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.

Author: alexcrichton

crates/backend/src/ast.rs

@@ -126,6 +126,7 @@ pub struct ImportType {
     pub attrs: Vec<syn::Attribute>,
     pub doc_comment: Option<String>,
     pub instanceof_shim: String,
+    pub extends: Vec<Ident>,
 }
 
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]

crates/backend/src/codegen.rs

@@ -656,6 +656,31 @@ impl ToTokens for ast::ImportType {
                 ()
             };
         }).to_tokens(tokens);
+
+        for superclass in self.extends.iter() {
+            (quote! {
+                impl From<#name> for #superclass {
+                    fn from(obj: #name) -> #superclass {
+                        use wasm_bindgen::JsCast;
+                        #superclass::unchecked_from_js(obj.into())
+                    }
+                }
+
+                impl AsRef<#superclass> for #name {
+                    fn as_ref(&self) -> &#superclass {
+                        use wasm_bindgen::JsCast;
+                        #superclass::unchecked_from_js_ref(self.as_ref())
+                    }
+                }
+
+                impl AsMut<#superclass> for #name {
+                    fn as_mut(&mut self) -> &mut #superclass {
+                        use wasm_bindgen::JsCast;
+                        #superclass::unchecked_from_js_mut(self.as_mut())
+                    }
+                }
+            }).to_tokens(tokens);
+        }
     }
 }
 

crates/macro-support/src/parser.rs

@@ -169,6 +169,16 @@ impl BindgenAttrs {
             })
             .next()
     }
+
+    /// Return the list of classes that a type extends
+    fn extends(&self) -> impl Iterator<Item = &Ident> {
+        self.attrs
+            .iter()
+            .filter_map(|a| match a {
+                BindgenAttr::Extends(s) => Some(s),
+                _ => None,
+            })
+    }
 }
 
 impl syn::synom::Synom for BindgenAttrs {
@@ -202,6 +212,7 @@ pub enum BindgenAttr {
     Readonly,
     JsName(Ident),
     JsClass(String),
+    Extends(Ident),
 }
 
 impl syn::synom::Synom for BindgenAttr {
@@ -277,6 +288,13 @@ impl syn::synom::Synom for BindgenAttr {
             s: syn!(syn::LitStr) >>
             (s.value())
         )=> { BindgenAttr::JsClass }
+        |
+        do_parse!(
+            call!(term, "extends") >>
+            punct!(=) >>
+            ns: call!(term2ident) >>
+            (ns)
+        )=> { BindgenAttr::Extends }
     ));
 }
 
@@ -490,17 +508,18 @@ impl<'a> ConvertToAst<(BindgenAttrs, &'a Option<String>)> for syn::ForeignItemFn
     }
 }
 
-impl ConvertToAst<()> for syn::ForeignItemType {
+impl ConvertToAst<BindgenAttrs> for syn::ForeignItemType {
     type Target = ast::ImportKind;
 
-    fn convert(self, (): ()) -> Result<Self::Target, Diagnostic> {
+    fn convert(self, attrs: BindgenAttrs) -> Result<Self::Target, Diagnostic> {
         let shim = format!("__wbg_instanceof_{}_{}", self.ident, ShortHash(&self.ident));
         Ok(ast::ImportKind::Type(ast::ImportType {
             vis: self.vis,
             attrs: self.attrs,
             doc_comment: None,
             instanceof_shim: shim,
             name: self.ident,
+            extends: attrs.extends().cloned().collect(),
         }))
     }
 }
@@ -907,7 +926,7 @@ impl<'a> MacroParse<&'a BindgenAttrs> for syn::ForeignItem {
         let js_namespace = item_opts.js_namespace().or(opts.js_namespace()).cloned();
         let kind = match self {
             syn::ForeignItem::Fn(f) => f.convert((item_opts, &module))?,
-            syn::ForeignItem::Type(t) => t.convert(())?,
+            syn::ForeignItem::Type(t) => t.convert(item_opts)?,
             syn::ForeignItem::Static(s) => s.convert(item_opts)?,
             _ => panic!("only foreign functions/types allowed for now"),
         };

crates/webidl/src/first_pass.rs

@@ -14,6 +14,7 @@ use std::{
 use webidl;
 
 use super::Result;
+use util::camel_case_ident;
 
 /// Collection of constructs that may use partial.
 #[derive(Default)]
@@ -33,6 +34,7 @@ pub(crate) struct InterfaceData {
     pub(crate) partial: bool,
     pub(crate) operations: BTreeMap<OperationId, OperationData>,
     pub(crate) global: bool,
+    pub(crate) superclass: Option<String>,
 }
 
 #[derive(PartialEq, Eq, PartialOrd, Ord)]
@@ -178,6 +180,7 @@ impl FirstPass<()> for webidl::ast::NonPartialInterface {
             .and_modify(|interface_data| {
                 if interface_data.partial {
                     interface_data.partial = false;
+                    interface_data.superclass = self.inherits.clone();
                 } else {
                     warn!("Encountered multiple declarations of {}", self.name);
                 }
@@ -187,6 +190,7 @@ impl FirstPass<()> for webidl::ast::NonPartialInterface {
                     partial: false,
                     operations: Default::default(),
                     global: false,
+                    superclass: self.inherits.clone(),
                 },
             );
 
@@ -216,6 +220,7 @@ impl FirstPass<()> for webidl::ast::PartialInterface {
                     partial: true,
                     operations: Default::default(),
                     global: false,
+                    superclass: None,
                 },
             );
 
@@ -386,3 +391,27 @@ impl FirstPass<()> for webidl::ast::Typedef {
         Ok(())
     }
 }
+
+impl<'a> FirstPassRecord<'a> {
+    pub fn all_superclasses<'me>(&'me self, interface: &str)
+        -> impl Iterator<Item = String> + 'me
+    {
+        let mut set = BTreeSet::new();
+        self.fill_superclasses(interface, &mut set);
+        set.into_iter()
+    }
+
+    fn fill_superclasses(&self, interface: &str, set: &mut BTreeSet<String>) {
+        let data = match self.interfaces.get(interface) {
+            Some(data) => data,
+            None => return,
+        };
+        let superclass = match &data.superclass {
+            Some(class) => class,
+            None => return,
+        };
+        if set.insert(camel_case_ident(superclass)) {
+            self.fill_superclasses(superclass, set);
+        }
+    }
+}

crates/webidl/src/lib.rs

@@ -38,6 +38,7 @@ use backend::defined::{ImportedTypeDefinitions, RemoveUndefinedImports};
 use backend::util::{ident_ty, rust_ident, wrap_import_function};
 use failure::{ResultExt, Fail};
 use heck::{ShoutySnakeCase};
+use proc_macro2::{Ident, Span};
 
 use first_pass::{FirstPass, FirstPassRecord};
 use util::{ApplyTypedefs, public, webidl_const_ty_to_syn_ty, webidl_const_v_to_backend_const_v, camel_case_ident, mdn_doc};
@@ -252,6 +253,9 @@ impl WebidlParse<()> for webidl::ast::NonPartialInterface {
                 attrs: Vec::new(),
                 doc_comment,
                 instanceof_shim: format!("__widl_instanceof_{}", self.name),
+                extends: first_pass.all_superclasses(&self.name)
+                    .map(|name| Ident::new(&name, Span::call_site()))
+                    .collect(),
             }),
         });
 

guide/src/design/import-customization.md

@@ -199,3 +199,25 @@ possibilities!
 
   All of these functions will call `console.log` in JS, but each identifier
   will have only one signature in Rust.
+
+* `extends = Bar` - this can be used to say that an imported type extends
+  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 `extends = ...` attribute can be specified multiple times for longer
+  inheritance chains, and `AsRef` and such impls will be generated for each of
+  the types.

tests/wasm/jscast.js

@@ -4,15 +4,25 @@ class JsCast1 {
   }
   myval() { return this.val; }
 }
+
 class JsCast2 {
 }
+
 class JsCast3 extends JsCast1 {
   constructor() {
     super();
     this.val = 3;
   }
 }
 
+class JsCast4 extends JsCast3 {
+  constructor() {
+    super();
+    this.val = 4;
+  }
+}
+
 exports.JsCast1 = JsCast1;
 exports.JsCast2 = JsCast2;
 exports.JsCast3 = JsCast3;
+exports.JsCast4 = JsCast4;

tests/wasm/jscast.rs

@@ -14,9 +14,15 @@ extern {
     #[wasm_bindgen(constructor)]
     fn new() -> JsCast2;
 
+    #[wasm_bindgen(extends = JsCast1)]
     type JsCast3;
     #[wasm_bindgen(constructor)]
     fn new() -> JsCast3;
+
+    #[wasm_bindgen(extends = JsCast1, extends = JsCast3)]
+    type JsCast4;
+    #[wasm_bindgen(constructor)]
+    fn new() -> JsCast4;
 }
 
 #[wasm_bindgen_test]
@@ -65,4 +71,18 @@ fn method_calling() {
     assert_eq!(a.myval(), 1);
     assert_eq!(b.dyn_ref::<JsCast1>().unwrap().myval(), 3);
     assert_eq!(b.unchecked_ref::<JsCast1>().myval(), 3);
+    let c: &JsCast1 = b.as_ref();
+    assert_eq!(c.myval(), 3);
+}
+
+#[wasm_bindgen_test]
+fn multiple_layers_of_inheritance() {
+    let a = JsCast4::new();
+    assert!(a.is_instance_of::<JsCast4>());
+    assert!(a.is_instance_of::<JsCast3>());
+    assert!(a.is_instance_of::<JsCast1>());
+
+    let _: &JsCast3 = a.as_ref();
+    let b: &JsCast1 = a.as_ref();
+    assert_eq!(b.myval(), 4);
 }