also allow arrays of allowed types

Author: RalfJung

compiler/rustc_passes/src/stability.rs

@@ -785,7 +785,12 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                             // allow tuples of allowed types
                             tys.iter().all(|ty| allowed_union_field(tcx, param_env, ty))
                         }
+                        ty::Array(elem, _len) => {
+                            // Like `Copy`, we do *not* special-case length 0.
+                            allowed_union_field(tcx, param_env, *elem)
+                        }
                         _ => {
+                            // Fallback case: allow `ManuallyDrop` and things that are `Copy`.
                             ty.ty_adt_def().map_or(false, |adt_def| adt_def.is_manually_drop())
                                 || ty.is_copy_modulo_regions(tcx.at(DUMMY_SP), param_env)
                         }

src/test/ui/feature-gates/feature-gate-untagged_unions.rs

@@ -17,6 +17,10 @@ union U23<T> { // OK
     a: (ManuallyDrop<T>, i32),
 }
 
+union U24<T> { // OK
+    a: [ManuallyDrop<T>; 2],
+}
+
 union U3 {
     a: String, //~ ERROR unions cannot contain fields that may need dropping
 }

src/test/ui/feature-gates/feature-gate-untagged_unions.stderr

@@ -1,5 +1,5 @@
 error[E0658]: unions with non-`Copy` fields other than `ManuallyDrop<T>`, references, and tuples of such types are unstable
-  --> $DIR/feature-gate-untagged_unions.rs:25:5
+  --> $DIR/feature-gate-untagged_unions.rs:29:5
    |
 LL |     a: std::cell::RefCell<i32>,
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -8,7 +8,7 @@ LL |     a: std::cell::RefCell<i32>,
    = help: add `#![feature(untagged_unions)]` to the crate attributes to enable
 
 error[E0740]: unions cannot contain fields that may need dropping
-  --> $DIR/feature-gate-untagged_unions.rs:21:5
+  --> $DIR/feature-gate-untagged_unions.rs:25:5
    |
 LL |     a: String,
    |     ^^^^^^^^^
@@ -20,7 +20,7 @@ LL |     a: std::mem::ManuallyDrop<String>,
    |        +++++++++++++++++++++++      +
 
 error[E0740]: unions cannot contain fields that may need dropping
-  --> $DIR/feature-gate-untagged_unions.rs:29:5
+  --> $DIR/feature-gate-untagged_unions.rs:33:5
    |
 LL |     a: T,
    |     ^^^^
@@ -32,7 +32,7 @@ LL |     a: std::mem::ManuallyDrop<T>,
    |        +++++++++++++++++++++++ +
 
 error[E0740]: unions cannot contain fields that may need dropping
-  --> $DIR/feature-gate-untagged_unions.rs:41:5
+  --> $DIR/feature-gate-untagged_unions.rs:45:5
    |
 LL |     nest: U5,
    |     ^^^^^^^^