warn about uninit multi-variant enums

src/librustc_lint/builtin.rs

@@ -1922,6 +1922,14 @@ impl<'tcx> LateLintPass<'tcx> for InvalidValue {
             None
         }
 
+        /// Test if this enum has several actually "existing" variants.
+        /// Zero-sized uninhabited variants do not always have a tag assigned and thus do not "exist".
+        fn is_multi_variant(adt: &ty::AdtDef) -> bool {
+            // As an approximation, we only count dataless variants. Those are definitely inhabited.
+            let existing_variants = adt.variants.iter().filter(|v| v.fields.is_empty()).count();
+            existing_variants > 1
+        }
+
         /// Return `Some` only if we are sure this type does *not*
         /// allow zero initialization.
         fn ty_find_init_error<'tcx>(
@@ -1950,7 +1958,7 @@ impl<'tcx> LateLintPass<'tcx> for InvalidValue {
                 }
                 // Recurse and checks for some compound types.
                 Adt(adt_def, substs) if !adt_def.is_union() => {
-                    // First check f this ADT has a layout attribute (like `NonNull` and friends).
+                    // First check if this ADT has a layout attribute (like `NonNull` and friends).
                     use std::ops::Bound;
                     match tcx.layout_scalar_valid_range(adt_def.did) {
                         // We exploit here that `layout_scalar_valid_range` will never
@@ -2001,10 +2009,20 @@ impl<'tcx> LateLintPass<'tcx> for InvalidValue {
                                 )
                             })
                         }
-                        // Multi-variant enums are tricky: if all but one variant are
-                        // uninhabited, we might actually do layout like for a single-variant
-                        // enum, and then even leaving them uninitialized could be okay.
-                        _ => None, // Conservative fallback for multi-variant enum.
+                        // Multi-variant enum.
+                        _ => {
+                            if init == InitKind::Uninit && is_multi_variant(adt_def) {
+                                let span = tcx.def_span(adt_def.did);
+                                Some((
+                                    "enums have to be initialized to a variant".to_string(),
+                                    Some(span),
+                                ))
+                            } else {
+                                // In principle, for zero-initialization we could figure out which variant corresponds
+                                // to tag 0, and check that... but for now we just accept all zero-initializations.
+                                None
+                            }
+                        }
                     }
                 }
                 Tuple(..) => {

src/test/ui/lint/uninitialized-zeroed.rs

@@ -23,6 +23,18 @@ enum WrapEnum<T> { Wrapped(T) }
 #[repr(transparent)]
 pub(crate) struct NonBig(u64);
 
+/// A two-variant enum, thus needs a tag and may not remain uninitialized.
+enum Fruit {
+    Apple,
+    Banana,
+}
+
+/// Looks like two variants but really only has one.
+enum OneFruit {
+    Apple(!),
+    Banana,
+}
+
 #[allow(unused)]
 fn generic<T: 'static>() {
     unsafe {
@@ -80,6 +92,9 @@ fn main() {
         let _val: NonBig = mem::zeroed();
         let _val: NonBig = mem::uninitialized(); //~ ERROR: does not permit being left uninitialized
 
+        let _val: Fruit = mem::zeroed();
+        let _val: Fruit = mem::uninitialized(); //~ ERROR: does not permit being left uninitialized
+
         // Transmute-from-0
         let _val: &'static i32 = mem::transmute(0usize); //~ ERROR: does not permit zero-initialization
         let _val: &'static [i32] = mem::transmute((0usize, 0usize)); //~ ERROR: does not permit zero-initialization
@@ -96,5 +111,9 @@ fn main() {
         let _val: MaybeUninit<&'static i32> = mem::zeroed();
         let _val: i32 = mem::zeroed();
         let _val: bool = MaybeUninit::zeroed().assume_init();
+        // Some things that happen to work due to rustc implementation details,
+        // but are not guaranteed to keep working.
+        let _val: i32 = mem::uninitialized();
+        let _val: OneFruit = mem::uninitialized();
     }
 }