Don't use the global stack for string lengths

This commit updates the `Abi` associated type for all slice types to a
`WasmSlice` type, an aggregate of two `u32` integers. This translates to an ABI
where when passed as a function argument it expands to two integer arguments,
and when passed as a return value it passes a return pointer as the first
argument to get filled in.

This is hopefully more forwards-compatible with the host bindings proposal which
uses this strategy for passing string arguments at least. It's a little sketchy
what we're doing as there's not really a stable ABI yet, but hopefully this'll
all be relatively stable for awhile!

crates/cli-support/src/js/js2rust.rs

@@ -23,10 +23,6 @@ pub struct Js2Rust<'a, 'b: 'a> {
     /// finishes. This is scheduled in a `finally` block.
     finally: String,
 
-    /// Next global index to write to when passing arguments via the single
-    /// global stack.
-    global_idx: usize,
-
     /// Index of the next argument for unique name generation purposes.
     arg_idx: usize,
 
@@ -52,7 +48,6 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
             js_arguments: Vec::new(),
             prelude: String::new(),
             finally: String::new(),
-            global_idx: 0,
             arg_idx: 0,
             ret_ty: String::new(),
             ret_expr: String::new(),
@@ -102,21 +97,23 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
         self
     }
 
+    fn abi_arg(&mut self) -> String {
+        let s = format!("arg{}", self.arg_idx);
+        self.arg_idx += 1;
+        s
+    }
+
     pub fn argument(&mut self, arg: &Descriptor) -> Result<&mut Self, Error> {
         let i = self.arg_idx;
-        self.arg_idx += 1;
+        let name = self.abi_arg();
-        let name = format!("arg{}", i);
 
         if let Some(kind) = arg.vector_kind() {
             self.js_arguments.push((name.clone(), kind.js_ty().to_string()));
 
             let func = self.cx.pass_to_wasm_function(kind)?;
-            self.cx.expose_set_global_argument()?;
-            let global_idx = self.global_idx();
             self.prelude(&format!("\
                 const [ptr{i}, len{i}] = {func}({arg});\n\
-                setGlobalArgument(len{i}, {global_idx});\n\
+            ", i = i, func = func, arg = name));
-            ", i = i, func = func, arg = name, global_idx = global_idx));
             if arg.is_by_ref() {
                 if arg.is_mut_ref() {
                     let get = self.cx.memview_function(kind);
@@ -133,6 +130,7 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
                 self.cx.require_internal_export("__wbindgen_free")?;
             }
             self.rust_arguments.push(format!("ptr{}", i));
+            self.rust_arguments.push(format!("len{}", i));
             return Ok(self)
         }
 
@@ -187,7 +185,7 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
                         _assertBoolean({name});\n\
                     ", name = name));
                 }
-                self.rust_arguments.push(format!("arg{i} ? 1 : 0", i = i));
+                self.rust_arguments.push(format!("{} ? 1 : 0", name));
             }
             Descriptor::Anyref => {
                 self.js_arguments.push((name.clone(), "any".to_string()));
@@ -225,13 +223,18 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
         if let Some(ty) = ty.vector_kind() {
             self.ret_ty = ty.js_ty().to_string();
             let f = self.cx.expose_get_vector_from_wasm(ty);
-            self.cx.expose_get_global_argument()?;
+            self.cx.expose_global_argument_ptr()?;
+            self.cx.expose_uint32_memory();
             self.cx.require_internal_export("__wbindgen_free")?;
+            self.prelude("const retptr = globalArgumentPtr();");
+            self.rust_arguments.insert(0, "retptr".to_string());
             self.ret_expr = format!("\
-                const ret = RET;\n\
+                RET;\n\
-                const len = getGlobalArgument(0);\n\
+                const mem = getUint32Memory();\n\
-                const realRet = {}(ret, len);\n\
+                const ptr = mem[retptr / 4];\n\
-                wasm.__wbindgen_free(ret, len * {});\n\
+                const len = mem[retptr / 4 + 1];\n\
+                const realRet = {}(ptr, len);\n\
+                wasm.__wbindgen_free(ptr, len * {});\n\
                 return realRet;\n\
             ", f, ty.size());
             return Ok(self)
@@ -309,10 +312,4 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
         let ts = format!("{} {}({}): {};\n", prefix, self.js_name, ts_args, self.ret_ty);
         (js, ts)
     }
-
-    fn global_idx(&mut self) -> usize {
-        let ret = self.global_idx;
-        self.global_idx += 1;
-        ret
-    }
 }

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

@@ -1318,21 +1318,6 @@ impl<'a> Context<'a> {
         }
     }
 
-    fn expose_set_global_argument(&mut self) -> Result<(), Error> {
-        if !self.exposed_globals.insert("set_global_argument") {
-            return Ok(());
-        }
-        self.expose_uint32_memory();
-        self.expose_global_argument_ptr()?;
-        self.global("
-            function setGlobalArgument(arg, i) {
-                const idx = globalArgumentPtr() / 4 + i;
-                getUint32Memory()[idx] = arg;
-            }
-        ");
-        Ok(())
-    }
-
     fn expose_get_global_argument(&mut self) -> Result<(), Error> {
         if !self.exposed_globals.insert("get_global_argument") {
             return Ok(());

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

@@ -72,31 +72,32 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         Ok(self)
     }
 
-    fn argument(&mut self, arg: &Descriptor) -> Result<(), Error> {
+    fn shim_argument(&mut self) -> String {
-        let i = self.arg_idx;
+        let s = format!("arg{}", self.arg_idx);
         self.arg_idx += 1;
+        self.shim_arguments.push(s.clone());
+        s
+    }
 
-        self.shim_arguments.push(format!("arg{}", i));
+    fn argument(&mut self, arg: &Descriptor) -> Result<(), Error> {
+        let abi = self.shim_argument();
 
         if let Some(ty) = arg.vector_kind() {
+            let abi2 = self.shim_argument();
             let f = self.cx.expose_get_vector_from_wasm(ty);
-            self.cx.expose_get_global_argument()?;
+            self.prelude(&format!("let v{0} = {func}({0}, {1});",
-            let next_global = self.global_idx();
+                                  abi, abi2, func = f));
-            self.prelude(&format!("\
-                let len{0} = getGlobalArgument({next_global});\n\
-                let v{0} = {func}(arg{0}, len{0});\n\
-            ", i, func = f, next_global = next_global));
 
             if !arg.is_by_ref() {
                 self.prelude(&format!("\
-                    wasm.__wbindgen_free(arg{0}, len{0} * {size});\
+                    wasm.__wbindgen_free({0}, {1} * {size});\
-                ", i, size = ty.size()));
+                ", abi, abi2, size = ty.size()));
                 self.cx.require_internal_export("__wbindgen_free")?;
             } else if arg.is_mut_ref() {
                 let f = self.cx.expose_commit_slice_to_wasm(ty)?;
-                self.finally(&format!("{}(arg{i}, v{i});", f, i = i));
+                self.finally(&format!("{}({1}, v{1});", f, abi));
             }
-            self.js_arguments.push(format!("v{}", i));
+            self.js_arguments.push(format!("v{}", abi));
             return Ok(())
         }
 
@@ -104,9 +105,9 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
             if arg.is_by_ref() {
                 bail!("cannot invoke JS functions with custom ref types yet")
             }
-            let assign = format!("let c{0} = {1}.__construct(arg{0});", i, class);
+            let assign = format!("let c{0} = {1}.__construct({0});", abi, class);
             self.prelude(&assign);
-            self.js_arguments.push(format!("c{}", i));
+            self.js_arguments.push(format!("c{}", abi));
             return Ok(())
         }
 
@@ -132,12 +133,12 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
             self.global_idx();
             self.prelude(&format!("\
                 let cb{0} = {js};\n\
-                cb{0}.f = wasm.__wbg_function_table.get(arg{0});\n\
+                cb{0}.f = wasm.__wbg_function_table.get({0});\n\
                 cb{0}.a = getGlobalArgument({next_global});\n\
                 cb{0}.b = getGlobalArgument({next_global} + 1);\n\
-            ", i, js = js, next_global = next_global));
+            ", abi, js = js, next_global = next_global));
-            self.finally(&format!("cb{0}.a = cb{0}.b = 0;", i));
+            self.finally(&format!("cb{0}.a = cb{0}.b = 0;", abi));
-            self.js_arguments.push(format!("cb{0}.bind(cb{0})", i));
+            self.js_arguments.push(format!("cb{0}.bind(cb{0})", abi));
             return Ok(())
         }
 
@@ -168,35 +169,35 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
                 cb{0}.f = wasm.__wbg_function_table.get(getGlobalArgument({c}));\n\
                 let real = cb{0}.bind(cb{0});\n\
                 real.original = cb{0};\n\
-                idx{0} = getUint32Memory()[arg{0} / 4] = addHeapObject(real);\n\
+                idx{0} = getUint32Memory()[{0} / 4] = addHeapObject(real);\n\
             ",
-                i,
+                abi,
                 js = js,
                 a = self.global_idx(),
                 b = self.global_idx(),
                 c = self.global_idx(),
             );
             self.prelude(&format!("\
-                let idx{0} = getUint32Memory()[arg{0} / 4];\n\
+                let idx{0} = getUint32Memory()[{0} / 4];\n\
                 if (idx{0} === 0xffffffff) {{\n\
                 {1}\
                 }}\n\
-            ", i, indent(&reset_idx)));
+            ", abi, indent(&reset_idx)));
             self.cx.expose_get_object();
-            self.js_arguments.push(format!("getObject(idx{})", i));
+            self.js_arguments.push(format!("getObject(idx{})", abi));
             return Ok(())
         }
 
         let invoc_arg = match *arg {
-            ref d if d.is_number() => format!("arg{}", i),
+            ref d if d.is_number() => abi,
-            Descriptor::Boolean => format!("arg{} !== 0", i),
+            Descriptor::Boolean => format!("{} !== 0", abi),
             Descriptor::Anyref => {
                 self.cx.expose_take_object();
-                format!("takeObject(arg{})", i)
+                format!("takeObject({})", abi)
             }
             ref d if d.is_ref_anyref() => {
                 self.cx.expose_get_object();
-                format!("getObject(arg{})", i)
+                format!("getObject({})", abi)
             }
             _ => bail!("unimplemented argument type in imported function: {:?}", arg),
         };
@@ -218,11 +219,12 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
         if let Some(ty) = ty.vector_kind() {
             let f = self.cx.pass_to_wasm_function(ty)?;
             self.cx.expose_uint32_memory();
-            self.cx.expose_set_global_argument()?;
+            self.shim_arguments.insert(0, "ret".to_string());
             self.ret_expr = format!("\
                 const [retptr, retlen] = {}(JS);\n\
-                setGlobalArgument(retlen, 0);\n\
+                const mem = getUint32Memory();
-                return retptr;\n\
+                mem[ret / 4] = retptr;
+                mem[ret / 4 + 1] = retlen;
             ", f);
             return Ok(())
         }

src/convert.rs

@@ -61,6 +61,14 @@ unsafe impl WasmAbi for i64 {}
 unsafe impl WasmAbi for f32 {}
 unsafe impl WasmAbi for f64 {}
 
+#[repr(C)]
+pub struct WasmSlice {
+    pub ptr: u32,
+    pub len: u32,
+}
+
+unsafe impl WasmAbi for WasmSlice {}
+
 macro_rules! simple {
     ($($t:tt)*) => ($(
         impl IntoWasmAbi for $t {
@@ -137,67 +145,71 @@ macro_rules! vectors {
     ($($t:ident)*) => ($(
         #[cfg(feature = "std")]
         impl IntoWasmAbi for Box<[$t]> {
-            type Abi = u32;
+            type Abi = WasmSlice;
 
-            fn into_abi(self, extra: &mut Stack) -> u32 {
+            fn into_abi(self, extra: &mut Stack) -> WasmSlice {
                 let ptr = self.as_ptr();
                 let len = self.len();
                 mem::forget(self);
-                extra.push(len as u32);
+                WasmSlice {
-                ptr.into_abi(extra)
+                    ptr: ptr.into_abi(extra),
+                    len: len as u32,
+                }
             }
         }
 
         #[cfg(feature = "std")]
         impl FromWasmAbi for Box<[$t]> {
-            type Abi = u32;
+            type Abi = WasmSlice;
 
-            unsafe fn from_abi(js: u32, extra: &mut Stack) -> Self {
+            unsafe fn from_abi(js: WasmSlice, extra: &mut Stack) -> Self {
-                let ptr = <*mut $t>::from_abi(js, extra);
+                let ptr = <*mut $t>::from_abi(js.ptr, extra);
-                let len = extra.pop() as usize;
+                let len = js.len as usize;
                 Vec::from_raw_parts(ptr, len, len).into_boxed_slice()
             }
         }
 
         impl<'a> IntoWasmAbi for &'a [$t] {
-            type Abi = u32;
+            type Abi = WasmSlice;
 
-            fn into_abi(self, extra: &mut Stack) -> u32 {
+            fn into_abi(self, extra: &mut Stack) -> WasmSlice {
-                let ptr = self.as_ptr();
+                WasmSlice {
-                let len = self.len();
+                    ptr: self.as_ptr().into_abi(extra),
-                extra.push(len as u32);
+                    len: self.len() as u32,
-                ptr.into_abi(extra)
+                }
             }
         }
 
         impl<'a> IntoWasmAbi for &'a mut [$t] {
-            type Abi = u32;
+            type Abi = WasmSlice;
 
-            fn into_abi(self, extra: &mut Stack) -> u32 {
+            fn into_abi(self, extra: &mut Stack) -> WasmSlice {
                 (&*self).into_abi(extra)
             }
         }
 
         impl RefFromWasmAbi for [$t] {
-            type Abi = u32;
+            type Abi = WasmSlice;
             type Anchor = &'static [$t];
 
-            unsafe fn ref_from_abi(js: u32, extra: &mut Stack) -> &'static [$t] {
+            unsafe fn ref_from_abi(js: WasmSlice, extra: &mut Stack) -> &'static [$t] {
                 slice::from_raw_parts(
-                    <*const $t>::from_abi(js, extra),
+                    <*const $t>::from_abi(js.ptr, extra),
-                    extra.pop() as usize,
+                    js.len as usize,
                 )
             }
         }
 
         impl RefMutFromWasmAbi for [$t] {
-            type Abi = u32;
+            type Abi = WasmSlice;
             type Anchor = &'static mut [$t];
 
-            unsafe fn ref_mut_from_abi(js: u32, extra: &mut Stack) -> &'static mut [$t] {
+            unsafe fn ref_mut_from_abi(js: WasmSlice, extra: &mut Stack)
+                -> &'static mut [$t]
+            {
                 slice::from_raw_parts_mut(
-                    <*mut $t>::from_abi(js, extra),
+                    <*mut $t>::from_abi(js.ptr, extra),
-                    extra.pop() as usize,
+                    js.len as usize,
                 )
             }
         }
@@ -225,17 +237,17 @@ if_std! {
     }
 
     impl IntoWasmAbi for String {
-        type Abi = u32;
+        type Abi = <Vec<u8> as IntoWasmAbi>::Abi;
 
-        fn into_abi(self, extra: &mut Stack) -> u32 {
+        fn into_abi(self, extra: &mut Stack) -> Self::Abi {
             self.into_bytes().into_abi(extra)
         }
     }
 
     impl FromWasmAbi for String {
-        type Abi = u32;
+        type Abi = <Vec<u8> as FromWasmAbi>::Abi;
 
-        unsafe fn from_abi(js: u32, extra: &mut Stack) -> Self {
+        unsafe fn from_abi(js: Self::Abi, extra: &mut Stack) -> Self {
             String::from_utf8_unchecked(<Vec<u8>>::from_abi(js, extra))
         }
     }
@@ -294,23 +306,25 @@ impl RefFromWasmAbi for JsValue {
 
 if_std! {
     impl IntoWasmAbi for Box<[JsValue]> {
-        type Abi = u32;
+        type Abi = WasmSlice;
 
-        fn into_abi(self, extra: &mut Stack) -> u32 {
+        fn into_abi(self, extra: &mut Stack) -> WasmSlice {
             let ptr = self.as_ptr();
             let len = self.len();
             mem::forget(self);
-            extra.push(len as u32);
+            WasmSlice {
-            ptr.into_abi(extra)
+                ptr: ptr.into_abi(extra),
+                len: len as u32,
+            }
         }
     }
 
     impl FromWasmAbi for Box<[JsValue]> {
-        type Abi = u32;
+        type Abi = WasmSlice;
 
-        unsafe fn from_abi(js: u32, extra: &mut Stack) -> Self {
+        unsafe fn from_abi(js: WasmSlice, extra: &mut Stack) -> Self {
-            let ptr = <*mut JsValue>::from_abi(js, extra);
+            let ptr = <*mut JsValue>::from_abi(js.ptr, extra);
-            let len = extra.pop() as usize;
+            let len = js.len as usize;
             Vec::from_raw_parts(ptr, len, len).into_boxed_slice()
         }
     }

tests/all/imports.rs

@@ -113,6 +113,40 @@ fn unused() {
         .test();
 }
 
+#[test]
+fn string_ret() {
+    project()
+        .file("src/lib.rs", r#"
+            #![feature(proc_macro, wasm_custom_section, wasm_import_module)]
+
+            extern crate wasm_bindgen;
+
+            use wasm_bindgen::prelude::*;
+
+            #[wasm_bindgen(module = "./test")]
+            extern {
+                fn foo() -> String;
+            }
+
+            #[wasm_bindgen]
+            pub fn run() {
+                assert_eq!(foo(), "bar");
+            }
+        "#)
+        .file("test.ts", r#"
+            import * as wasm from "./out";
+
+            export function foo(): string {
+                return 'bar';
+            }
+
+            export function test() {
+                wasm.run();
+            }
+        "#)
+        .test();
+}
+
 #[test]
 fn strings() {
     project()