Implement #[wasm_bindgen(extends = ...)]

This commit implements the `extends` attribute for `#[wasm_bindgen]` to
statically draw the inheritance hierarchy in the generated bindings, generating
appropriate `AsRef`, `AsMut`, and `From` implementations.

guide/src/SUMMARY.md

@@ -20,6 +20,7 @@
     - [On JavaScript Imports](./reference/attributes/on-js-imports/index.md)
       - [`catch`](./reference/attributes/on-js-imports/catch.md)
       - [`constructor`](./reference/attributes/on-js-imports/constructor.md)
+      - [`extends`](./reference/attributes/on-js-imports/extends.md)
       - [`getter` and `setter`](./reference/attributes/on-js-imports/getter-and-setter.md)
       - [`js_class = "Blah"`](./reference/attributes/on-js-imports/js_class.md)
       - [`js_name`](./reference/attributes/on-js-imports/js_name.md)

guide/src/design/import-customization.md

@@ -0,0 +1,170 @@
+# Customizing import behavior
+
+The `#[wasm_bindgen]` macro supports a good amount of configuration for
+controlling precisely how imports are imported and what they map to in JS. This
+section is intended to hopefully be an exhaustive reference of the
+possibilities!
+
+* `catch` - this attribute allows catching a JS exception. This can be attached
+  to any imported function and the function must return a `Result` where the
+  `Err` payload is a `JsValue`, like so:
+
+  ```rust
+  #[wasm_bindgen]
+  extern {
+      #[wasm_bindgen(catch)]
+      fn foo() -> Result<(), JsValue>;
+  }
+  ```
+
+  If the imported function throws an exception then `Err` will be returned with
+  the exception that was raised, and otherwise `Ok` is returned with the result
+  of the function.
+
+  By default `wasm-bindgen` will take no action when wasm calls a JS function
+  which ends up throwing an exception. The wasm spec right now doesn't support
+  stack unwinding and as a result Rust code **will not execute destructors**.
+  This can unfortunately cause memory leaks in Rust right now, but as soon as
+  wasm implements catching exceptions we'll be sure to add support as well!
+
+* `constructor` - this is used to indicate that the function being bound should
+  actually translate to a `new` constructor in JS. The final argument must be a
+  type that's imported from JS, and it's what'll get used in JS:
+
+  ```rust
+  #[wasm_bindgen]
+  extern {
+      type Foo;
+      #[wasm_bindgen(constructor)]
+      fn new() -> Foo;
+  }
+  ```
+
+  This will attach the `new` function to the `Foo` type (implied by
+  `constructor`) and in JS when this function is called it will be equivalent to
+  `new Foo()`.
+
+* `method` - this is the gateway to adding methods to imported objects or
+  otherwise accessing properties on objects via methods and such. This should be
+  done for doing the equivalent of expressions like `foo.bar()` in JS.
+
+  ```rust
+  #[wasm_bindgen]
+  extern {
+      type Foo;
+      #[wasm_bindgen(method)]
+      fn work(this: &Foo);
+  }
+  ```
+
+  The first argument of a `method` annotation must be a borrowed reference (not
+  mutable, shared) to the type that the method is attached to. In this case
+  we'll be able to call this method like `foo.work()` in JS (where `foo` has
+  type `Foo`).
+
+  In JS this invocation will correspond to accessing `Foo.prototype.work` and
+  then calling that when the import is called. Note that `method` by default
+  implies going through `prototype` to get a function pointer.
+
+* `js_namespace` - this attribute indicates that the JS type is accessed through
+  a particular namespace. For example the `WebAssembly.Module` APIs are all
+  accessed through the `WebAssembly` namespace. The `js_namespace` can be
+  applied to any import and whenever the generated JS attempts to reference a
+  name (like a class or function name) it'll be accessed through this namespace.
+
+  ```rust
+  #[wasm_bindgen]
+  extern {
+      #[wasm_bindgen(js_namespace = console)]
+      fn log(s: &str);
+  }
+  ```
+
+  This is an example of how to bind `console.log(x)` in Rust. The `log` function
+  will be available in the Rust module and will be invoked as `console.log` in
+  JS.
+
+* `getter` and `setter` - these two attributes can be combined with `method` to
+  indicate that this is a getter or setter method. A `getter`-tagged function by
+  default accesses the JS property with the same name as the getter function. A
+  `setter`'s function name is currently required to start with "set\_" and the
+  property it accesses is the suffix after "set\_".
+
+  ```rust
+  #[wasm_bindgen]
+  extern {
+      type Foo;
+      #[wasm_bindgen(method, getter)]
+      fn property(this: &Foo) -> u32;
+      #[wasm_bindgen(method, setter)]
+      fn set_property(this: &Foo, val: u32);
+  }
+  ```
+
+  Here we're importing the `Foo` type and defining the ability to access each
+  object's `property` property. The first function here is a getter and will be
+  available in Rust as `foo.property()`, and the latter is the setter which is
+  accessible as `foo.set_property(2)`. Note that both functions have a `this`
+  argument as they're tagged with `method`.
+
+  Finally, you can also pass an argument to the `getter` and `setter`
+  properties to configure what property is accessed. When the property is
+  explicitly specified then there is no restriction on the method name. For
+  example the below is equivalent to the above:
+
+  ```rust
+  #[wasm_bindgen]
+  extern {
+      type Foo;
+      #[wasm_bindgen(method, getter = property)]
+      fn assorted_method_name(this: &Foo) -> u32;
+      #[wasm_bindgen(method, setter = "property")]
+      fn some_other_method_name(this: &Foo, val: u32);
+  }
+  ```
+
+  Properties in JS are accessed through `Object.getOwnPropertyDescriptor`. Note
+  that this typically only works for class-like-defined properties which aren't
+  just attached properties on any old object. For accessing any old property on
+  an object we can use...
+
+* `structural` - this is a flag to `method` annotations which indicates that the
+  method being accessed (or property with getters/setters) should be accessed in
+  a structural fashion. For example methods are *not* accessed through
+  `prototype` and properties are accessed on the object directly rather than
+  through `Object.getOwnPropertyDescriptor`.
+
+  ```rust
+  #[wasm_bindgen]
+  extern {
+      type Foo;
+      #[wasm_bindgen(method, structural)]
+      fn bar(this: &Foo);
+      #[wasm_bindgen(method, getter, structural)]
+      fn baz(this: &Foo) -> u32;
+  }
+  ```
+
+  The type here, `Foo`, is not required to exist in JS (it's not referenced).
+  Instead wasm-bindgen will generate shims that will access the passed in JS
+  value's `bar` property to or the `baz` property (depending on the function).
+
+* `js_name = foo` - this can be used to bind to a different function in JS than
+  the identifier that's defined in Rust. For example you can also define
+  multiple signatures for a polymorphic function in JS as well:
+
+  ```rust
+  #[wasm_bindgen]
+  extern {
+      type Foo;
+      #[wasm_bindgen(js_namespace = console, js_name = log)]
+      fn log_string(s: &str);
+      #[wasm_bindgen(js_namespace = console, js_name = log)]
+      fn log_u32(n: u32);
+      #[wasm_bindgen(js_namespace = console, js_name = log)]
+      fn log_many(a: u32, b: JsValue);
+  }
+  ```
+
+  All of these functions will call `console.log` in JS, but each identifier
+  will have only one signature in Rust.

guide/src/reference/attributes/on-js-imports/extends.md

@@ -0,0 +1,49 @@
+# `extends = Class`
+
+The `extends` attribute can be used to say that an imported type extends (in the
+JS class hierarchy sense) another type. This will generate `AsRef`, `AsMut`, and
+`From` impls for converting a type into another given that we statically know
+the inheritance hierarchy:
+
+```rust
+#[wasm_bindgen]
+extern {
+    type Foo;
+
+    #[wasm_bindgen(extends = Foo)]
+    type Bar;
+}
+
+let x: &Bar = ...;
+let y: &Foo = x.as_ref(); // zero cost cast
+```
+
+The trait implementations generated for the above block are:
+
+```rust
+impl From<Bar> for Foo { ... }
+impl AsRef<Foo> for Bar { ... }
+impl AsMut<Foo> for Bar { ... }
+```
+
+
+The `extends = ...` attribute can be specified multiple times for longer
+inheritance chains, and `AsRef` and such impls will be generated for each of
+the types.
+
+```rust
+#[wasm_bindgen]
+extern {
+    type Foo;
+
+    #[wasm_bindgen(extends = Foo)]
+    type Bar;
+
+    #[wasm_bindgen(extends = Foo, extends = Bar)]
+    type Baz;
+}
+
+let x: &Baz = ...;
+let y1: &Bar = x.as_ref();
+let y2: &Foo = x.as_ref();
+```