Assert that DOM structs have the correct first field

DOM structs embed their parent type as their first field. This
introduces a `.parent()` method to the DOM struct that returns its first
field, and codegens a type assert that ensures that `.parent()` returns
the parent struct.

This generates:

On `#[dom_struct]`:

```rust
impl HasParent for Type {
    type Parent = ParentType;
    fn as_parent(&self) -> ParentType {
        &self.first_field
    }
}
```

In the codegen files:

```rust
impl Type {
    fn __assert_parent_type(&self) {
        let _: &ParentType = self.as_parent();
    }
}
````

components/dom_struct/Cargo.toml

@@ -1,9 +1,13 @@
 [package]
-name = "dom_struct"
-version = "0.0.1"
 authors = ["The Servo Project Developers"]
 license = "MPL-2.0"
+name = "dom_struct"
 publish = false
+version = "0.0.1"
+
+[dependencies]
+quote = "0.6.3"
+syn = { version = "0.14.2", features = ["full"] }
 
 [lib]
 path = "lib.rs"

components/dom_struct/lib.rs

@@ -7,7 +7,12 @@
 
 extern crate proc_macro;
 
-use proc_macro::{TokenStream, quote};
+#[macro_use]
+extern crate quote;
+extern crate syn;
+
+use proc_macro::TokenStream;
+use syn::*;
 
 #[proc_macro_attribute]
 pub fn dom_struct(args: TokenStream, input: TokenStream) -> TokenStream {
@@ -23,5 +28,38 @@ pub fn dom_struct(args: TokenStream, input: TokenStream) -> TokenStream {
     // Work around https://github.com/rust-lang/rust/issues/46489
     let attributes: TokenStream = attributes.to_string().parse().unwrap();
 
-    attributes.into_iter().chain(input.into_iter()).collect()
+
+    let output: TokenStream = attributes.into_iter().chain(input.into_iter()).collect();
+
+    let item: Item = syn::parse(output).unwrap();
+
+    if let Item::Struct(s) = item {
+        let s2 = s.clone();
+        if s.generics.params.len() > 0 {
+            return quote!(#s2).into();
+        }
+        if let Fields::Named(ref f) = s.fields {
+            let f = f.named.first().expect("Must have at least one field").into_value();
+            let ident = f.ident.as_ref().expect("Must have named fields");
+            let name = &s.ident;
+            let ty = &f.ty;
+
+            quote! (
+                #s2
+
+                impl ::dom::bindings::inheritance::HasParent for #name {
+                    type Parent = #ty;
+                    /// This is used in a type assertion to ensure that
+                    /// the source and webidls agree as to what the parent type is
+                    fn as_parent(&self) -> &#ty {
+                        &self.#ident
+                    }
+                }
+            ).into()
+        } else {
+            panic!("#[dom_struct] only applies to structs with named fields");
+        }
+    } else {
+        panic!("#[dom_struct] only applies to structs");
+    }
 }