Add support for optional slice types (#507)

* Shard the `convert.rs` module into sub-modules

Hopefully this'll make the organization a little nicer over time!

* Start adding support for optional types

This commit starts adding support for optional types to wasm-bindgen as
arguments/return values to functions. The strategy here is to add two new
traits, `OptionIntoWasmAbi` and `OptionFromWasmAbi`. These two traits are used
as a blanket impl to implement `IntoWasmAbi` and `FromWasmAbi` for `Option<T>`.

Some consequences of this design:

* It should be possible to ensure `Option<SomeForeignType>` implements to/from
  wasm traits. This is because the option-based traits can be implemented for
  foreign types.
* A specialized implementation is possible for all types, so there's no need for
  `Option<T>` to introduce unnecessary overhead.
* Two new traits is a bit unforutnate but I can't currently think of an
  alternative design that works for the above two constraints, although it
  doesn't mean one doesn't exist!
* The error messages for "can't use this type here" is actually halfway decent
  because it says these new traits need to be implemented, which provides a good
  place to document and talk about what's going on here!
* Nested references like `Option<&T>` can't implement `FromWasmAbi`. This means
  that you can't define a function in Rust which takes `Option<&str>`. It may be
  possible to do this one day but it'll likely require more trait trickery than
  I'm capable of right now.

* Add support for optional slices

This commit adds support for optional slice types, things like strings and
arrays. The null representation of these has a pointer value of 0, which should
never happen in normal Rust. Otherwise the various plumbing is done throughout
the tooling to enable these types in all locations.

* Fix `takeObject` on global sentinels

These don't have a reference count as they're always expected to work, so avoid
actually dropping a reference on them.

* Remove some no longer needed bindings

* Add support for optional anyref types

This commit adds support for optional imported class types. Each type imported
with `#[wasm_bindgen]` automatically implements the relevant traits and now
supports `Option<Foo>` in various argument/return positions.

* Fix building without the `std` feature

* Actually fix the build...

* Add support for optional types to WebIDL

Closes #502

crates/cli-support/src/js/mod.rs

@@ -53,6 +53,8 @@ pub struct SubContext<'a, 'b: 'a> {
     pub cx: &'a mut Context<'b>,
 }
 
+const INITIAL_SLAB_VALUES: &[&str] = &["undefined", "null", "true", "false"];
+
 impl<'a> Context<'a> {
     fn export(&mut self, name: &str, contents: &str, comments: Option<String>) {
         let contents = contents.trim();
@@ -183,28 +185,6 @@ impl<'a> Context<'a> {
             ))
         })?;
 
-        self.bind("__wbindgen_undefined_new", &|me| {
-            me.expose_add_heap_object();
-            Ok(String::from(
-                "
-                function() {
-                    return addHeapObject(undefined);
-                }
-                ",
-            ))
-        })?;
-
-        self.bind("__wbindgen_null_new", &|me| {
-            me.expose_add_heap_object();
-            Ok(String::from(
-                "
-                function() {
-                    return addHeapObject(null);
-                }
-                ",
-            ))
-        })?;
-
         self.bind("__wbindgen_is_null", &|me| {
             me.expose_get_object();
             Ok(String::from(
@@ -227,17 +207,6 @@ impl<'a> Context<'a> {
             ))
         })?;
 
-        self.bind("__wbindgen_boolean_new", &|me| {
-            me.expose_add_heap_object();
-            Ok(String::from(
-                "
-                function(v) {
-                    return addHeapObject(v === 1);
-                }
-                ",
-            ))
-        })?;
-
         self.bind("__wbindgen_boolean_get", &|me| {
             me.expose_get_object();
             Ok(String::from(
@@ -782,14 +751,16 @@ impl<'a> Context<'a> {
             "
             function dropRef(idx) {{
                 {}
-                let obj = slab[idx >> 1];
+                idx = idx >> 1;
+                if (idx < {}) return;
+                let obj = slab[idx];
                 {}
                 // If we hit 0 then free up our space in the slab
-                slab[idx >> 1] = slab_next;
-                slab_next = idx >> 1;
+                slab[idx] = slab_next;
+                slab_next = idx;
             }}
             ",
-            validate_owned, dec_ref
+            validate_owned, INITIAL_SLAB_VALUES.len(), dec_ref
         ));
     }
 
@@ -820,12 +791,9 @@ impl<'a> Context<'a> {
         if !self.exposed_globals.insert("slab") {
             return;
         }
-        let initial_values = [
-            "{ obj: null }",
-            "{ obj: undefined }",
-            "{ obj: true }",
-            "{ obj: false }",
-        ];
+        let initial_values = INITIAL_SLAB_VALUES.iter()
+            .map(|s| format!("{{ obj: {} }}", s))
+            .collect::<Vec<_>>();
         self.global(&format!("const slab = [{}];", initial_values.join(", ")));
         if self.config.debug {
             self.export(
@@ -1575,6 +1543,17 @@ impl<'a> Context<'a> {
         name
     }
 
+    fn expose_is_like_none(&mut self) {
+        if !self.exposed_globals.insert("is_like_none") {
+            return
+        }
+        self.global("
+            function isLikeNone(x) {
+                return x === undefined || x === null;
+            }
+        ");
+    }
+
     fn gc(&mut self) -> Result<(), Error> {
         let module = mem::replace(self.module, Module::default());
         let wasm_bytes = parity_wasm::serialize(module)?;