Support variadic javascript function parameters

crates/backend/src/ast.rs

@@ -85,6 +85,7 @@ pub struct ImportFunction {
     pub rust_name: Ident,
     pub js_ret: Option<syn::Type>,
     pub catch: bool,
+    pub variadic: bool,
     pub structural: bool,
     pub kind: ImportFunctionKind,
     pub shim: Ident,
@@ -463,6 +464,7 @@ impl ImportFunction {
         shared::ImportFunction {
             shim: self.shim.to_string(),
             catch: self.catch,
+            variadic: self.variadic,
             method,
             structural: self.structural,
             function: self.function.shared(),

crates/cli-support/src/js/mod.rs

@@ -1733,7 +1733,6 @@ impl<'a> Context<'a> {
     }
 
     fn global(&mut self, s: &str) {
-        let s = s;
         let s = s.trim();
 
         // Ensure a blank line between adjacent items, and ensure everything is
@@ -1963,8 +1962,9 @@ impl<'a, 'b> SubContext<'a, 'b> {
 
         let js = Rust2Js::new(self.cx)
             .catch(import.catch)
+            .variadic(import.variadic)
             .process(descriptor.unwrap_function())?
-            .finish(&target);
+            .finish(&target)?;
         self.cx.export(&import.shim, &js, None);
         Ok(())
     }

crates/cli-support/src/js/rust2js.rs

@@ -1,4 +1,4 @@
-use failure::Error;
+use failure::{self, Error};
 
 use super::{Context, Js2Rust};
 use descriptor::{Descriptor, Function};
@@ -36,6 +36,9 @@ pub struct Rust2Js<'a, 'b: 'a> {
 
     /// Whether or not we're catching JS exceptions
     catch: bool,
+
+    /// Whether or not the last argument is a slice representing variadic arguments.
+    variadic: bool,
 }
 
 impl<'a, 'b> Rust2Js<'a, 'b> {
@@ -50,6 +53,7 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
             arg_idx: 0,
             ret_expr: String::new(),
             catch: false,
+            variadic: false,
         }
     }
 
@@ -62,6 +66,15 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         self
     }
 
+    pub fn variadic(&mut self, variadic: bool) -> &mut Self {
+        if variadic {
+            self.cx.expose_uint32_memory();
+            self.cx.expose_add_heap_object();
+        }
+        self.variadic = variadic;
+        self
+    }
+
     /// Generates all bindings necessary for the signature in `Function`,
     /// creating necessary argument conversions and return value processing.
     pub fn process(&mut self, function: &Function) -> Result<&mut Self, Error> {
@@ -72,6 +85,7 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         Ok(self)
     }
 
+    /// Get a generated name for an argument.
     fn shim_argument(&mut self) -> String {
         let s = format!("arg{}", self.arg_idx);
         self.arg_idx += 1;
@@ -515,7 +529,7 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         Ok(())
     }
 
-    pub fn finish(&self, invoc: &str) -> String {
+    pub fn finish(&self, invoc: &str) -> Result<String, Error> {
         let mut ret = String::new();
         ret.push_str("function(");
         ret.push_str(&self.shim_arguments.join(", "));
@@ -528,10 +542,24 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         ret.push_str(") {\n");
         ret.push_str(&self.prelude);
 
-        let mut invoc = self.ret_expr.replace(
-            "JS",
-            &format!("{}({})", invoc, self.js_arguments.join(", ")),
-        );
+        let mut invoc = if self.variadic {
+            if self.js_arguments.is_empty() {
+                return Err(failure::err_msg("a function with no arguments cannot be variadic"));
+            }
+            let last_arg = self.js_arguments.len() - 1; // check implies >= 0
+            self.ret_expr.replace(
+                "JS",
+                &format!("{}({}, ...{})",
+                         invoc,
+                         self.js_arguments[..last_arg].join(", "),
+                         self.js_arguments[last_arg])
+            )
+        } else {
+            self.ret_expr.replace(
+                "JS",
+                &format!("{}({})", invoc, self.js_arguments.join(", ")),
+            )
+        };
         if self.catch {
             let catch = "\
                          const view = getUint32Memory();\n\
@@ -566,7 +594,7 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         ret.push_str(&invoc);
 
         ret.push_str("\n}\n");
-        return ret;
+        Ok(ret)
     }
 
     fn global_idx(&mut self) -> usize {

crates/macro-support/src/parser.rs

@@ -183,6 +183,14 @@ impl BindgenAttrs {
             _ => None,
         })
     }
+
+    /// Whether the variadic attributes is present
+    fn variadic(&self) -> bool {
+        self.attrs.iter().any(|a| match *a {
+            BindgenAttr::Variadic => true,
+            _ => false,
+        })
+    }
 }
 
 impl syn::synom::Synom for BindgenAttrs {
@@ -219,6 +227,7 @@ pub enum BindgenAttr {
     JsName(String),
     JsClass(String),
     Extends(Ident),
+    Variadic,
 }
 
 impl syn::synom::Synom for BindgenAttr {
@@ -304,6 +313,8 @@ impl syn::synom::Synom for BindgenAttr {
             ns: call!(term2ident) >>
             (ns)
         )=> { BindgenAttr::Extends }
+        |
+        call!(term, "variadic") => { |_| BindgenAttr::Variadic }
     ));
 }
 
@@ -365,6 +376,7 @@ impl<'a> ConvertToAst<()> for &'a mut syn::ItemStruct {
                 let getter = shared::struct_field_get(&ident, &name_str);
                 let setter = shared::struct_field_set(&ident, &name_str);
                 let opts = BindgenAttrs::find(&mut field.attrs)?;
+                assert_not_variadic(&opts, &field)?;
                 let comments = extract_doc_comments(&field.attrs);
                 fields.push(ast::StructField {
                     name: name.clone(),
@@ -395,6 +407,7 @@ impl<'a> ConvertToAst<(BindgenAttrs, &'a Option<String>)> for syn::ForeignItemFn
     ) -> Result<Self::Target, Diagnostic> {
         let default_name = self.ident.to_string();
         let js_name = opts.js_name().unwrap_or(&default_name);
+
         let wasm = function_from_decl(
             js_name,
             self.decl.clone(),
@@ -404,6 +417,7 @@ impl<'a> ConvertToAst<(BindgenAttrs, &'a Option<String>)> for syn::ForeignItemFn
             None,
         )?.0;
         let catch = opts.catch();
+        let variadic = opts.variadic();
         let js_ret = if catch {
             // TODO: this assumes a whole bunch:
             //
@@ -533,6 +547,7 @@ impl<'a> ConvertToAst<(BindgenAttrs, &'a Option<String>)> for syn::ForeignItemFn
             kind,
             js_ret,
             catch,
+            variadic,
             structural: opts.structural(),
             rust_name: self.ident.clone(),
             shim: Ident::new(&shim, Span::call_site()),
@@ -545,6 +560,7 @@ impl ConvertToAst<BindgenAttrs> for syn::ForeignItemType {
     type Target = ast::ImportKind;
 
     fn convert(self, attrs: BindgenAttrs) -> Result<Self::Target, Diagnostic> {
+        assert_not_variadic(&attrs, &self)?;
         let js_name = attrs
             .js_name()
             .map_or_else(|| self.ident.to_string(), |s| s.to_string());
@@ -570,6 +586,7 @@ impl<'a> ConvertToAst<(BindgenAttrs, &'a Option<String>)> for syn::ForeignItemSt
         if self.mutability.is_some() {
             bail_span!(self.mutability, "cannot import mutable globals yet")
         }
+        assert_not_variadic(&opts, &self)?;
         let default_name = self.ident.to_string();
         let js_name = opts.js_name().unwrap_or(&default_name);
         let shim = format!(
@@ -604,6 +621,7 @@ impl ConvertToAst<BindgenAttrs> for syn::ItemFn {
         if self.unsafety.is_some() {
             bail_span!(self.unsafety, "can only #[wasm_bindgen] safe functions");
         }
+        assert_not_variadic(&attrs, &self)?;
 
         let default_name = self.ident.to_string();
         let name = attrs.js_name().unwrap_or(&default_name);
@@ -1074,6 +1092,15 @@ fn assert_no_lifetimes(decl: &syn::FnDecl) -> Result<(), Diagnostic> {
     Diagnostic::from_vec(walk.diagnostics)
 }
 
+/// This method always fails if the BindgenAttrs contain variadic
+fn assert_not_variadic(attrs: &BindgenAttrs, span: &dyn ToTokens) -> Result<(), Diagnostic> {
+    if attrs.variadic() {
+        bail_span!(span, "the `variadic` attribute can only be applied to imported \
+            (`extern`) functions")
+    }
+    Ok(())
+}
+
 /// If the path is a single ident, return it.
 fn extract_path_ident(path: &syn::Path) -> Result<Ident, Diagnostic> {
     if path.leading_colon.is_some() {

crates/shared/src/lib.rs

@@ -43,6 +43,7 @@ pub enum ImportKind {
 pub struct ImportFunction {
     pub shim: String,
     pub catch: bool,
+    pub variadic: bool,
     pub method: Option<MethodData>,
     pub structural: bool,
     pub function: Function,

crates/webidl/src/util.rs

@@ -277,6 +277,7 @@ impl<'src> FirstPassRecord<'src> {
             },
             rust_name: rust_ident(rust_name),
             js_ret: js_ret.clone(),
+            variadic: false,
             catch,
             structural,
             shim:{

guide/src/SUMMARY.md

@@ -42,6 +42,7 @@
       - [`module = "blah"`](./reference/attributes/on-js-imports/module.md)
       - [`static_method_of = Blah`](./reference/attributes/on-js-imports/static_method_of.md)
       - [`structural`](./reference/attributes/on-js-imports/structural.md)
+      - [variadic](./reference/attributes/on-js-imports/variadic.md)
     - [On Rust Exports](./reference/attributes/on-rust-exports/index.md)
       - [`constructor`](./reference/attributes/on-rust-exports/constructor.md)
       - [`js_name = Blah`](./reference/attributes/on-rust-exports/js_name.md)

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

@@ -0,0 +1,38 @@
+# Variadic Parameters
+
+In javascript, both the types of function arguments, and the number of function arguments are
+dynamic. For example
+
+```js
+function sum(...rest) {
+    let i;
+    // the old way
+    let old_way = 0;
+    for (i=0; i<arguments.length; i++) {
+        old_way += arguments[i];
+    }
+    // the new way
+    let new_way = 0;
+    for (i=0; i<rest.length; i++) {
+        new_way += rest[i];
+    }
+    // both give the same answer
+    assert(old_way === new_way);
+    return new_way;
+}
+```
+
+This function doesn't translate directly into rust, since we don't currently support variadic
+arguments on the wasm target. To bind to it, we use a slice as the last argument, and annotate the
+function as variadic:
+
+```rust
+#[wasm_bindgen]
+extern {
+    #[wasm_bindgen(variadic)]
+    fn sum(args: &[i32]) -> i32;
+}
+```
+
+when we call this function, the last argument will be expanded as the javascript expects.
+

guide/src/whirlwind-tour/what-else-can-we-do.md

@@ -64,6 +64,10 @@ extern {
     fn new() -> Awesome;
     #[wasm_bindgen(method)]
     fn get_internal(this: &Awesome) -> u32;
+    // We can call javascript functions that have a dynamic number of arguments,
+    // e.g. rust `sum(&[1, 2, 3])` will be called like `sum(1, 2, 3)`
+    #[wasm_bindgen(variadic)]
+    fn sum(vals: &[u32]) -> u32;
 }
 
 #[wasm_bindgen]
@@ -143,5 +147,13 @@ export class Awesome {
   }
 }
 
+export function sum(...args) {
+    let answer = 0;
+    for(var i=0; i<args.length; i++) {
+        answer += args[i];
+    }
+    return answer;
+}
+
 booted.then(main);
 ```

tests/wasm/main.rs

@@ -31,3 +31,4 @@ pub mod slice;
 pub mod structural;
 pub mod u64;
 pub mod validate_prt;
+pub mod variadic;

tests/wasm/variadic.js

@@ -0,0 +1,59 @@
+const assert = require('assert');
+
+// a function for testing numbers
+function variadic_sum(...args) {
+    let answer = 0;
+    for(var i=0; i<args.length; i++) {
+        answer += args[i];
+    }
+    return answer;
+}
+
+exports.variadic_sum_u8 = variadic_sum;
+exports.variadic_sum_u16 = variadic_sum;
+exports.variadic_sum_u32 = variadic_sum;
+exports.variadic_sum_u64 = variadic_sum;
+exports.variadic_sum_i8 = variadic_sum;
+exports.variadic_sum_i16 = variadic_sum;
+exports.variadic_sum_i32 = variadic_sum;
+exports.variadic_sum_i64 = variadic_sum;
+exports.variadic_sum_f32 = variadic_sum;
+exports.variadic_sum_f64 = variadic_sum;
+exports.variadic_sum_rest_vec = variadic_sum;
+
+// a function for testing nullable numbers
+function variadic_sum_opt(...args) {
+    let answer = 0;
+    for(var i=0; i<args.length; i++) {
+        if(args[i] != null) {
+            answer += args[i];
+        }
+    }
+    return answer;
+}
+
+exports.variadic_sum_opt = variadic_sum_opt;
+
+// a function for testing strings
+function variadic_concat(...args) {
+    let answer = "";
+    for(var i=0; i<args.length; i++) {
+        answer = `${answer}${args[i]}`;
+    }
+    return answer;
+}
+
+exports.variadic_concat_str = variadic_concat;
+exports.variadic_concat_string = variadic_concat;
+
+// a test that takes any type of arguments (for testing JsValue).
+function variadic_compare_pairs(...args) {
+    assert(args.length % 2 == 0, "args must be sequence of pairs");
+    for(var i=0; i<args.length; i++) {
+        const first = args[i++];
+        const second = args[i];
+        assert.equal(first, second);
+    }
+}
+
+exports.variadic_compare_pairs = variadic_compare_pairs;