Disallow single-variant enums

Couldn't find documentation supporting that single-variant
`#[repr(Rust)]` enums with RHS assigned work as expected with this
change.

```rust
enum Variants {
  A = 17,
} // Would this be zero sized optimized guaranteed?
```

Author: MasterAwesome

compiler/rustc_lint/src/types.rs

@@ -1102,22 +1102,24 @@ fn get_nullable_type<'tcx>(
 /// A type is niche_optimization_candiate iff:
 /// - Is a zero-sized type with alignment 1 (a “1-ZST”).
 /// - Has no fields.
-/// - Does not have the #[non_exhaustive] attribute.
+/// - Does not have the `#[non_exhaustive]` attribute.
 fn is_niche_optimization_candidate<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     ty: Ty<'tcx>,
 ) -> bool {
-    if !tcx.layout_of(param_env.and(ty)).is_ok_and(|layout| layout.is_1zst()) {
+    if tcx.layout_of(param_env.and(ty)).is_ok_and(|layout| !layout.is_1zst()) {
         return false;
     }
 
     match ty.kind() {
         ty::Adt(ty_def, _) => {
             let non_exhaustive = ty_def.is_variant_list_non_exhaustive();
-            let contains_no_fields = ty_def.all_fields().next().is_none();
+            // Should single-variant enums be allowed?
+            let empty = (ty_def.is_struct() && ty_def.all_fields().next().is_none())
+                || (ty_def.is_enum() && ty_def.variants().is_empty());
 
-            !non_exhaustive && contains_no_fields
+            !non_exhaustive && empty
         }
         ty::Tuple(tys) => tys.is_empty(),
         _ => false,

tests/ui/lint/lint-ctypes-enum.rs

@@ -123,6 +123,7 @@ extern "C" {
     fn result_phantom_t(x: Result<num::NonZero<u8>, std::marker::PhantomData<()>>);
     fn result_1zst_exhaustive_no_variant_t(x: Result<num::NonZero<u8>, Z>);
     fn result_1zst_exhaustive_single_variant_t(x: Result<num::NonZero<u8>, U>);
+    //~^ ERROR `extern` block uses type
     fn result_1zst_exhaustive_multiple_variant_t(x: Result<num::NonZero<u8>, B>);
     //~^ ERROR `extern` block uses type
     fn result_1zst_non_exhaustive_no_variant_t(x: Result<num::NonZero<u8>, NonExhaustive>);
@@ -160,6 +161,7 @@ extern "C" {
     fn result_phantom_e(x: Result<num::NonZero<u8>, std::marker::PhantomData<()>>);
     fn result_1zst_exhaustive_no_variant_e(x: Result<Z, num::NonZero<u8>>);
     fn result_1zst_exhaustive_single_variant_e(x: Result<U, num::NonZero<u8>>);
+    //~^ ERROR `extern` block uses type
     fn result_1zst_exhaustive_multiple_variant_e(x: Result<B, num::NonZero<u8>>);
     //~^ ERROR `extern` block uses type
     fn result_1zst_non_exhaustive_no_variant_e(x: Result<NonExhaustive, num::NonZero<u8>>);

tests/ui/lint/lint-ctypes-enum.stderr

@@ -113,8 +113,17 @@ LL |     fn result_repr_rust_t(x: Result<Rust<num::NonZero<u8>>, ()>);
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
 
+error: `extern` block uses type `Result<NonZero<u8>, U>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-enum.rs:125:51
+   |
+LL |     fn result_1zst_exhaustive_single_variant_t(x: Result<num::NonZero<u8>, U>);
+   |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
 error: `extern` block uses type `Result<NonZero<u8>, B>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:126:53
+  --> $DIR/lint-ctypes-enum.rs:127:53
    |
 LL |     fn result_1zst_exhaustive_multiple_variant_t(x: Result<num::NonZero<u8>, B>);
    |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -123,7 +132,7 @@ LL |     fn result_1zst_exhaustive_multiple_variant_t(x: Result<num::NonZero<u8>
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<NonZero<u8>, NonExhaustive>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:128:51
+  --> $DIR/lint-ctypes-enum.rs:129:51
    |
 LL |     fn result_1zst_non_exhaustive_no_variant_t(x: Result<num::NonZero<u8>, NonExhaustive>);
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -132,7 +141,7 @@ LL |     fn result_1zst_non_exhaustive_no_variant_t(x: Result<num::NonZero<u8>,
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<NonZero<u8>, Field>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:131:49
+  --> $DIR/lint-ctypes-enum.rs:132:49
    |
 LL |     fn result_1zst_exhaustive_single_field_t(x: Result<num::NonZero<u8>, Field>);
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -141,7 +150,7 @@ LL |     fn result_1zst_exhaustive_single_field_t(x: Result<num::NonZero<u8>, Fi
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<Result<(), NonZero<u8>>, ()>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:133:30
+  --> $DIR/lint-ctypes-enum.rs:134:30
    |
 LL |     fn result_cascading_t(x: Result<Result<(), num::NonZero<u8>>, ()>);
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -150,23 +159,23 @@ LL |     fn result_cascading_t(x: Result<Result<(), num::NonZero<u8>>, ()>);
    = note: enum has no representation hint
 
 error: `extern` block uses type `u128`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:144:33
+  --> $DIR/lint-ctypes-enum.rs:145:33
    |
 LL |     fn result_nonzero_u128_e(x: Result<(), num::NonZero<u128>>);
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
    |
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `i128`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:151:33
+  --> $DIR/lint-ctypes-enum.rs:152:33
    |
 LL |     fn result_nonzero_i128_e(x: Result<(), num::NonZero<i128>>);
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
    |
    = note: 128-bit integers don't currently have a known stable ABI
 
 error: `extern` block uses type `Result<(), TransparentUnion<NonZero<u8>>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:156:38
+  --> $DIR/lint-ctypes-enum.rs:157:38
    |
 LL |     fn result_transparent_union_e(x: Result<(), TransparentUnion<num::NonZero<u8>>>);
    |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -175,16 +184,25 @@ LL |     fn result_transparent_union_e(x: Result<(), TransparentUnion<num::NonZe
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<(), Rust<NonZero<u8>>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:158:30
+  --> $DIR/lint-ctypes-enum.rs:159:30
    |
 LL |     fn result_repr_rust_e(x: Result<(), Rust<num::NonZero<u8>>>);
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
    |
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
 
+error: `extern` block uses type `Result<U, NonZero<u8>>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-enum.rs:163:51
+   |
+LL |     fn result_1zst_exhaustive_single_variant_e(x: Result<U, num::NonZero<u8>>);
+   |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
 error: `extern` block uses type `Result<B, NonZero<u8>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:163:53
+  --> $DIR/lint-ctypes-enum.rs:165:53
    |
 LL |     fn result_1zst_exhaustive_multiple_variant_e(x: Result<B, num::NonZero<u8>>);
    |                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -193,7 +211,7 @@ LL |     fn result_1zst_exhaustive_multiple_variant_e(x: Result<B, num::NonZero<
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<NonExhaustive, NonZero<u8>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:165:51
+  --> $DIR/lint-ctypes-enum.rs:167:51
    |
 LL |     fn result_1zst_non_exhaustive_no_variant_e(x: Result<NonExhaustive, num::NonZero<u8>>);
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -202,7 +220,7 @@ LL |     fn result_1zst_non_exhaustive_no_variant_e(x: Result<NonExhaustive, num
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<Field, NonZero<u8>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:168:49
+  --> $DIR/lint-ctypes-enum.rs:170:49
    |
 LL |     fn result_1zst_exhaustive_single_field_e(x: Result<Field, num::NonZero<u8>>);
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
@@ -211,13 +229,13 @@ LL |     fn result_1zst_exhaustive_single_field_e(x: Result<Field, num::NonZero<
    = note: enum has no representation hint
 
 error: `extern` block uses type `Result<(), Result<(), NonZero<u8>>>`, which is not FFI-safe
-  --> $DIR/lint-ctypes-enum.rs:170:30
+  --> $DIR/lint-ctypes-enum.rs:172:30
    |
 LL |     fn result_cascading_e(x: Result<(), Result<(), num::NonZero<u8>>>);
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
    |
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
 
-error: aborting due to 23 previous errors
+error: aborting due to 25 previous errors