diff --git a/phases/ephemeral/docs.md b/phases/ephemeral/docs.md
index fed63a44e..a046e9ce8 100644
--- a/phases/ephemeral/docs.md
+++ b/phases/ephemeral/docs.md
@@ -687,8 +687,11 @@ The state of the file descriptor.
 The contents of an $event.
 
 ### Union variants
-- <a href="#event_u.fd_readwrite" name="event_u.fd_readwrite"></a> `fd_readwrite`: [`event_fd_readwrite`](#event_fd_readwrite)
-When type is [`eventtype::fd_read`](#eventtype.fd_read) or [`eventtype::fd_write`](#eventtype.fd_write):
+- <a href="#event_u.fd_read" name="event_u.fd_read"></a> `fd_read`: [`event_fd_readwrite`](#event_fd_readwrite)
+
+- <a href="#event_u.fd_write" name="event_u.fd_write"></a> `fd_write`: [`event_fd_readwrite`](#event_fd_readwrite)
+
+- <a href="#event_u.clock" name="event_u.clock"></a> `clock`
 
 ## <a href="#event" name="event"></a> `event`: Struct
 An event that occurred.
@@ -700,11 +703,8 @@ User-provided value that got attached to [`subscription::userdata`](#subscriptio
 - <a href="#event.error" name="event.error"></a> `error`: [`errno`](#errno)
 If non-zero, an error that occurred while processing the subscription request.
 
-- <a href="#event.type" name="event.type"></a> `type`: [`eventtype`](#eventtype)
-The type of the event that occurred.
-
 - <a href="#event.u" name="event.u"></a> `u`: [`event_u`](#event_u)
-The contents of the event.
+The type of the event that occurred, and the contents of the event
 
 ## <a href="#subclockflags" name="subclockflags"></a> `subclockflags`: Flags(`u16`)
 Flags determining how to interpret the timestamp provided in
@@ -748,10 +748,10 @@ The contents of a $subscription.
 
 ### Union variants
 - <a href="#subscription_u.clock" name="subscription_u.clock"></a> `clock`: [`subscription_clock`](#subscription_clock)
-When type is [`eventtype::clock`](#eventtype.clock):
 
-- <a href="#subscription_u.fd_readwrite" name="subscription_u.fd_readwrite"></a> `fd_readwrite`: [`subscription_fd_readwrite`](#subscription_fd_readwrite)
-When type is [`eventtype::fd_read`](#eventtype.fd_read) or [`eventtype::fd_write`](#eventtype.fd_write):
+- <a href="#subscription_u.fd_read" name="subscription_u.fd_read"></a> `fd_read`: [`subscription_fd_readwrite`](#subscription_fd_readwrite)
+
+- <a href="#subscription_u.fd_write" name="subscription_u.fd_write"></a> `fd_write`: [`subscription_fd_readwrite`](#subscription_fd_readwrite)
 
 ## <a href="#subscription" name="subscription"></a> `subscription`: Struct
 Subscription to an event.
@@ -761,11 +761,8 @@ Subscription to an event.
 User-provided value that is attached to the subscription in the
 implementation and returned through [`event::userdata`](#event.userdata).
 
-- <a href="#subscription.type" name="subscription.type"></a> `type`: [`eventtype`](#eventtype)
-The type of the event to which to subscribe.
-
 - <a href="#subscription.u" name="subscription.u"></a> `u`: [`subscription_u`](#subscription_u)
-The contents of the subscription.
+The type of the event to which to subscribe, and the contents of the subscription.
 
 ## <a href="#exitcode" name="exitcode"></a> `exitcode`: `u32`
 Exit code generated by a process when exiting.
@@ -815,22 +812,12 @@ The contents of a $prestat when its $pr_type is [`preopentype::dir`](#preopentyp
 - <a href="#prestat_dir.pr_name_len" name="prestat_dir.pr_name_len"></a> `pr_name_len`: [`size`](#size)
 The length of the directory name for use with `fd_prestat_dir_name`.
 
-## <a href="#prestat_u" name="prestat_u"></a> `prestat_u`: Union
-The contents of an $prestat.
-
-### Union variants
-- <a href="#prestat_u.dir" name="prestat_u.dir"></a> `dir`: [`prestat_dir`](#prestat_dir)
-When $pr_type of the $prestat is [`preopentype::dir`](#preopentype.dir):
-
-## <a href="#prestat" name="prestat"></a> `prestat`: Struct
+## <a href="#prestat" name="prestat"></a> `prestat`: Union
 Information about a pre-opened capability.
 
-### Struct members
-- <a href="#prestat.pr_type" name="prestat.pr_type"></a> `pr_type`: [`preopentype`](#preopentype)
-The type of the pre-opened capability.
-
-- <a href="#prestat.u" name="prestat.u"></a> `u`: [`prestat_u`](#prestat_u)
-The contents of the information.
+### Union variants
+- <a href="#prestat.dir" name="prestat.dir"></a> `dir`: [`prestat_dir`](#prestat_dir)
+When type is [`preopentype::dir`](#preopentype.dir):
 
 # Modules
 ## <a href="#wasi_ephemeral_args" name="wasi_ephemeral_args"></a> wasi_ephemeral_args
diff --git a/phases/ephemeral/witx/typenames.witx b/phases/ephemeral/witx/typenames.witx
index 38ca67b1c..a7c55abb2 100644
--- a/phases/ephemeral/witx/typenames.witx
+++ b/phases/ephemeral/witx/typenames.witx
@@ -560,9 +560,10 @@
 
 ;;; The contents of an $event.
 (typename $event_u
-  (union
-    ;;; When type is `eventtype::fd_read` or `eventtype::fd_write`:
-    (field $fd_readwrite $event_fd_readwrite)
+  (union $eventtype
+    (field $fd_read $event_fd_readwrite)
+    (field $fd_write $event_fd_readwrite)
+    (empty $clock)
   )
 )
 
@@ -573,9 +574,7 @@
     (field $userdata $userdata)
     ;;; If non-zero, an error that occurred while processing the subscription request.
     (field $error $errno)
-    ;;; The type of the event that occurred.
-    (field $type $eventtype)
-    ;;; The contents of the event.
+    ;;; The type of the event that occurred, and the contents of the event
     (field $u $event_u)
   )
 )
@@ -619,11 +618,10 @@
 
 ;;; The contents of a $subscription.
 (typename $subscription_u
-  (union
-    ;;; When type is `eventtype::clock`:
+  (union $eventtype
     (field $clock $subscription_clock)
-    ;;; When type is `eventtype::fd_read` or `eventtype::fd_write`:
-    (field $fd_readwrite $subscription_fd_readwrite)
+    (field $fd_read $subscription_fd_readwrite)
+    (field $fd_write $subscription_fd_readwrite)
   )
 )
 
@@ -633,9 +631,7 @@
     ;;; User-provided value that is attached to the subscription in the
     ;;; implementation and returned through `event::userdata`.
     (field $userdata $userdata)
-    ;;; The type of the event to which to subscribe.
-    (field $type $eventtype)
-    ;;; The contents of the subscription.
+    ;;; The type of the event to which to subscribe, and the contents of the subscription.
     (field $u $subscription_u)
   )
 )
@@ -691,20 +687,10 @@
   )
 )
 
-;;; The contents of an $prestat.
-(typename $prestat_u
-  (union
-    ;;; When $pr_type of the $prestat is `preopentype::dir`:
-    (field $dir $prestat_dir)
-  )
-)
-
 ;;; Information about a pre-opened capability.
 (typename $prestat
-  (struct
-    ;;; The type of the pre-opened capability.
-    (field $pr_type $preopentype)
-    ;;; The contents of the information.
-    (field $u $prestat_u)
+  (union $preopentype
+    ;;; When type is `preopentype::dir`:
+    (field $dir $prestat_dir)
   )
 )
diff --git a/phases/old/snapshot_0/docs.md b/phases/old/snapshot_0/docs.md
index 360cff906..3466d13ad 100644
--- a/phases/old/snapshot_0/docs.md
+++ b/phases/old/snapshot_0/docs.md
@@ -627,8 +627,8 @@ The state of the file descriptor subscribed to with
 The peer of this socket has closed or disconnected.
 
 ## <a href="#event_fd_readwrite" name="event_fd_readwrite"></a> `event_fd_readwrite`: Struct
-The contents of an $event when type is [`eventtype::fd_read`](#eventtype.fd_read) or
-[`eventtype::fd_write`](#eventtype.fd_write).
+The contents of an $event for the [`eventtype::fd_read`](#eventtype.fd_read) and
+[`eventtype::fd_write`](#eventtype.fd_write) variants
 
 ### Struct members
 - <a href="#event_fd_readwrite.nbytes" name="event_fd_readwrite.nbytes"></a> `nbytes`: [`filesize`](#filesize)
@@ -637,13 +637,6 @@ The number of bytes available for reading or writing.
 - <a href="#event_fd_readwrite.flags" name="event_fd_readwrite.flags"></a> `flags`: [`eventrwflags`](#eventrwflags)
 The state of the file descriptor.
 
-## <a href="#event_u" name="event_u"></a> `event_u`: Union
-The contents of an $event.
-
-### Union variants
-- <a href="#event_u.fd_readwrite" name="event_u.fd_readwrite"></a> `fd_readwrite`: [`event_fd_readwrite`](#event_fd_readwrite)
-When type is [`eventtype::fd_read`](#eventtype.fd_read) or [`eventtype::fd_write`](#eventtype.fd_write):
-
 ## <a href="#event" name="event"></a> `event`: Struct
 An event that occurred.
 
@@ -655,10 +648,11 @@ User-provided value that got attached to [`subscription::userdata`](#subscriptio
 If non-zero, an error that occurred while processing the subscription request.
 
 - <a href="#event.type" name="event.type"></a> `type`: [`eventtype`](#eventtype)
-The type of the event that occurred.
+The type of event that occured
 
-- <a href="#event.u" name="event.u"></a> `u`: [`event_u`](#event_u)
-The contents of the event.
+- <a href="#event.fd_readwrite" name="event.fd_readwrite"></a> `fd_readwrite`: [`event_fd_readwrite`](#event_fd_readwrite)
+The contents of the event, if it is an [`eventtype::fd_read`](#eventtype.fd_read) or
+[`eventtype::fd_write`](#eventtype.fd_write). [`eventtype::clock`](#eventtype.clock) events ignore this field.
 
 ## <a href="#subclockflags" name="subclockflags"></a> `subclockflags`: Flags(`u16`)
 Flags determining how to interpret the timestamp provided in
@@ -693,7 +687,7 @@ to coalesce with other events.
 Flags specifying whether the timeout is absolute or relative
 
 ## <a href="#subscription_fd_readwrite" name="subscription_fd_readwrite"></a> `subscription_fd_readwrite`: Struct
-The contents of a $subscription when type is type is
+The contents of a $subscription when the variant is
 [`eventtype::fd_read`](#eventtype.fd_read) or [`eventtype::fd_write`](#eventtype.fd_write).
 
 ### Struct members
@@ -705,10 +699,10 @@ The contents of a $subscription.
 
 ### Union variants
 - <a href="#subscription_u.clock" name="subscription_u.clock"></a> `clock`: [`subscription_clock`](#subscription_clock)
-When type is [`eventtype::clock`](#eventtype.clock):
 
-- <a href="#subscription_u.fd_readwrite" name="subscription_u.fd_readwrite"></a> `fd_readwrite`: [`subscription_fd_readwrite`](#subscription_fd_readwrite)
-When type is [`eventtype::fd_read`](#eventtype.fd_read) or [`eventtype::fd_write`](#eventtype.fd_write):
+- <a href="#subscription_u.fd_read" name="subscription_u.fd_read"></a> `fd_read`: [`subscription_fd_readwrite`](#subscription_fd_readwrite)
+
+- <a href="#subscription_u.fd_write" name="subscription_u.fd_write"></a> `fd_write`: [`subscription_fd_readwrite`](#subscription_fd_readwrite)
 
 ## <a href="#subscription" name="subscription"></a> `subscription`: Struct
 Subscription to an event.
@@ -718,11 +712,8 @@ Subscription to an event.
 User-provided value that is attached to the subscription in the
 implementation and returned through [`event::userdata`](#event.userdata).
 
-- <a href="#subscription.type" name="subscription.type"></a> `type`: [`eventtype`](#eventtype)
-The type of the event to which to subscribe.
-
 - <a href="#subscription.u" name="subscription.u"></a> `u`: [`subscription_u`](#subscription_u)
-The contents of the subscription.
+The type of the event to which to subscribe.
 
 ## <a href="#exitcode" name="exitcode"></a> `exitcode`: `u32`
 Exit code generated by a process when exiting.
@@ -900,22 +891,11 @@ The contents of a $prestat when type is `PREOPENTYPE_DIR`.
 - <a href="#prestat_dir.pr_name_len" name="prestat_dir.pr_name_len"></a> `pr_name_len`: [`size`](#size)
 The length of the directory name for use with [`fd_prestat_dir_name`](#fd_prestat_dir_name).
 
-## <a href="#prestat_u" name="prestat_u"></a> `prestat_u`: Union
-The contents of an $prestat.
-
-### Union variants
-- <a href="#prestat_u.dir" name="prestat_u.dir"></a> `dir`: [`prestat_dir`](#prestat_dir)
-When type is `PREOPENTYPE_DIR`:
-
-## <a href="#prestat" name="prestat"></a> `prestat`: Struct
+## <a href="#prestat" name="prestat"></a> `prestat`: Union
 Information about a pre-opened capability.
 
-### Struct members
-- <a href="#prestat.pr_type" name="prestat.pr_type"></a> `pr_type`: [`preopentype`](#preopentype)
-The type of the pre-opened capability.
-
-- <a href="#prestat.u" name="prestat.u"></a> `u`: [`prestat_u`](#prestat_u)
-The contents of the information.
+### Union variants
+- <a href="#prestat.dir" name="prestat.dir"></a> `dir`: [`prestat_dir`](#prestat_dir)
 
 # Modules
 ## <a href="#wasi_unstable" name="wasi_unstable"></a> wasi_unstable
diff --git a/phases/old/snapshot_0/witx/typenames.witx b/phases/old/snapshot_0/witx/typenames.witx
index 9a7fc0bb2..493bda050 100644
--- a/phases/old/snapshot_0/witx/typenames.witx
+++ b/phases/old/snapshot_0/witx/typenames.witx
@@ -503,8 +503,8 @@
   )
 )
 
-;;; The contents of an $event when type is `eventtype::fd_read` or
-;;; `eventtype::fd_write`.
+;;; The contents of an $event for the `eventtype::fd_read` and
+;;; `eventtype::fd_write` variants
 (typename $event_fd_readwrite
   (struct
     ;;; The number of bytes available for reading or writing.
@@ -514,14 +514,6 @@
   )
 )
 
-;;; The contents of an $event.
-(typename $event_u
-  (union
-    ;;; When type is `eventtype::fd_read` or `eventtype::fd_write`:
-    (field $fd_readwrite $event_fd_readwrite)
-  )
-)
-
 ;;; An event that occurred.
 (typename $event
   (struct
@@ -529,10 +521,11 @@
     (field $userdata $userdata)
     ;;; If non-zero, an error that occurred while processing the subscription request.
     (field $error $errno)
-    ;;; The type of the event that occurred.
+    ;;; The type of event that occured
     (field $type $eventtype)
-    ;;; The contents of the event.
-    (field $u $event_u)
+    ;;; The contents of the event, if it is an `eventtype::fd_read` or
+    ;;; `eventtype::fd_write`. `eventtype::clock` events ignore this field.
+    (field $fd_readwrite $event_fd_readwrite)
   )
 )
 
@@ -566,7 +559,7 @@
   )
 )
 
-;;; The contents of a $subscription when type is type is
+;;; The contents of a $subscription when the variant is
 ;;; `eventtype::fd_read` or `eventtype::fd_write`.
 (typename $subscription_fd_readwrite
   (struct
@@ -577,11 +570,10 @@
 
 ;;; The contents of a $subscription.
 (typename $subscription_u
-  (union
-    ;;; When type is `eventtype::clock`:
+  (union $eventtype
     (field $clock $subscription_clock)
-    ;;; When type is `eventtype::fd_read` or `eventtype::fd_write`:
-    (field $fd_readwrite $subscription_fd_readwrite)
+    (field $fd_read $subscription_fd_readwrite)
+    (field $fd_write $subscription_fd_readwrite)
   )
 )
 
@@ -592,8 +584,6 @@
     ;;; implementation and returned through `event::userdata`.
     (field $userdata $userdata)
     ;;; The type of the event to which to subscribe.
-    (field $type $eventtype)
-    ;;; The contents of the subscription.
     (field $u $subscription_u)
   )
 )
@@ -748,20 +738,9 @@
   )
 )
 
-;;; The contents of an $prestat.
-(typename $prestat_u
-  (union
-    ;;; When type is `PREOPENTYPE_DIR`:
-    (field $dir $prestat_dir)
-  )
-)
-
 ;;; Information about a pre-opened capability.
 (typename $prestat
-  (struct
-    ;;; The type of the pre-opened capability.
-    (field $pr_type $preopentype)
-    ;;; The contents of the information.
-    (field $u $prestat_u)
+  (union $preopentype
+    (field $dir $prestat_dir)
   )
 )
diff --git a/phases/snapshot/docs.md b/phases/snapshot/docs.md
index 09575008e..0b3dec378 100644
--- a/phases/snapshot/docs.md
+++ b/phases/snapshot/docs.md
@@ -639,13 +639,6 @@ The number of bytes available for reading or writing.
 - <a href="#event_fd_readwrite.flags" name="event_fd_readwrite.flags"></a> `flags`: [`eventrwflags`](#eventrwflags)
 The state of the file descriptor.
 
-## <a href="#event_u" name="event_u"></a> `event_u`: Union
-The contents of an $event.
-
-### Union variants
-- <a href="#event_u.fd_readwrite" name="event_u.fd_readwrite"></a> `fd_readwrite`: [`event_fd_readwrite`](#event_fd_readwrite)
-When type is [`eventtype::fd_read`](#eventtype.fd_read) or [`eventtype::fd_write`](#eventtype.fd_write):
-
 ## <a href="#event" name="event"></a> `event`: Struct
 An event that occurred.
 
@@ -657,10 +650,11 @@ User-provided value that got attached to [`subscription::userdata`](#subscriptio
 If non-zero, an error that occurred while processing the subscription request.
 
 - <a href="#event.type" name="event.type"></a> `type`: [`eventtype`](#eventtype)
-The type of the event that occurred.
+The type of event that occured
 
-- <a href="#event.u" name="event.u"></a> `u`: [`event_u`](#event_u)
-The contents of the event.
+- <a href="#event.fd_readwrite" name="event.fd_readwrite"></a> `fd_readwrite`: [`event_fd_readwrite`](#event_fd_readwrite)
+The contents of the event, if it is an [`eventtype::fd_read`](#eventtype.fd_read) or
+[`eventtype::fd_write`](#eventtype.fd_write). [`eventtype::clock`](#eventtype.clock) events ignore this field.
 
 ## <a href="#subclockflags" name="subclockflags"></a> `subclockflags`: Flags(`u16`)
 Flags determining how to interpret the timestamp provided in
@@ -704,10 +698,10 @@ The contents of a $subscription.
 
 ### Union variants
 - <a href="#subscription_u.clock" name="subscription_u.clock"></a> `clock`: [`subscription_clock`](#subscription_clock)
-When type is [`eventtype::clock`](#eventtype.clock):
 
-- <a href="#subscription_u.fd_readwrite" name="subscription_u.fd_readwrite"></a> `fd_readwrite`: [`subscription_fd_readwrite`](#subscription_fd_readwrite)
-When type is [`eventtype::fd_read`](#eventtype.fd_read) or [`eventtype::fd_write`](#eventtype.fd_write):
+- <a href="#subscription_u.fd_read" name="subscription_u.fd_read"></a> `fd_read`: [`subscription_fd_readwrite`](#subscription_fd_readwrite)
+
+- <a href="#subscription_u.fd_write" name="subscription_u.fd_write"></a> `fd_write`: [`subscription_fd_readwrite`](#subscription_fd_readwrite)
 
 ## <a href="#subscription" name="subscription"></a> `subscription`: Struct
 Subscription to an event.
@@ -717,11 +711,8 @@ Subscription to an event.
 User-provided value that is attached to the subscription in the
 implementation and returned through [`event::userdata`](#event.userdata).
 
-- <a href="#subscription.type" name="subscription.type"></a> `type`: [`eventtype`](#eventtype)
-The type of the event to which to subscribe.
-
 - <a href="#subscription.u" name="subscription.u"></a> `u`: [`subscription_u`](#subscription_u)
-The contents of the subscription.
+The type of the event to which to subscribe, and its contents
 
 ## <a href="#exitcode" name="exitcode"></a> `exitcode`: `u32`
 Exit code generated by a process when exiting.
@@ -899,22 +890,11 @@ The contents of a $prestat when type is [`preopentype::dir`](#preopentype.dir).
 - <a href="#prestat_dir.pr_name_len" name="prestat_dir.pr_name_len"></a> `pr_name_len`: [`size`](#size)
 The length of the directory name for use with [`fd_prestat_dir_name`](#fd_prestat_dir_name).
 
-## <a href="#prestat_u" name="prestat_u"></a> `prestat_u`: Union
-The contents of an $prestat.
-
-### Union variants
-- <a href="#prestat_u.dir" name="prestat_u.dir"></a> `dir`: [`prestat_dir`](#prestat_dir)
-When type is [`preopentype::dir`](#preopentype.dir):
-
-## <a href="#prestat" name="prestat"></a> `prestat`: Struct
+## <a href="#prestat" name="prestat"></a> `prestat`: Union
 Information about a pre-opened capability.
 
-### Struct members
-- <a href="#prestat.pr_type" name="prestat.pr_type"></a> `pr_type`: [`preopentype`](#preopentype)
-The type of the pre-opened capability.
-
-- <a href="#prestat.u" name="prestat.u"></a> `u`: [`prestat_u`](#prestat_u)
-The contents of the information.
+### Union variants
+- <a href="#prestat.dir" name="prestat.dir"></a> `dir`: [`prestat_dir`](#prestat_dir)
 
 # Modules
 ## <a href="#wasi_snapshot_preview1" name="wasi_snapshot_preview1"></a> wasi_snapshot_preview1
diff --git a/phases/snapshot/witx/typenames.witx b/phases/snapshot/witx/typenames.witx
index ee8c652ec..eb8cb8c64 100644
--- a/phases/snapshot/witx/typenames.witx
+++ b/phases/snapshot/witx/typenames.witx
@@ -516,14 +516,6 @@
   )
 )
 
-;;; The contents of an $event.
-(typename $event_u
-  (union
-    ;;; When type is `eventtype::fd_read` or `eventtype::fd_write`:
-    (field $fd_readwrite $event_fd_readwrite)
-  )
-)
-
 ;;; An event that occurred.
 (typename $event
   (struct
@@ -531,10 +523,11 @@
     (field $userdata $userdata)
     ;;; If non-zero, an error that occurred while processing the subscription request.
     (field $error $errno)
-    ;;; The type of the event that occurred.
+    ;;; The type of event that occured
     (field $type $eventtype)
-    ;;; The contents of the event.
-    (field $u $event_u)
+    ;;; The contents of the event, if it is an `eventtype::fd_read` or
+    ;;; `eventtype::fd_write`. `eventtype::clock` events ignore this field.
+    (field $fd_readwrite $event_fd_readwrite)
   )
 )
 
@@ -577,11 +570,10 @@
 
 ;;; The contents of a $subscription.
 (typename $subscription_u
-  (union
-    ;;; When type is `eventtype::clock`:
+  (union $eventtype
     (field $clock $subscription_clock)
-    ;;; When type is `eventtype::fd_read` or `eventtype::fd_write`:
-    (field $fd_readwrite $subscription_fd_readwrite)
+    (field $fd_read  $subscription_fd_readwrite)
+    (field $fd_write $subscription_fd_readwrite)
   )
 )
 
@@ -591,9 +583,7 @@
     ;;; User-provided value that is attached to the subscription in the
     ;;; implementation and returned through `event::userdata`.
     (field $userdata $userdata)
-    ;;; The type of the event to which to subscribe.
-    (field $type $eventtype)
-    ;;; The contents of the subscription.
+    ;;; The type of the event to which to subscribe, and its contents
     (field $u $subscription_u)
   )
 )
@@ -748,20 +738,10 @@
   )
 )
 
-;;; The contents of an $prestat.
-(typename $prestat_u
-  (union
-    ;;; When type is `preopentype::dir`:
-    (field $dir $prestat_dir)
-  )
-)
-
 ;;; Information about a pre-opened capability.
 (typename $prestat
-  (struct
-    ;;; The type of the pre-opened capability.
-    (field $pr_type $preopentype)
-    ;;; The contents of the information.
-    (field $u $prestat_u)
+  (union $preopentype
+    (field $dir $prestat_dir)
   )
 )
+
diff --git a/tools/witx/Cargo.toml b/tools/witx/Cargo.toml
index 959c22143..32f0cbea9 100644
--- a/tools/witx/Cargo.toml
+++ b/tools/witx/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "witx"
-version = "0.7.0"
+version = "0.8.0"
 description = "Parse and validate witx file format"
 homepage = "https://github.com/WebAssembly/WASI"
 repository = "https://github.com/WebAssembly/WASI"
diff --git a/tools/witx/src/ast.rs b/tools/witx/src/ast.rs
index fcb33ad44..564d4ea1d 100644
--- a/tools/witx/src/ast.rs
+++ b/tools/witx/src/ast.rs
@@ -247,13 +247,14 @@ pub struct StructMember {
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct UnionDatatype {
+    pub tag: Rc<NamedType>,
     pub variants: Vec<UnionVariant>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct UnionVariant {
     pub name: Id,
-    pub tref: TypeRef,
+    pub tref: Option<TypeRef>,
     pub docs: String,
 }
 
diff --git a/tools/witx/src/docs/ast.rs b/tools/witx/src/docs/ast.rs
index a5f2ca952..10dc9f7c2 100644
--- a/tools/witx/src/docs/ast.rs
+++ b/tools/witx/src/docs/ast.rs
@@ -211,7 +211,11 @@ impl ToMarkdown for UnionDatatype {
                 name,
                 &variant.docs,
             ));
-            variant.tref.generate(n.clone());
+            if let Some(ref tref) = variant.tref {
+                tref.generate(n.clone());
+            } else {
+                n.content_ref_mut::<MdNamedType>().r#type = None;
+            }
         }
 
         node.content_ref_mut::<MdNamedType>().r#type = Some(MdType::Union);
diff --git a/tools/witx/src/layout.rs b/tools/witx/src/layout.rs
index cd3bd7da9..3e2f6ee09 100644
--- a/tools/witx/src/layout.rs
+++ b/tools/witx/src/layout.rs
@@ -22,7 +22,37 @@ impl TypeRef {
         if let Some(hit) = cache.get(self) {
             return *hit;
         }
-        let layout = match &*self.type_() {
+        let layout = match &self {
+            TypeRef::Name(nt) => nt.layout(cache),
+            TypeRef::Value(v) => v.layout(cache),
+        };
+        cache.insert(self.clone(), layout);
+        layout
+    }
+}
+
+impl Layout for TypeRef {
+    fn mem_size_align(&self) -> SizeAlign {
+        let mut cache = HashMap::new();
+        self.layout(&mut cache)
+    }
+}
+
+impl NamedType {
+    fn layout(&self, cache: &mut HashMap<TypeRef, SizeAlign>) -> SizeAlign {
+        self.tref.layout(cache)
+    }
+}
+impl Layout for NamedType {
+    fn mem_size_align(&self) -> SizeAlign {
+        let mut cache = HashMap::new();
+        self.layout(&mut cache)
+    }
+}
+
+impl Type {
+    fn layout(&self, cache: &mut HashMap<TypeRef, SizeAlign>) -> SizeAlign {
+        match &self {
             Type::Enum(e) => e.repr.mem_size_align(),
             Type::Int(i) => i.repr.mem_size_align(),
             Type::Flags(f) => f.repr.mem_size_align(),
@@ -32,18 +62,17 @@ impl TypeRef {
             Type::Array { .. } => BuiltinType::String.mem_size_align(),
             Type::Pointer { .. } | Type::ConstPointer { .. } => BuiltinType::U32.mem_size_align(),
             Type::Builtin(b) => b.mem_size_align(),
-        };
-        cache.insert(self.clone(), layout);
-        layout
+        }
     }
 }
 
-impl Layout for TypeRef {
+impl Layout for Type {
     fn mem_size_align(&self) -> SizeAlign {
         let mut cache = HashMap::new();
         self.layout(&mut cache)
     }
 }
+
 impl Layout for IntRepr {
     fn mem_size_align(&self) -> SizeAlign {
         match self {
@@ -132,25 +161,51 @@ mod test {
     }
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct UnionLayout {
+    pub tag_size: usize,
+    pub tag_align: usize,
+    pub contents_offset: usize,
+    pub contents_size: usize,
+    pub contents_align: usize,
+}
+
+impl Layout for UnionLayout {
+    fn mem_size_align(&self) -> SizeAlign {
+        let align = std::cmp::max(self.tag_align, self.contents_align);
+        let size = align_to(self.contents_offset + self.contents_size, align);
+        SizeAlign { size, align }
+    }
+}
+
 impl UnionDatatype {
-    fn layout(&self, cache: &mut HashMap<TypeRef, SizeAlign>) -> SizeAlign {
-        let sas = self
+    pub fn union_layout(&self) -> UnionLayout {
+        let mut cache = HashMap::new();
+        self.union_layout_(&mut cache)
+    }
+    fn union_layout_(&self, cache: &mut HashMap<TypeRef, SizeAlign>) -> UnionLayout {
+        let tag = self.tag.layout(cache);
+
+        let variant_sas = self
             .variants
             .iter()
-            .map(|v| v.tref.layout(cache))
+            .filter_map(|v| v.tref.as_ref().map(|t| t.layout(cache)))
             .collect::<Vec<SizeAlign>>();
-        let size = sas
-            .iter()
-            .map(|sa| sa.size)
-            .max()
-            .expect("nonzero variants");
-        let align = sas
-            .iter()
-            .map(|sa| sa.align)
-            .max()
-            .expect("nonzero variants");
-        let size = align_to(size, align);
-        SizeAlign { size, align }
+
+        let contents_size = variant_sas.iter().map(|sa| sa.size).max().unwrap_or(0);
+        let contents_align = variant_sas.iter().map(|sa| sa.align).max().unwrap_or(1);
+
+        UnionLayout {
+            tag_size: tag.size,
+            tag_align: tag.align,
+            contents_offset: align_to(tag.size, contents_align),
+            contents_size,
+            contents_align,
+        }
+    }
+
+    fn layout(&self, cache: &mut HashMap<TypeRef, SizeAlign>) -> SizeAlign {
+        self.union_layout_(cache).mem_size_align()
     }
 }
 
diff --git a/tools/witx/src/parser.rs b/tools/witx/src/parser.rs
index 08bc9af5b..85f8c8d09 100644
--- a/tools/witx/src/parser.rs
+++ b/tools/witx/src/parser.rs
@@ -24,6 +24,7 @@ mod kw {
     wast::custom_keyword!(f32);
     wast::custom_keyword!(f64);
     wast::custom_keyword!(field);
+    wast::custom_keyword!(empty);
     wast::custom_keyword!(flags);
     wast::custom_keyword!(handle);
     wast::custom_keyword!(int);
@@ -477,20 +478,48 @@ impl<'a> Parse<'a> for FieldSyntax<'a> {
     }
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum VariantSyntax<'a> {
+    Field(FieldSyntax<'a>),
+    Empty(wast::Id<'a>),
+}
+
+impl<'a> Parse<'a> for VariantSyntax<'a> {
+    fn parse(parser: Parser<'a>) -> Result<Self> {
+        parser.parens(|p| {
+            let mut l = p.lookahead1();
+            if l.peek::<kw::field>() {
+                parser.parse::<kw::field>()?;
+                let name = p.parse()?;
+                let type_ = p.parse()?;
+                Ok(VariantSyntax::Field(FieldSyntax { name, type_ }))
+            } else if l.peek::<kw::empty>() {
+                parser.parse::<kw::empty>()?;
+                let name = p.parse()?;
+                Ok(VariantSyntax::Empty(name))
+            } else {
+                Err(l.error())
+            }
+        })
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct UnionSyntax<'a> {
-    pub fields: Vec<Documented<'a, FieldSyntax<'a>>>,
+    pub tag: wast::Id<'a>,
+    pub fields: Vec<Documented<'a, VariantSyntax<'a>>>,
 }
 
 impl<'a> Parse<'a> for UnionSyntax<'a> {
     fn parse(parser: Parser<'a>) -> Result<Self> {
         parser.parse::<kw::r#union>()?;
+        let tag = parser.parse()?;
         let mut fields = Vec::new();
         fields.push(parser.parse()?);
         while !parser.is_empty() {
             fields.push(parser.parse()?);
         }
-        Ok(UnionSyntax { fields })
+        Ok(UnionSyntax { tag, fields })
     }
 }
 
diff --git a/tools/witx/src/render.rs b/tools/witx/src/render.rs
index a1a6ff58a..64b429f8b 100644
--- a/tools/witx/src/render.rs
+++ b/tools/witx/src/render.rs
@@ -206,19 +206,26 @@ impl StructDatatype {
 
 impl UnionDatatype {
     pub fn to_sexpr(&self) -> SExpr {
-        let header = vec![SExpr::word("union")];
+        let header = vec![SExpr::word("union"), self.tag.name.to_sexpr()];
         let variants = self
             .variants
             .iter()
             .map(|v| {
-                SExpr::docs(
-                    &v.docs,
-                    SExpr::Vec(vec![
-                        SExpr::word("field"),
-                        v.name.to_sexpr(),
-                        v.tref.to_sexpr(),
-                    ]),
-                )
+                if let Some(ref tref) = v.tref {
+                    SExpr::docs(
+                        &v.docs,
+                        SExpr::Vec(vec![
+                            SExpr::word("field"),
+                            v.name.to_sexpr(),
+                            tref.to_sexpr(),
+                        ]),
+                    )
+                } else {
+                    SExpr::docs(
+                        &v.docs,
+                        SExpr::Vec(vec![SExpr::word("empty"), v.name.to_sexpr()]),
+                    )
+                }
             })
             .collect::<Vec<SExpr>>();
         SExpr::Vec([header, variants].concat())
diff --git a/tools/witx/src/representation.rs b/tools/witx/src/representation.rs
index 3bdd6a792..0f12217af 100644
--- a/tools/witx/src/representation.rs
+++ b/tools/witx/src/representation.rs
@@ -177,7 +177,21 @@ impl Representable for UnionDatatype {
         }
         for v in self.variants.iter() {
             if let Some(byv) = by.variants.iter().find(|byv| byv.name == v.name) {
-                if v.tref.type_().representable(&*byv.tref.type_()) != RepEquality::Eq {
+                if v.tref.is_none() && byv.tref.is_none() {
+                    // Both empty is OK
+                } else if v.tref.is_some() && byv.tref.is_some() {
+                    if v.tref
+                        .as_ref()
+                        .unwrap()
+                        .type_()
+                        .representable(&*byv.tref.as_ref().unwrap().type_())
+                        != RepEquality::Eq
+                    {
+                        // Fields must be Eq
+                        return RepEquality::NotEq;
+                    }
+                } else {
+                    // Either one empty means not representable
                     return RepEquality::NotEq;
                 }
             } else {
@@ -185,9 +199,19 @@ impl Representable for UnionDatatype {
             }
         }
         if by.variants.len() > self.variants.len() {
-            RepEquality::Superset
+            // By is a superset of self only if the tags are as well:
+            if self.tag.type_().representable(&*by.tag.type_()) == RepEquality::Superset {
+                RepEquality::Superset
+            } else {
+                RepEquality::NotEq
+            }
         } else {
-            RepEquality::Eq
+            // By and self have matching variants, so they are equal if tags are:
+            if self.tag.type_().representable(&*by.tag.type_()) == RepEquality::Eq {
+                RepEquality::Eq
+            } else {
+                RepEquality::NotEq
+            }
         }
     }
 }
@@ -287,16 +311,25 @@ mod test {
 
     #[test]
     fn union() {
-        let base = def_type("a", "(typename $a (union (field $b u32) (field $c f32)))");
+        let base = def_type(
+            "a",
+            "(typename $tag (enum u8 $b $c))
+            (typename $a (union $tag (field $b u32) (field $c f32)))",
+        );
         let extra_variant = def_type(
             "a",
-            "(typename $a (union (field $b u32) (field $c f32) (field $d f64)))",
+            "(typename $tag (enum u8 $b $c $d))
+            (typename $a (union $tag (field $b u32) (field $c f32) (field $d f64)))",
         );
 
         assert_eq!(base.representable(&extra_variant), RepEquality::Superset);
         assert_eq!(extra_variant.representable(&base), RepEquality::NotEq);
 
-        let other_ordering = def_type("a", "(typename $a (union (field $c f32) (field $b u32)))");
+        let other_ordering = def_type(
+            "a",
+            "(typename $tag (enum u8 $b $c))
+            (typename $a (union $tag (field $c f32) (field $b u32)))",
+        );
         assert_eq!(base.representable(&other_ordering), RepEquality::Eq);
         assert_eq!(other_ordering.representable(&base), RepEquality::Eq);
         assert_eq!(
diff --git a/tools/witx/src/validate.rs b/tools/witx/src/validate.rs
index 4bb92aa89..17be21e82 100644
--- a/tools/witx/src/validate.rs
+++ b/tools/witx/src/validate.rs
@@ -3,6 +3,7 @@ use crate::{
     parser::{
         CommentSyntax, DeclSyntax, Documented, EnumSyntax, FlagsSyntax, HandleSyntax,
         ImportTypeSyntax, IntSyntax, ModuleDeclSyntax, StructSyntax, TypedefSyntax, UnionSyntax,
+        VariantSyntax,
     },
     BuiltinType, Definition, Entry, EnumDatatype, EnumVariant, FlagsDatatype, FlagsMember,
     HandleDatatype, Id, IntConst, IntDatatype, IntRepr, InterfaceFunc, InterfaceFuncParam,
@@ -10,7 +11,7 @@ use crate::{
     ModuleImportVariant, NamedType, StructDatatype, StructMember, Type, TypePassedBy, TypeRef,
     UnionDatatype, UnionVariant,
 };
-use std::collections::HashMap;
+use std::collections::{hash_map, HashMap};
 use std::path::Path;
 use std::rc::Rc;
 use thiserror::Error;
@@ -43,6 +44,12 @@ pub enum ValidationError {
     InvalidFirstResultType { location: Location },
     #[error("Anonymous structured types (struct, union, enum, flags, handle) are not permitted")]
     AnonymousStructure { location: Location },
+    #[error("Invalid union field `{name}`: {reason}")]
+    InvalidUnionField {
+        name: String,
+        reason: String,
+        location: Location,
+    },
 }
 
 impl ValidationError {
@@ -54,7 +61,8 @@ impl ValidationError {
             | Recursive { location, .. }
             | InvalidRepr { location, .. }
             | InvalidFirstResultType { location, .. }
-            | AnonymousStructure { location, .. } => {
+            | AnonymousStructure { location, .. }
+            | InvalidUnionField { location, .. } => {
                 format!("{}\n{}", location.highlight_source_with(witxio), &self)
             }
             NameAlreadyExists {
@@ -348,22 +356,100 @@ impl DocValidationScope<'_> {
     fn validate_union(
         &self,
         syntax: &UnionSyntax,
-        _span: wast::Span,
+        span: wast::Span,
     ) -> Result<UnionDatatype, ValidationError> {
         let mut variant_scope = IdentValidation::new();
+        let tag_id = self.get(&syntax.tag)?;
+        let (tag, mut variant_name_uses) = match self.doc.entries.get(&tag_id) {
+            Some(Entry::Typename(weak_ref)) => {
+                let named_dt = weak_ref.upgrade().expect("weak backref to defined type");
+                match &*named_dt.type_() {
+                    Type::Enum(e) => {
+                        let uses = e
+                            .variants
+                            .iter()
+                            .map(|v| (v.name.clone(), false))
+                            .collect::<HashMap<Id, bool>>();
+                        Ok((named_dt, uses))
+                    }
+                    other => Err(ValidationError::WrongKindName {
+                        name: syntax.tag.name().to_string(),
+                        location: self.location(syntax.tag.span()),
+                        expected: "enum",
+                        got: other.kind(),
+                    }),
+                }
+            }
+            other => Err(ValidationError::WrongKindName {
+                name: syntax.tag.name().to_string(),
+                location: self.location(syntax.tag.span()),
+                expected: "enum",
+                got: match other {
+                    Some(e) => e.kind(),
+                    None => "unknown",
+                },
+            }),
+        }?;
+
         let variants = syntax
             .fields
             .iter()
-            .map(|f| {
+            .map(|v| {
+                let variant_name = match v.item {
+                    VariantSyntax::Field(ref f) => &f.name,
+                    VariantSyntax::Empty(ref name) => name,
+                };
                 let name = variant_scope
-                    .introduce(f.item.name.name(), self.location(f.item.name.span()))?;
-                let tref = self.validate_datatype(&f.item.type_, false, f.item.name.span())?;
-                let docs = f.comments.docs();
+                    .introduce(variant_name.name(), self.location(variant_name.span()))?;
+                let tref = match &v.item {
+                    VariantSyntax::Field(f) => {
+                        Some(self.validate_datatype(&f.type_, false, variant_name.span())?)
+                    }
+                    VariantSyntax::Empty { .. } => None,
+                };
+                let docs = v.comments.docs();
+                match variant_name_uses.entry(name.clone()) {
+                    hash_map::Entry::Occupied(mut e) => {
+                        if *e.get() {
+                            Err(ValidationError::InvalidUnionField {
+                                name: variant_name.name().to_string(),
+                                reason: "variant already defined".to_owned(),
+                                location: self.location(variant_name.span()),
+                            })?;
+                        } else {
+                            e.insert(true);
+                        }
+                    }
+                    hash_map::Entry::Vacant { .. } => Err(ValidationError::InvalidUnionField {
+                        name: variant_name.name().to_string(),
+                        reason: format!(
+                            "does not correspond to variant in tag `{}`",
+                            tag.name.as_str()
+                        ),
+                        location: self.location(variant_name.span()),
+                    })?,
+                }
                 Ok(UnionVariant { name, tref, docs })
             })
             .collect::<Result<Vec<UnionVariant>, _>>()?;
 
-        Ok(UnionDatatype { variants })
+        let unused_variants = variant_name_uses
+            .iter()
+            .filter(|(_k, used)| **used == false)
+            .map(|(k, _)| k.clone())
+            .collect::<Vec<Id>>();
+        if !unused_variants.is_empty() {
+            Err(ValidationError::InvalidUnionField {
+                name: unused_variants
+                    .iter()
+                    .map(|i| i.as_str())
+                    .collect::<Vec<_>>()
+                    .join(", "),
+                reason: format!("missing variants from tag `{}`", tag.name.as_str()),
+                location: self.location(span),
+            })?;
+        }
+        Ok(UnionDatatype { tag, variants })
     }
 
     fn validate_handle(
diff --git a/tools/witx/tests/anonymous.rs b/tools/witx/tests/anonymous.rs
index e1976a261..6bf11d883 100644
--- a/tools/witx/tests/anonymous.rs
+++ b/tools/witx/tests/anonymous.rs
@@ -12,7 +12,9 @@ fn anonymous_types() {
     let pointer_to_struct = witx::parse("(typename $a (@witx pointer (struct (field $b u8))))");
     assert!(is_anonymous_struct_err(pointer_to_struct));
 
-    let pointer_to_union = witx::parse("(typename $a (@witx pointer (union (field $b u8))))");
+    let pointer_to_union = witx::parse(
+        "(typename $tag (enum u8 $b)) (typename $a (@witx pointer (union $tag (field $b u8))))",
+    );
     assert!(is_anonymous_struct_err(pointer_to_union));
 
     let pointer_to_enum = witx::parse("(typename $a (@witx pointer (enum u32 $b)))");
@@ -33,7 +35,9 @@ fn anonymous_types() {
     let struct_in_struct = witx::parse("(typename $a (struct (field $b (struct (field $c u8)))))");
     assert!(is_anonymous_struct_err(struct_in_struct));
 
-    let union_in_struct = witx::parse("(typename $a (struct (field $b (union (field $c u8)))))");
+    let union_in_struct = witx::parse(
+        "(typename $tag (enum u8 $c)) (typename $a (struct (field $b (union $tag (field $c u8)))))",
+    );
     assert!(is_anonymous_struct_err(union_in_struct));
 
     let pointer_in_struct = witx::parse("(typename $a (struct (field $b (@witx pointer u8))))");
diff --git a/tools/witx/tests/union.rs b/tools/witx/tests/union.rs
new file mode 100644
index 000000000..732b5c007
--- /dev/null
+++ b/tools/witx/tests/union.rs
@@ -0,0 +1,183 @@
+// Every worker needs a union! Time to organize
+use witx::{Id, Representable};
+
+#[test]
+fn one_variant_union() {
+    let d = witx::parse(
+        "(typename $tag (enum u8 $c))
+         (typename $u (union $tag (field $c u8)))",
+    );
+    assert!(d.is_ok());
+}
+
+#[test]
+fn two_variant_union() {
+    let d1 = witx::parse(
+        "(typename $tag (enum u8 $a $b))
+         (typename $u (union $tag (field $a u8) (field $b u16)))",
+    );
+    assert!(d1.is_ok(), "d1 is ok");
+
+    // Fields can come in whatever order:
+    let d2 = witx::parse(
+        "(typename $tag (enum u8 $a $b))
+         (typename $u (union $tag (field $b u16) (field $a u8)))",
+    );
+    assert!(d2.is_ok(), "d2 is ok");
+
+    // These two unions should be represented the same:
+    let u1 = d1.unwrap().typename(&Id::new("u")).unwrap().type_();
+    let u2 = d2.unwrap().typename(&Id::new("u")).unwrap().type_();
+
+    assert_eq!(
+        u1.representable(&u2),
+        witx::RepEquality::Eq,
+        "u1 can represent u2"
+    );
+    assert_eq!(
+        u2.representable(&u1),
+        witx::RepEquality::Eq,
+        "u2 can represent u1"
+    );
+
+    // Tag order doesnt matter for validation, but does for rep equality
+    let d3 = witx::parse(
+        "(typename $tag (enum u8 $b $a))
+         (typename $u (union $tag (field $b u16) (field $a u8)))",
+    );
+    assert!(d3.is_ok(), "d2 is ok");
+    let u3 = d3.unwrap().typename(&Id::new("u")).unwrap().type_();
+    assert_eq!(
+        u3.representable(&u1),
+        witx::RepEquality::NotEq,
+        "u3 cannot represent u1"
+    );
+}
+
+#[test]
+fn empty_variant_unions() {
+    let d1 = witx::parse(
+        "(typename $tag (enum u8 $a $b))
+         (typename $u (union $tag (empty $a) (field $b u16)))",
+    );
+    assert!(d1.is_ok(), "d1 is ok");
+
+    let d2 = witx::parse(
+        "(typename $tag (enum u8 $a $b))
+         (typename $u (union $tag (empty $a) (empty $b)))",
+    );
+    assert!(d2.is_ok(), "d2 is ok");
+}
+
+#[test]
+fn many_variant_unions() {
+    let d1 = witx::parse(
+        "(typename $tag (enum u32 $a $b $c $d $e $f $g $h $i $j $k $l $m))
+         (typename $u
+           (union $tag
+            (field $a u8)
+            (field $b u16)
+            (field $c u32)
+            (field $d u64)
+            (field $e s8)
+            (field $f s16)
+            (field $g s32)
+            (field $h s64)
+            (field $i f32)
+            (field $j f64)
+            (field $k (@witx usize))
+            (field $l char8)
+            (empty $m)
+           )
+         )",
+    );
+    assert!(d1.is_ok(), "d1 is ok");
+}
+
+#[test]
+fn no_tag_union() {
+    let d = witx::parse("(typename $u (union $tag (field $a u8) (field $b u16)))");
+    assert!(d.is_err());
+}
+
+#[test]
+fn wrong_kind_tag_union() {
+    let d = witx::parse(
+        "(typename $tag u32)
+            (typename $u (union $tag (field $a u8) (field $b u16)))",
+    );
+    let (expected, got) = wrong_kind_name_err(d).expect("wrong kind of tag");
+    assert_eq!(expected, "enum");
+    assert_eq!(got, "builtin");
+}
+
+#[test]
+fn bad_field_unions() {
+    let d = witx::parse(
+        "(typename $tag (enum u8 $c))
+         (typename $u (union $tag (field $b u8)))",
+    );
+    let (name, reason) = union_field_err(d).expect("bad field union 1");
+    assert_eq!(name, "b", "bad field name union 1");
+    assert_eq!(
+        reason, "does not correspond to variant in tag `tag`",
+        "reason union 1"
+    );
+
+    let d = witx::parse(
+        "(typename $tag (enum u8 $c))
+         (typename $u (union $tag (field $c f32) (field $b u8)))",
+    );
+    let (name, reason) = union_field_err(d).expect("bad field union 2");
+    assert_eq!(name, "b", "bad field name union 2");
+    assert_eq!(
+        reason, "does not correspond to variant in tag `tag`",
+        "reason union 2"
+    );
+
+    let d = witx::parse(
+        "(typename $tag (enum u8 $c $d))
+         (typename $u (union $tag (field $c f32)))",
+    );
+    let (name, reason) = union_field_err(d).expect("bad field union 3");
+    assert_eq!(name, "d", "bad field name union 3");
+    assert_eq!(reason, "missing variants from tag `tag`", "reason union 3");
+}
+
+fn wrong_kind_name_err(
+    r: Result<witx::Document, witx::WitxError>,
+) -> Option<(&'static str, &'static str)> {
+    match r {
+        Err(witx::WitxError::Validation(witx::ValidationError::WrongKindName {
+            expected,
+            got,
+            ..
+        })) => Some((expected, got)),
+        Err(e) => {
+            eprintln!("expected WrongKindName ValidationError, got: {:?}", e);
+            None
+        }
+        Ok(_) => {
+            eprintln!("expected WrongKindName ValidationError: Ok(witx::Document)");
+            None
+        }
+    }
+}
+
+fn union_field_err(r: Result<witx::Document, witx::WitxError>) -> Option<(String, String)> {
+    match r {
+        Err(witx::WitxError::Validation(witx::ValidationError::InvalidUnionField {
+            name,
+            reason,
+            ..
+        })) => Some((name, reason)),
+        Err(e) => {
+            eprintln!("expected InvalidUnionField ValidationError, got: {:?}", e);
+            None
+        }
+        Ok(_) => {
+            eprintln!("expected InvalidUnionField ValidationError, got: Ok(witx::Document)");
+            None
+        }
+    }
+}