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.

3 files changed, 25 insertions, 6 deletions

diff --git a/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/auxiliary/types.rs b/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/auxiliary/types.rs
index d6251fcb768..4dc5932feab 100644
--- a/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/auxiliary/types.rs
+++ b/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,
+}
diff --git a/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/extern_crate_improper.rs b/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/extern_crate_improper.rs
index 7a9b465bb56..c7f470fb787 100644
--- a/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/extern_crate_improper.rs
+++ b/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/extern_crate_improper.rs
@@ -6,7 +6,10 @@ 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::{
+    NonExhaustiveCLikeEnum, NonExhaustiveEnum, NonExhaustiveVariants,
+    NormalStruct, TupleStruct, UnitStruct,
+};
 
 extern "C" {
     pub fn non_exhaustive_enum(_: NonExhaustiveEnum);
@@ -21,4 +24,9 @@ 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() {}
diff --git a/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/extern_crate_improper.stderr b/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/extern_crate_improper.stderr
index 43c8e1015e6..afc3d3838ad 100644
--- a/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/extern_crate_improper.stderr
+++ b/tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/extern_crate_improper.stderr
@@ -1,5 +1,5 @@
 error: `extern` block uses type `NonExhaustiveEnum`, which is not FFI-safe
-  --> $DIR/extern_crate_improper.rs:12:35
+  --> $DIR/extern_crate_improper.rs:15:35
    |
 LL |     pub fn non_exhaustive_enum(_: NonExhaustiveEnum);
    |                                   ^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -12,7 +12,7 @@ LL | #![deny(improper_ctypes)]
    |         ^^^^^^^^^^^^^^^
 
 error: `extern` block uses type `NormalStruct`, which is not FFI-safe
-  --> $DIR/extern_crate_improper.rs:14:44
+  --> $DIR/extern_crate_improper.rs:17:44
    |
 LL |     pub fn non_exhaustive_normal_struct(_: NormalStruct);
    |                                            ^^^^^^^^^^^^ not FFI-safe
@@ -20,7 +20,7 @@ LL |     pub fn non_exhaustive_normal_struct(_: NormalStruct);
    = note: this struct is non-exhaustive
 
 error: `extern` block uses type `UnitStruct`, which is not FFI-safe
-  --> $DIR/extern_crate_improper.rs:16:42
+  --> $DIR/extern_crate_improper.rs:19:42
    |
 LL |     pub fn non_exhaustive_unit_struct(_: UnitStruct);
    |                                          ^^^^^^^^^^ not FFI-safe
@@ -28,7 +28,7 @@ LL |     pub fn non_exhaustive_unit_struct(_: UnitStruct);
    = note: this struct is non-exhaustive
 
 error: `extern` block uses type `TupleStruct`, which is not FFI-safe
-  --> $DIR/extern_crate_improper.rs:18:43
+  --> $DIR/extern_crate_improper.rs:21:43
    |
 LL |     pub fn non_exhaustive_tuple_struct(_: TupleStruct);
    |                                           ^^^^^^^^^^^ not FFI-safe
@@ -36,7 +36,7 @@ LL |     pub fn non_exhaustive_tuple_struct(_: TupleStruct);
    = note: this struct is non-exhaustive
 
 error: `extern` block uses type `NonExhaustiveVariants`, which is not FFI-safe
-  --> $DIR/extern_crate_improper.rs:20:38
+  --> $DIR/extern_crate_improper.rs:23:38
    |
 LL |     pub fn non_exhaustive_variant(_: NonExhaustiveVariants);
    |                                      ^^^^^^^^^^^^^^^^^^^^^ not FFI-safe