Union current state. III. Discussion record.

Author: alercah

active_discussion/unions.md

@@ -1,3 +1,29 @@
 # Unions
 
-TBD
+## Outstanding questions
+
+* Is `#[repr(Rust)]` the bag-o-bytes union repr, or do we want to propose a new repr?
+    * *Discussion:* [#73: Validity of unions][#73]
+* The following questions are all implicitly answered if `#[repr(Rust)]` is the bag-o-bytes repr, but remain open if not:
+    * Do `#[repr(Rust)]` unions guarantee all fields at offset 0?
+        * *Discussion*: [#353: Offsets of union fields][#353]
+    * Do `#[repr(Rust)]` unions have internal padding?
+        * *Discussion*: [#354: Do #[repr(Rust)] unions have internal padding?][#354]
+* Do `#[repr(transparent)]` unions ever have niches?
+    * *Discussion*: [#364: What is the value model/validity invariant for transparent unions?][#364]
+
+## Closed discussion issues:
+
+* [#13: Representation of unions][#13]
+* [#156: Layout of repr(C) unions has padding][#156]
+* [#298: Is `repr(transparent)` completely transparent within `repr(Rust)` types?][#298]
+* [#352: What is the safety invariant, if any, for unions?][#352]
+
+[#13]: https://github.com/rust-lang/unsafe-code-guidelines/issues/13
+[#156]: https://github.com/rust-lang/unsafe-code-guidelines/issues/156
+[#298]: https://github.com/rust-lang/unsafe-code-guidelines/issues/298
+[#352]: https://github.com/rust-lang/unsafe-code-guidelines/issues/352
+[#353]: https://github.com/rust-lang/unsafe-code-guidelines/issues/353
+[#354]: https://github.com/rust-lang/unsafe-code-guidelines/issues/354
+[#364]: https://github.com/rust-lang/unsafe-code-guidelines/issues/364
+[#73]: https://github.com/rust-lang/unsafe-code-guidelines/issues/73

active_discussion/validity.md

@@ -140,8 +140,7 @@ To start, we will create threads for each major category of types.
     allow the remaining bits to be uninitialized?
 
 * Unions
-  * Do we make any restrictions here, or are unions just "bags of bits" that may
-    contain anything?  That would mean we can do no layout optimizations.
+  * See [this page](unions.md).
 
 * Enums
   * Is there anything to say besides: The discriminant must be valid, and all