Merge pull request #713 from alexcrichton/weakref

Add experimental support for `WeakRef`

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

@@ -490,6 +490,39 @@ impl<'a> Context<'a> {
         let mut dst = format!("class {} {{\n", name);
         let mut ts_dst = format!("export {}", dst);
 
+        let (mkweakref, freeref) = if self.config.weak_refs {
+            // When weak refs are enabled we use them to automatically free the
+            // contents of an exported rust class when it's gc'd. Note that a
+            // manual `free` function still exists for deterministic
+            // destruction.
+            //
+            // This is implemented by using a `WeakRefGroup` to run finalizers
+            // for all `WeakRef` objects that it creates. Upon construction of
+            // a new wasm object we use `makeRef` with "holdings" of a thunk to
+            // free the wasm instance.  Once the `this` (the instance we're
+            // creating) is gc'd then the finalizer will run with the
+            // `WeakRef`, and we'll pull out the `holdings`, our pointer.
+            //
+            // Note, though, that if manual finalization happens we want to
+            // cancel the `WeakRef`-generated finalization, so we retain the
+            // `WeakRef` in a global map. This global map is then used to
+            // `drop()` the `WeakRef` (cancel finalization) whenever it is
+            // finalized.
+            self.expose_cleanup_groups();
+            let mk = format!("
+                const cleanup_ptr = this.ptr;
+                const ref = CLEANUPS.makeRef(this, () => free{}(cleanup_ptr));
+                CLEANUPS_MAP.set(this.ptr, ref);
+            ", name);
+            let free = "
+                CLEANUPS_MAP.get(ptr).drop();
+                CLEANUPS_MAP.delete(ptr);
+            ";
+            (mk, free)
+        } else {
+            (String::new(), "")
+        };
+
         if self.config.debug || class.constructor.is_some() {
             self.expose_constructor_token();
 
@@ -516,9 +549,11 @@ impl<'a> Context<'a> {
                     // This invocation of new will call this constructor with a ConstructorToken
                     let instance = {class}.{constructor}(...args);
                     this.ptr = instance.ptr;
+                    {mkweakref}
                     ",
                     class = name,
-                    constructor = constructor
+                    constructor = constructor,
+                    mkweakref = mkweakref,
                 ));
             } else {
                 dst.push_str(
@@ -537,9 +572,11 @@ impl<'a> Context<'a> {
 
                 constructor(ptr) {{
                     this.ptr = ptr;
+                    {}
                 }}
                 ",
-                name
+                name,
+                mkweakref,
             ));
         }
 
@@ -599,16 +636,29 @@ impl<'a> Context<'a> {
             }
         }
 
-        dst.push_str(&format!(
+        self.global(&format!(
             "
-            free() {{
-                const ptr = this.ptr;
-                this.ptr = 0;
+            function free{}(ptr) {{
+                {}
                 wasm.{}(ptr);
             }}
             ",
+            name,
+            freeref,
             shared::free_function(&name)
         ));
+        dst.push_str(&format!(
+            "
+            free() {{
+                if (this.ptr === 0)
+                    return;
+                const ptr = this.ptr;
+                this.ptr = 0;
+                free{}(this.ptr);
+            }}
+            ",
+            name,
+        ));
         ts_dst.push_str("free(): void;\n");
         dst.push_str(&class.contents);
         ts_dst.push_str(&class.typescript);
@@ -1594,6 +1644,18 @@ impl<'a> Context<'a> {
         ");
     }
 
+    fn expose_cleanup_groups(&mut self) {
+        if !self.exposed_globals.insert("cleanup_groups") {
+            return
+        }
+        self.global(
+            "
+                const CLEANUPS = new WeakRefGroup(x => x.holdings());
+                const CLEANUPS_MAP = new Map();
+            "
+        );
+    }
+
     fn gc(&mut self) -> Result<(), Error> {
         let module = mem::replace(self.module, Module::default());
         let module = module.parse_names().unwrap_or_else(|p| p.1);

crates/cli-support/src/lib.rs

@@ -10,6 +10,7 @@ extern crate failure;
 
 use std::any::Any;
 use std::collections::BTreeSet;
+use std::env;
 use std::fmt;
 use std::fs;
 use std::mem;
@@ -33,6 +34,9 @@ pub struct Bindgen {
     typescript: bool,
     demangle: bool,
     keep_debug: bool,
+    // Experimental support for `WeakRefGroup`, an upcoming ECMAScript feature.
+    // Currently only enable-able through an env var.
+    weak_refs: bool,
 }
 
 enum Input {
@@ -55,6 +59,7 @@ impl Bindgen {
             typescript: false,
             demangle: true,
             keep_debug: false,
+            weak_refs: env::var("WASM_BINDGEN_WEAKREF").is_ok(),
         }
     }