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/js2rust.rs

@@ -122,20 +122,31 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
         let i = self.arg_idx;
         let name = self.abi_arg();
 
+        let (arg, optional) = match arg {
+            Descriptor::Option(t) => (&**t, true),
+            _ => (arg, false),
+        };
+
         if let Some(kind) = arg.vector_kind() {
             self.js_arguments
                 .push((name.clone(), kind.js_ty().to_string()));
 
             let func = self.cx.pass_to_wasm_function(kind)?;
+            let val = if optional {
+                self.cx.expose_is_like_none();
+                format!("isLikeNone({}) ? [0, 0] : {}({})", name, func, name)
+            } else {
+                format!("{}({})", func, name)
+            };
             self.prelude(&format!(
-                "\
-                 const [ptr{i}, len{i}] = {func}({arg});\n\
-                 ",
+                "const [ptr{i}, len{i}] = {val};",
                 i = i,
-                func = func,
-                arg = name
+                val = val,
             ));
             if arg.is_by_ref() {
+                if optional {
+                    bail!("optional slices aren't currently supported");
+                }
                 if arg.is_mut_ref() {
                     let get = self.cx.memview_function(kind);
                     self.finally(&format!(
@@ -165,6 +176,25 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
             return Ok(self);
         }
 
+        if arg.is_anyref() {
+            self.js_arguments.push((name.clone(), "any".to_string()));
+            self.cx.expose_add_heap_object();
+            if optional {
+                self.cx.expose_is_like_none();
+                self.rust_arguments.push(format!(
+                    "isLikeNone({0}) ? 0 : addHeapObject({0})",
+                    name,
+                ));
+            } else {
+                self.rust_arguments.push(format!("addHeapObject({})", name));
+            }
+            return Ok(self);
+        }
+
+        if optional {
+            bail!("unsupported optional argument to rust function {:?}", arg);
+        }
+
         if let Some(s) = arg.rust_struct() {
             self.js_arguments.push((name.clone(), s.to_string()));
 
@@ -262,11 +292,6 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
                 self.js_arguments.push((name.clone(), "string".to_string()));
                 self.rust_arguments.push(format!("{}.codePointAt(0)", name))
             }
-            Descriptor::Anyref => {
-                self.js_arguments.push((name.clone(), "any".to_string()));
-                self.cx.expose_add_heap_object();
-                self.rust_arguments.push(format!("addHeapObject({})", name));
-            }
             _ => bail!("unsupported argument to rust function {:?}", arg),
         }
         Ok(self)
@@ -282,16 +307,10 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
             }
         };
 
-        if ty.is_ref_anyref() {
-            self.ret_ty = "any".to_string();
-            self.cx.expose_get_object();
-            self.ret_expr = format!("return getObject(RET);");
-            return Ok(self);
-        }
-
-        if ty.is_by_ref() {
-            bail!("cannot return references from Rust to JS yet")
-        }
+        let (ty, optional) = match ty {
+            Descriptor::Option(t) => (&**t, true),
+            _ => (ty, false),
+        };
 
         if let Some(ty) = ty.vector_kind() {
             self.ret_ty = ty.js_ty().to_string();
@@ -307,16 +326,42 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
                  const mem = getUint32Memory();\n\
                  const ptr = mem[retptr / 4];\n\
                  const len = mem[retptr / 4 + 1];\n\
+                 {guard}
                  const realRet = {}(ptr, len).slice();\n\
                  wasm.__wbindgen_free(ptr, len * {});\n\
                  return realRet;\n\
                  ",
                 f,
-                ty.size()
+                ty.size(),
+                guard = if optional { "if (ptr === 0) return;" } else { "" },
             );
             return Ok(self);
         }
 
+        // No need to worry about `optional` here, the abi representation means
+        // that `takeObject` will naturally pluck out `undefined`.
+        if ty.is_anyref() {
+            self.ret_ty = "any".to_string();
+            self.cx.expose_take_object();
+            self.ret_expr = format!("return takeObject(RET);");
+            return Ok(self);
+        }
+
+        if optional {
+            bail!("unsupported optional argument to rust function {:?}", ty);
+        }
+
+        if ty.is_ref_anyref() {
+            self.ret_ty = "any".to_string();
+            self.cx.expose_get_object();
+            self.ret_expr = format!("return getObject(RET);");
+            return Ok(self);
+        }
+
+        if ty.is_by_ref() {
+            bail!("cannot return references from Rust to JS yet")
+        }
+
         if let Some(name) = ty.rust_struct() {
             self.ret_ty = name.to_string();
             self.ret_expr = format!("return {name}.__construct(RET);", name = name);
@@ -360,11 +405,6 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
                 self.ret_ty = "string".to_string();
                 self.ret_expr = format!("return String.fromCodePoint(RET);")
             }
-            Descriptor::Anyref => {
-                self.ret_ty = "any".to_string();
-                self.cx.expose_take_object();
-                self.ret_expr = format!("return takeObject(RET);");
-            }
             _ => bail!("unsupported return from Rust to JS {:?}", ty),
         }
         Ok(self)