariants, NormalStruct, TupleStruct, UnitStruct};
+use types::{NonExhaustiveEnum, NonExhaustiveCLikeEnum, NonExhaustiveVariants, NormalStruct, TupleStruct, UnitStruct};
 
 extern "C" {
     pub fn non_exhaustive_enum(_: NonExhaustiveEnum);
@@ -21,4 +21,9 @@ extern "C" {
     //~^ ERROR `extern` block uses type `NonExhaustiveVariants`, which is not FFI-safe
 }
 
+// These should pass without remark, as they're C-compatible, despite being "non-exhaustive".
+extern "C" {
+    pub fn non_exhaustive_c_compat_enum(_: NonExhaustiveCLikeEnum);
+}
+
 fn main() {}