Add no_deref attribute to opt out of generating deref impls for imported types (#2652)

* Add `no_deref` attribute to opt out of `Deref` impl

* Document `no_deref`

Author: Jules-Bertholet

crates/backend/src/ast.rs

@@ -261,6 +261,8 @@ pub struct ImportType {
     pub extends: Vec<syn::Path>,
     /// A custom prefix to add and attempt to fall back to, if the type isn't found
     pub vendor_prefixes: Vec<Ident>,
+    /// If present, don't generate a `Deref` impl
+    pub no_deref: bool,
 }
 
 /// The metadata for an Enum being imported

crates/backend/src/codegen.rs

@@ -618,6 +618,8 @@ impl ToTokens for ast::ImportType {
             }
         });
 
+        let no_deref = self.no_deref;
+
         (quote! {
             #[allow(bad_style)]
             #(#attrs)*
@@ -644,15 +646,6 @@ impl ToTokens for ast::ImportType {
                     }
                 }
 
-                impl core::ops::Deref for #rust_name {
-                    type Target = #internal_obj;
-
-                    #[inline]
-                    fn deref(&self) -> &#internal_obj {
-                        &self.obj
-                    }
-                }
-
                 impl IntoWasmAbi for #rust_name {
                     type Abi = <JsValue as IntoWasmAbi>::Abi;
 
@@ -779,6 +772,21 @@ impl ToTokens for ast::ImportType {
         })
         .to_tokens(tokens);
 
+        if !no_deref {
+            (quote! {
+                #[allow(clippy::all)]
+                impl core::ops::Deref for #rust_name {
+                    type Target = #internal_obj;
+
+                    #[inline]
+                    fn deref(&self) -> &#internal_obj {
+                        &self.obj
+                    }
+                }
+            })
+            .to_tokens(tokens);
+        }
+
         for superclass in self.extends.iter() {
             (quote! {
                 #[allow(clippy::all)]

crates/macro-support/src/parser.rs

@@ -52,6 +52,7 @@ macro_rules! attrgen {
             (inspectable, Inspectable(Span)),
             (is_type_of, IsTypeOf(Span, syn::Expr)),
             (extends, Extends(Span, syn::Path)),
+            (no_deref, NoDeref(Span)),
             (vendor_prefix, VendorPrefix(Span, Ident)),
             (variadic, Variadic(Span)),
             (typescript_custom_section, TypescriptCustomSection(Span)),
@@ -612,6 +613,7 @@ impl ConvertToAst<BindgenAttrs> for syn::ForeignItemType {
         let shim = format!("__wbg_instanceof_{}_{}", self.ident, ShortHash(&self.ident));
         let mut extends = Vec::new();
         let mut vendor_prefixes = Vec::new();
+        let no_deref = attrs.no_deref().is_some();
         for (used, attr) in attrs.attrs.iter() {
             match attr {
                 BindgenAttr::Extends(_, e) => {
@@ -637,6 +639,7 @@ impl ConvertToAst<BindgenAttrs> for syn::ForeignItemType {
             js_name,
             extends,
             vendor_prefixes,
+            no_deref,
         }))
     }
 }

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

@@ -0,0 +1,24 @@
+# `no_deref`
+
+The `no_deref` attribute can be used to say that no `Deref` impl should be
+generated for an imported type. If this attribute is not present, a `Deref` impl
+will be generated with a `Target` of the type's first `extends` attribute, or
+`Target = JsValue` if there are no `extends` attributes.
+
+```rust
+#[wasm_bindgen]
+extern "C" {
+    type Foo;
+
+    #[wasm_bindgen(method)]
+    fn baz(this: &Foo)
+
+    #[wasm_bindgen(extends = Foo, no_deref)]
+    type Bar;
+}
+
+fn do_stuff(bar: &Bar) {
+    bar.baz() // Does not compile
+}
+
+```