update to use tagged unions (#41)

* update to use tagged unions

* latest wasi repo lays out event as flat struct

* rebased wasi tagged unions branch

* fix for namedtype as union tag

* regenerate lib_generated.rs

* restore `size` -> `usize` hack in generate-raw

along with a comment describing when its safe to remove

* WASI submodule: master with tagged_unions PR merged.

Author: Pat Hickey

crates/generate-raw/WASI

@@ -97,16 +97,27 @@ fn render_const(src: &mut String, name: &str, c: &IntDatatype) {
 fn render_union(src: &mut String, name: &str, u: &UnionDatatype) {
     src.push_str("#[repr(C)]\n");
     src.push_str("#[derive(Copy, Clone)]\n");
-    src.push_str(&format!("pub union {} {{\n", name.to_camel_case()));
+    src.push_str(&format!("pub union {}U {{\n", name.to_camel_case()));
     for variant in u.variants.iter() {
-        rustdoc(&variant.docs, src);
-        src.push_str("pub ");
-        variant.name.render(src);
-        src.push_str(": ");
-        variant.tref.render(src);
-        src.push_str(",\n");
+        if let Some(ref tref) = variant.tref {
+            rustdoc(&variant.docs, src);
+            src.push_str("pub ");
+            variant.name.render(src);
+            src.push_str(": ");
+            tref.render(src);
+            src.push_str(",\n");
+        }
     }
-    src.push_str("}");
+    src.push_str("}\n");
+    src.push_str("#[repr(C)]\n");
+    src.push_str("#[derive(Copy, Clone)]\n");
+    src.push_str(&format!("pub struct {} {{\n", name.to_camel_case()));
+    src.push_str(&format!(
+        "pub tag: {},\n",
+        u.tag.name.as_str().to_camel_case()
+    ));
+    src.push_str(&format!("pub u: {}U,\n", name.to_camel_case()));
+    src.push_str("}\n");
 }
 
 fn render_struct(src: &mut String, name: &str, s: &StructDatatype) {
@@ -190,8 +201,9 @@ fn render_alias(src: &mut String, name: &str, dest: &TypeRef) {
     }
     src.push_str(" = ");
 
-    // Give `size_t` special treatment to translate it to `usize` in Rust
-    // instead of `u32`, makes things a bit nicer in Rust.
+    // Give `size` special treatment to translate it to `usize` in Rust instead of `u32`. Makes
+    // things a bit nicer for client libraries. We can remove this hack once WASI moves to a
+    // snapshot that uses BuiltinType::Size.
     if name == "size" {
         src.push_str("usize");
     } else {

crates/generate-raw/src/lib.rs

@@ -495,21 +495,16 @@ pub struct EventFdReadwrite {
 }
 #[repr(C)]
 #[derive(Copy, Clone)]
-pub union EventU {
-    /// When type is `eventtype::fd_read` or `eventtype::fd_write`:
-    pub fd_readwrite: EventFdReadwrite,
-}
-#[repr(C)]
-#[derive(Copy, Clone)]
 pub struct Event {
     /// User-provided value that got attached to `subscription::userdata`.
     pub userdata: Userdata,
     /// If non-zero, an error that occurred while processing the subscription request.
     pub error: Errno,
-    /// The type of the event that occurred.
+    /// The type of event that occured
     pub r#type: Eventtype,
-    /// The contents of the event.
-    pub u: EventU,
+    /// The contents of the event, if it is an `eventtype::fd_read` or
+    /// `eventtype::fd_write`. `eventtype::clock` events ignore this field.
+    pub fd_readwrite: EventFdReadwrite,
 }
 pub type Subclockflags = u16;
 /// If set, treat the timestamp provided in
@@ -539,21 +534,25 @@ pub struct SubscriptionFdReadwrite {
 }
 #[repr(C)]
 #[derive(Copy, Clone)]
-pub union SubscriptionU {
-    /// When type is `eventtype::clock`:
+pub union SubscriptionUU {
     pub clock: SubscriptionClock,
-    /// When type is `eventtype::fd_read` or `eventtype::fd_write`:
-    pub fd_readwrite: SubscriptionFdReadwrite,
+    pub fd_read: SubscriptionFdReadwrite,
+    pub fd_write: SubscriptionFdReadwrite,
 }
+#[repr(C)]
+#[derive(Copy, Clone)]
+pub struct SubscriptionU {
+    pub tag: Eventtype,
+    pub u: SubscriptionUU,
+}
+
 #[repr(C)]
 #[derive(Copy, Clone)]
 pub struct Subscription {
     /// User-provided value that is attached to the subscription in the
     /// implementation and returned through `event::userdata`.
     pub userdata: Userdata,
-    /// The type of the event to which to subscribe.
-    pub r#type: Eventtype,
-    /// The contents of the subscription.
+    /// The type of the event to which to subscribe, and its contents
     pub u: SubscriptionU,
 }
 pub type Exitcode = u32;
@@ -677,17 +676,15 @@ pub struct PrestatDir {
 #[repr(C)]
 #[derive(Copy, Clone)]
 pub union PrestatU {
-    /// When type is `preopentype::dir`:
     pub dir: PrestatDir,
 }
 #[repr(C)]
 #[derive(Copy, Clone)]
 pub struct Prestat {
-    /// The type of the pre-opened capability.
-    pub pr_type: Preopentype,
-    /// The contents of the information.
+    pub tag: Preopentype,
     pub u: PrestatU,
 }
+
 /// Read command-line argument data.
 /// The size of the array should match that returned by `args_sizes_get`
 pub unsafe fn args_get(argv: *mut *mut u8, argv_buf: *mut u8) -> Result<()> {