diff --git a/crates/backend/src/ast.rs b/crates/backend/src/ast.rs
index 34cdd9809..d336b6bf0 100644
--- a/crates/backend/src/ast.rs
+++ b/crates/backend/src/ast.rs
@@ -2,31 +2,54 @@ use proc_macro2::{Ident, Span};
 use shared;
 use syn;
 
+/// An abstract syntax tree representing a rust program. Contains
+/// extra information for joining up this rust code with javascript.
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq))]
 #[derive(Default)]
 pub struct Program {
+    /// rust -> js interfaces
     pub exports: Vec<Export>,
+    /// js -> rust interfaces
     pub imports: Vec<Import>,
+    /// rust enums
     pub enums: Vec<Enum>,
+    /// rust structs
     pub structs: Vec<Struct>,
+    /// rust type aliases
     pub type_aliases: Vec<TypeAlias>,
+    /// rust consts
     pub consts: Vec<Const>,
 }
 
+/// A rust to js interface. Allows interaction with rust objects/functions
+/// from javascript.
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]
 pub struct Export {
+    /// The javascript class name.
     pub class: Option<Ident>,
+    /// The type of `self` (either `self`, `&self`, or `&mut self`)
     pub method_self: Option<MethodSelf>,
+    /// The name of the constructor function (e.g. new).
+    ///
+    /// This allows javascript to expose an `Object` interface, where calling
+    /// the constructor maps correctly to rust.
     pub constructor: Option<String>,
+    /// The rust function
     pub function: Function,
+    /// Comments extracted from the rust source.
     pub comments: Vec<String>,
+    /// The name of the rust function/method on the rust side.
     pub rust_name: Ident,
 }
 
+/// The 3 types variations of `self`.
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]
 pub enum MethodSelf {
+    /// `self`
     ByValue,
+    /// `&mut self`
     RefMutable,
+    /// `&self`
     RefShared,
 }
 
@@ -219,6 +242,8 @@ impl Function {
 }
 
 impl Export {
+    /// Mangles a rust -> javascript export, so that the created Ident will be unique over function
+    /// name and class name, if the function belongs to a javascript class.
     pub(crate) fn rust_symbol(&self) -> Ident {
         let mut generated_name = String::from("__wasm_bindgen_generated");
         if let Some(class) = &self.class {
@@ -230,6 +255,9 @@ impl Export {
         Ident::new(&generated_name, Span::call_site())
     }
 
+    /// This is the name of the shim function that gets exported and takes the raw
+    /// ABI form of its arguments and converts them back into their normal,
+    /// "high level" form before calling the actual function.
     pub(crate) fn export_name(&self) -> String {
         let fn_name = self.function.name.to_string();
         match &self.class {
@@ -310,6 +338,7 @@ impl Import {
 }
 
 impl ImportKind {
+    /// Whether this type can be inside an `impl` block.
     pub fn fits_on_impl(&self) -> bool {
         match *self {
             ImportKind::Function(_) => true,
@@ -330,10 +359,14 @@ impl ImportKind {
 }
 
 impl ImportFunction {
+    /// If the rust object has a `fn xxx(&self) -> MyType` method, get the name for a getter in
+    /// javascript (in this case `xxx`, so you can write `val = obj.xxx`)
     fn infer_getter_property(&self) -> String {
         self.function.name.to_string()
     }
 
+    /// If the rust object has a `fn set_xxx(&mut self, MyType)` style method, get the name
+    /// for a setter in javascript (in this case `xxx`, so you can write `obj.xxx = val`)
     fn infer_setter_property(&self) -> String {
         let name = self.function.name.to_string();
         assert!(name.starts_with("set_"), "setters must start with `set_`");
diff --git a/crates/macro/src/parser.rs b/crates/macro/src/parser.rs
index 6955174e4..cb65fd2a3 100644
--- a/crates/macro/src/parser.rs
+++ b/crates/macro/src/parser.rs
@@ -5,13 +5,16 @@ use quote::ToTokens;
 use shared;
 use syn;
 
+/// Parsed attributes from a `#[wasm_bindgen(..)]`.
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]
 #[derive(Default)]
 pub struct BindgenAttrs {
+    /// List of parsed attributes
     pub attrs: Vec<BindgenAttr>,
 }
 
 impl BindgenAttrs {
+    /// Find and parse the wasm_bindgen attributes.
     fn find(attrs: &mut Vec<syn::Attribute>) -> BindgenAttrs {
         let pos = attrs
             .iter()
@@ -34,6 +37,7 @@ impl BindgenAttrs {
         syn::parse(tt.into()).expect("malformed #[wasm_bindgen] attribute")
     }
 
+    /// Get the first module attribute
     fn module(&self) -> Option<&str> {
         self.attrs
             .iter()
@@ -44,6 +48,7 @@ impl BindgenAttrs {
             .next()
     }
 
+    /// Get the first version attribute
     fn version(&self) -> Option<&str> {
         self.attrs
             .iter()
@@ -54,6 +59,7 @@ impl BindgenAttrs {
             .next()
     }
 
+    /// Whether the catch attribute is present
     fn catch(&self) -> bool {
         self.attrs.iter().any(|a| match a {
             BindgenAttr::Catch => true,
@@ -61,6 +67,7 @@ impl BindgenAttrs {
         })
     }
 
+    /// Whether the constructor attribute is present
     fn constructor(&self) -> bool {
         self.attrs.iter().any(|a| match a {
             BindgenAttr::Constructor => true,
@@ -68,6 +75,7 @@ impl BindgenAttrs {
         })
     }
 
+    /// Get the first static_method_of attribute
     fn static_method_of(&self) -> Option<&Ident> {
         self.attrs
             .iter()
@@ -78,6 +86,7 @@ impl BindgenAttrs {
             .next()
     }
 
+    /// Whether the method attributes is present
     fn method(&self) -> bool {
         self.attrs.iter().any(|a| match a {
             BindgenAttr::Method => true,
@@ -85,6 +94,7 @@ impl BindgenAttrs {
         })
     }
 
+    /// Get the first js_namespace attribute
     fn js_namespace(&self) -> Option<&Ident> {
         self.attrs
             .iter()
@@ -95,6 +105,7 @@ impl BindgenAttrs {
             .next()
     }
 
+    /// Get the first getter attribute
     fn getter(&self) -> Option<Option<Ident>> {
         self.attrs
             .iter()
@@ -105,6 +116,7 @@ impl BindgenAttrs {
             .next()
     }
 
+    /// Get the first setter attribute
     fn setter(&self) -> Option<Option<Ident>> {
         self.attrs
             .iter()
@@ -115,6 +127,7 @@ impl BindgenAttrs {
             .next()
     }
 
+    /// Whether the structural attributes is present
     fn structural(&self) -> bool {
         self.attrs.iter().any(|a| match *a {
             BindgenAttr::Structural => true,
@@ -122,6 +135,7 @@ impl BindgenAttrs {
         })
     }
 
+    /// Whether the readonly attributes is present
     fn readonly(&self) -> bool {
         self.attrs.iter().any(|a| match *a {
             BindgenAttr::Readonly => true,
@@ -129,6 +143,7 @@ impl BindgenAttrs {
         })
     }
 
+    /// Get the first js_name attribute
     fn js_name(&self) -> Option<&Ident> {
         self.attrs
             .iter()
@@ -139,6 +154,7 @@ impl BindgenAttrs {
             .next()
     }
 
+    /// Get the first js_name attribute
     fn js_class(&self) -> Option<&str> {
         self.attrs
             .iter()
@@ -165,6 +181,7 @@ impl syn::synom::Synom for BindgenAttrs {
     ));
 }
 
+/// The possible attributes in the `#[wasm_bindgen]`.
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]
 pub enum BindgenAttr {
     Catch,
@@ -258,6 +275,7 @@ impl syn::synom::Synom for BindgenAttr {
     ));
 }
 
+/// Consumes a `Ident` with the given name
 fn term<'a>(cursor: syn::buffer::Cursor<'a>, name: &str) -> syn::synom::PResult<'a, ()> {
     if let Some((ident, next)) = cursor.ident() {
         if ident == name {
@@ -267,6 +285,7 @@ fn term<'a>(cursor: syn::buffer::Cursor<'a>, name: &str) -> syn::synom::PResult<
     syn::parse_error()
 }
 
+/// Consumes a `Ident` and returns it.
 fn term2ident<'a>(cursor: syn::buffer::Cursor<'a>) -> syn::synom::PResult<'a, Ident> {
     match cursor.ident() {
         Some(pair) => Ok(pair),
@@ -274,8 +293,16 @@ fn term2ident<'a>(cursor: syn::buffer::Cursor<'a>) -> syn::synom::PResult<'a, Id
     }
 }
 
+/// Conversion trait with context.
+///
+/// Used to convert syn tokens into an AST, that we can then use to generate glue code. The context
+/// (`Ctx`) is used to pass in the attributes from the `#[wasm_bindgen]`, if needed.
 trait ConvertToAst<Ctx> {
+    /// What we are converting to.
     type Target;
+    /// Convert into our target.
+    ///
+    /// Since this is used in a procedural macro, use panic to fail.
     fn convert(self, context: Ctx) -> Self::Target;
 }
 
@@ -497,6 +524,7 @@ impl ConvertToAst<BindgenAttrs> for syn::ItemFn {
     }
 }
 
+/// Construct a function (and gets the self type if appropriate) for our AST from a syn function.
 fn function_from_decl(
     name: &Ident,
     mut decl: Box<syn::FnDecl>,
@@ -556,6 +584,10 @@ fn function_from_decl(
 }
 
 pub(crate) trait MacroParse<Ctx> {
+    /// Parse the contents of an object into our AST, with a context if necessary.
+    ///
+    /// The context is used to have access to the attributes on `#[wasm_bindgen]`, and to allow
+    /// writing to the output `TokenStream`.
     fn macro_parse(self, program: &mut ast::Program, context: Ctx);
 }
 
@@ -785,34 +817,36 @@ impl MacroParse<BindgenAttrs> for syn::ItemForeignMod {
     }
 }
 
-fn extract_first_ty_param(ty: Option<&syn::Type>) -> Option<Option<syn::Type>> {
+/// Get the first type parameter of a generic type, errors on incorrect input.
+fn extract_first_ty_param(ty: Option<&syn::Type>) -> Result<Option<syn::Type>, ()> {
     let t = match ty {
         Some(t) => t,
-        None => return Some(None),
+        None => return Ok(None),
     };
     let path = match *t {
         syn::Type::Path(syn::TypePath {
             qself: None,
             ref path,
         }) => path,
-        _ => return None,
+        _ => return Err(()),
     };
-    let seg = path.segments.last()?.into_value();
+    let seg = path.segments.last().ok_or(())?.into_value();
     let generics = match seg.arguments {
         syn::PathArguments::AngleBracketed(ref t) => t,
-        _ => return None,
+        _ => return Err(()),
     };
-    let ty = match *generics.args.first()?.into_value() {
+    let ty = match *generics.args.first().ok_or(())?.into_value() {
         syn::GenericArgument::Type(ref t) => t,
-        _ => return None,
+        _ => return Err(()),
     };
     match *ty {
-        syn::Type::Tuple(ref t) if t.elems.len() == 0 => return Some(None),
+        syn::Type::Tuple(ref t) if t.elems.len() == 0 => return Ok(None),
         _ => {}
     }
-    Some(Some(ty.clone()))
+    Ok(Some(ty.clone()))
 }
 
+/// Replace `Self` with the given name in `item`.
 fn replace_self(name: &Ident, item: &mut syn::ImplItem) {
     struct Walk<'a>(&'a Ident);
 
@@ -854,6 +888,7 @@ fn extract_doc_comments(attrs: &[syn::Attribute]) -> Vec<String> {
     .fold(vec![], |mut acc, a| {acc.extend(a); acc})
 }
 
+/// Check there are no lifetimes on the function.
 fn assert_no_lifetimes(decl: &mut syn::FnDecl) {
     struct Walk;
 
@@ -869,6 +904,7 @@ fn assert_no_lifetimes(decl: &mut syn::FnDecl) {
     syn::visit_mut::VisitMut::visit_fn_decl_mut(&mut Walk, decl);
 }
 
+/// If the path is a single ident, return it.
 fn extract_path_ident(path: &syn::Path) -> Option<Ident> {
     if path.leading_colon.is_some() {
         return None;
diff --git a/crates/webidl/src/first_pass.rs b/crates/webidl/src/first_pass.rs
index a4b063c8b..a2fd089a0 100644
--- a/crates/webidl/src/first_pass.rs
+++ b/crates/webidl/src/first_pass.rs
@@ -1,3 +1,12 @@
+//! Because some WebIDL constructs are defined in multiple places
+//! (keyword `partial` is used to add to an existing construct),
+//! We need to first walk the webidl to collect all non-partial
+//! constructs so that we have containers in which to put the
+//! partial ones.
+//!
+//! Only `interface`s, `dictionary`s, `enum`s and `mixin`s can
+//! be partial.
+
 use std::{
     collections::{BTreeMap, BTreeSet}, mem,
 };
@@ -6,21 +15,30 @@ use webidl;
 
 use super::Result;
 
+/// Collection of constructs that may use partial.
 #[derive(Default)]
 pub(crate) struct FirstPassRecord<'a> {
     pub(crate) interfaces: BTreeSet<String>,
     pub(crate) dictionaries: BTreeSet<String>,
     pub(crate) enums: BTreeSet<String>,
+    /// The mixins, mapping their name to the webidl ast node for the mixin.
     pub(crate) mixins: BTreeMap<String, MixinData<'a>>,
 }
 
+/// We need to collect mixin data during the first pass, to be used later.
 #[derive(Default)]
 pub(crate) struct MixinData<'a> {
+    /// The non partial mixin, if present. If there is more than one, we are
+    /// parsing is a malformed WebIDL file, but the parser will recover by
+    /// using the last parsed mixin.
     pub(crate) non_partial: Option<&'a webidl::ast::NonPartialMixin>,
+    /// 0 or more partial mixins.
     pub(crate) partials: Vec<&'a webidl::ast::PartialMixin>,
 }
 
+/// Implemented on an AST node to populate the `FirstPassRecord` struct.
 pub(crate) trait FirstPass {
+    /// Populate `record` with any constructs in `self`.
     fn first_pass<'a>(&'a self, record: &mut FirstPassRecord<'a>) -> Result<()>;
 }
 
diff --git a/crates/webidl/src/lib.rs b/crates/webidl/src/lib.rs
index d3de58a6d..129fbe43c 100644
--- a/crates/webidl/src/lib.rs
+++ b/crates/webidl/src/lib.rs
@@ -78,6 +78,7 @@ pub fn compile(webidl_source: &str) -> Result<String> {
     Ok(compile_ast(ast))
 }
 
+/// Run codegen on the AST to generate rust code.
 fn compile_ast(mut ast: backend::ast::Program) -> String {
     let mut defined = BTreeSet::from_iter(
         vec![
@@ -96,7 +97,9 @@ fn compile_ast(mut ast: backend::ast::Program) -> String {
     tokens.to_string()
 }
 
+/// The main trait for parsing WebIDL AST into wasm-bindgen AST.
 trait WebidlParse<Ctx> {
+    /// Parse `self` into wasm-bindgen AST, and insert it into `program`.
     fn webidl_parse(
         &self,
         program: &mut backend::ast::Program,
diff --git a/crates/webidl/src/util.rs b/crates/webidl/src/util.rs
index a2293c86b..6d276af10 100644
--- a/crates/webidl/src/util.rs
+++ b/crates/webidl/src/util.rs
@@ -10,6 +10,7 @@ use webidl::ast::ExtendedAttribute;
 
 use first_pass::FirstPassRecord;
 
+/// Take a type and create an immutable shared reference to that type.
 fn shared_ref(ty: syn::Type) -> syn::Type {
     syn::TypeReference {
         and_token: Default::default(),
@@ -19,6 +20,7 @@ fn shared_ref(ty: syn::Type) -> syn::Type {
     }.into()
 }
 
+/// For a webidl const type node, get the corresponding syn type node.
 pub fn webidl_const_ty_to_syn_ty(ty: &webidl::ast::ConstType) -> syn::Type {
     use webidl::ast::ConstType::*;
 
@@ -39,6 +41,7 @@ pub fn webidl_const_ty_to_syn_ty(ty: &webidl::ast::ConstType) -> syn::Type {
     }
 }
 
+/// Map a webidl const value to the correct wasm-bindgen const value
 pub fn webidl_const_v_to_backend_const_v(v: &webidl::ast::ConstValue) -> backend::ast::ConstValue {
     match *v {
         webidl::ast::ConstValue::BooleanLiteral(b) => backend::ast::ConstValue::BooleanLiteral(b),
@@ -49,6 +52,7 @@ pub fn webidl_const_v_to_backend_const_v(v: &webidl::ast::ConstValue) -> backend
     }
 }
 
+/// From `ident` and `Ty`, create `ident: Ty` for use in e.g. `fn(ident: Ty)`.
 fn simple_fn_arg(ident: Ident, ty: syn::Type) -> syn::ArgCaptured {
     syn::ArgCaptured {
         pat: syn::Pat::Ident(syn::PatIdent {
@@ -62,6 +66,7 @@ fn simple_fn_arg(ident: Ident, ty: syn::Type) -> syn::ArgCaptured {
     }
 }
 
+/// Create `()`.
 fn unit_ty() -> syn::Type {
     syn::Type::Tuple(syn::TypeTuple {
         paren_token: Default::default(),
@@ -69,6 +74,7 @@ fn unit_ty() -> syn::Type {
     })
 }
 
+/// From `T` create `Result<T, ::wasm_bindgen::JsValue>`.
 fn result_ty(t: syn::Type) -> syn::Type {
     let js_value = leading_colon_path_ty(vec![rust_ident("wasm_bindgen"), rust_ident("JsValue")]);
 
@@ -89,6 +95,7 @@ fn result_ty(t: syn::Type) -> syn::Type {
     ty.into()
 }
 
+/// From `T` create `[T]`.
 fn slice_ty(t: syn::Type) -> syn::Type {
     syn::TypeSlice {
         bracket_token: Default::default(),
@@ -96,6 +103,7 @@ fn slice_ty(t: syn::Type) -> syn::Type {
     }.into()
 }
 
+/// From `T` create `Vec<T>`.
 fn vec_ty(t: syn::Type) -> syn::Type {
     let arguments = syn::PathArguments::AngleBracketed(syn::AngleBracketedGenericArguments {
         colon2_token: None,
@@ -113,6 +121,7 @@ fn vec_ty(t: syn::Type) -> syn::Type {
     ty.into()
 }
 
+/// Possible positions for a type in a function signature.
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum TypePosition {
     Argument,
@@ -120,11 +129,14 @@ pub enum TypePosition {
 }
 
 impl<'a> FirstPassRecord<'a> {
+    /// Use information from the first pass to work out the correct Rust type to use for
+    /// a given WebIDL type.
     pub fn webidl_ty_to_syn_ty(
         &self,
         ty: &webidl::ast::Type,
         pos: TypePosition,
     ) -> Option<syn::Type> {
+        // Array type is borrowed for arguments (`&[T]`) and owned for return value (`Vec<T>`).
         let array = |base_ty: &str| {
             match pos {
                 TypePosition::Argument => {
@@ -220,6 +232,8 @@ impl<'a> FirstPassRecord<'a> {
                 return None;
             }
         };
+
+        // Map nullable to an option.
         if ty.nullable {
             let arguments = syn::PathArguments::AngleBracketed(syn::AngleBracketedGenericArguments {
                 colon2_token: None,
@@ -240,6 +254,10 @@ impl<'a> FirstPassRecord<'a> {
         }
     }
 
+    /// Use the first pass to convert webidl function arguments to rust arguments.
+    ///
+    /// `kind` is whether the function is a method, in which case we would need a `self`
+    /// parameter.
     fn webidl_arguments_to_syn_arg_captured<'b, I>(
         &self,
         arguments: I,
@@ -284,6 +302,7 @@ impl<'a> FirstPassRecord<'a> {
         Some(res)
     }
 
+    /// Create a wasm-bindgen function, if possible.
     pub fn create_function<'b, I>(
         &self,
         name: &str,
@@ -333,6 +352,7 @@ impl<'a> FirstPassRecord<'a> {
         })
     }
 
+    /// Create a wasm-bindgen method, if possible.
     pub fn create_basic_method(
         &self,
         arguments: &[webidl::ast::Argument],
@@ -384,6 +404,7 @@ impl<'a> FirstPassRecord<'a> {
         )
     }
 
+    /// Create a wasm-bindgen getter method, if possible.
     pub fn create_getter(
         &self,
         name: &str,
@@ -413,6 +434,7 @@ impl<'a> FirstPassRecord<'a> {
         self.create_function(name, iter::empty(), ret, kind, is_structural, catch)
     }
 
+    /// Create a wasm-bindgen setter method, if possible.
     pub fn create_setter(
         &self,
         name: &str,
@@ -442,6 +464,7 @@ impl<'a> FirstPassRecord<'a> {
     }
 }
 
+/// Search for an attribute by name in some webidl object's attributes.
 fn has_named_attribute(ext_attrs: &[Box<ExtendedAttribute>], attribute: &str) -> bool {
     ext_attrs.iter().any(|attr| match &**attr {
         ExtendedAttribute::NoArguments(webidl::ast::Other::Identifier(name)) => {
@@ -456,10 +479,12 @@ pub fn is_chrome_only(ext_attrs: &[Box<ExtendedAttribute>]) -> bool {
     has_named_attribute(ext_attrs, "ChromeOnly")
 }
 
+/// Whether a webidl object is marked as a no interface object.
 pub fn is_no_interface_object(ext_attrs: &[Box<ExtendedAttribute>]) -> bool {
     has_named_attribute(ext_attrs, "NoInterfaceObject")
 }
 
+/// Whether a webidl object is marked as structural.
 pub fn is_structural(attrs: &[Box<ExtendedAttribute>]) -> bool {
     attrs.iter().any(|attr| match &**attr {
         ExtendedAttribute::NoArguments(webidl::ast::Other::Identifier(name)) => {
@@ -469,6 +494,7 @@ pub fn is_structural(attrs: &[Box<ExtendedAttribute>]) -> bool {
     })
 }
 
+/// Whether a webidl object is marked as throwing.
 pub fn throws(attrs: &[Box<ExtendedAttribute>]) -> bool {
     attrs.iter().any(|attr| match &**attr {
         ExtendedAttribute::NoArguments(webidl::ast::Other::Identifier(name)) => name == "Throws",
@@ -476,6 +502,7 @@ pub fn throws(attrs: &[Box<ExtendedAttribute>]) -> bool {
     })
 }
 
+/// Create a syn `pub` token
 pub fn public() -> syn::Visibility {
     syn::Visibility::Public(syn::VisPublic {
         pub_token: Default::default(),