Mark JsValue idx as NonZeroU32

I noticed in the generated wasm for examples/dom that `JsStatic` takes two words of storage (for its `Option<JsValue>`) when it would seem to only need one, if we mandate that `0` is not a valid heap index. Changing the `.idx` type to `NonZeroU32` fixes this.

However, LLVM doesn't know that these values are non-zero yet (rust-lang/rust#49572) which means in the implementation of Option's `get_or_insert_with` (https://doc.rust-lang.org/nightly/src/core/option.rs.html#774) LLVM is no longer able to prove that the `unreachable!()` branch is never hit and thus JsStatic ends up pulling in all the panic infrastructure where it previously didn't. This results in the examples/dom output being much larger, rather than smaller as I had hoped. Probably makes sense to wait for the upstream issue to be fixed so that this can be landed without regressing anything.

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

@@ -631,15 +631,16 @@ impl<'a> Context<'a> {
         if !self.exposed_globals.insert("slab") {
             return;
         }
-        self.global(&format!("let slab = [];"));
+        self.global(&format!("let slab = [null];"));
     }
 
     fn expose_global_slab_next(&mut self) {
         if !self.exposed_globals.insert("slab_next") {
             return;
         }
         self.global(&format!("
-            let slab_next = 0;
+            // Avoid 0 as an index to enable optimizing Option<JsValue> layout
+            let slab_next = 1;
         "));
     }
 

src/closure.rs

@@ -7,6 +7,7 @@
 use std::cell::UnsafeCell;
 use std::marker::Unsize;
 use std::mem::{self, ManuallyDrop};
+use std::num::NonZeroU32;
 use std::prelude::v1::*;
 
 use JsValue;
@@ -95,9 +96,12 @@ impl<T> Closure<T>
     ///
     /// This is the function where the JS closure is manufactured.
     pub fn wrap(t: Box<T>) -> Closure<T> {
+        let idx = unsafe {
+            NonZeroU32::new_unchecked(!0)
+        };
         Closure {
             inner: UnsafeCell::new(t),
-            js: UnsafeCell::new(ManuallyDrop::new(JsValue { idx: !0 })),
+            js: UnsafeCell::new(ManuallyDrop::new(JsValue { idx })),
         }
     }
 
@@ -114,7 +118,7 @@ impl<T> Closure<T>
     /// cleanup as it can.
     pub fn forget(self) {
         unsafe {
-            let idx = (*self.js.get()).idx;
+            let idx = (*self.js.get()).idx.get();
             if idx != !0 {
                 super::__wbindgen_cb_forget(idx);
             }
@@ -142,7 +146,7 @@ impl<'a, T> IntoWasmAbi for &'a Closure<T>
         unsafe {
             let fnptr = WasmClosure::into_abi(&mut **self.inner.get(), extra);
             extra.push(fnptr);
-            &mut (*self.js.get()).idx as *const u32 as u32
+            &mut (*self.js.get()).idx.get() as *const u32 as u32
         }
     }
 }
@@ -162,7 +166,7 @@ impl<T> Drop for Closure<T>
 {
     fn drop(&mut self) {
         unsafe {
-            let idx = (*self.js.get()).idx;
+            let idx = (*self.js.get()).idx.get();
             if idx != !0 {
                 super::__wbindgen_cb_drop(idx);
             }

src/convert.rs

@@ -2,6 +2,7 @@
 //! at your own risk.
 
 use core::mem::{self, ManuallyDrop};
+use core::num::NonZeroU32;
 use core::ops::{Deref, DerefMut};
 use core::slice;
 use core::str;
@@ -242,7 +243,7 @@ impl IntoWasmAbi for JsValue {
     type Abi = u32;
 
     fn into_abi(self, _extra: &mut Stack) -> u32 {
-        let ret = self.idx;
+        let ret = self.idx.get();
         mem::forget(self);
         return ret
     }
@@ -252,14 +253,14 @@ impl FromWasmAbi for JsValue {
     type Abi = u32;
 
     unsafe fn from_abi(js: u32, _extra: &mut Stack) -> JsValue {
-        JsValue { idx: js }
+        JsValue { idx: NonZeroU32::new_unchecked(js) }
     }
 }
 
 impl<'a> IntoWasmAbi for &'a JsValue {
     type Abi = u32;
     fn into_abi(self, _extra: &mut Stack) -> u32 {
-        self.idx
+        self.idx.get()
     }
 }
 
@@ -268,7 +269,7 @@ impl RefFromWasmAbi for JsValue {
     type Anchor = ManuallyDrop<JsValue>;
 
     unsafe fn ref_from_abi(js: u32, _extra: &mut Stack) -> Self::Anchor {
-        ManuallyDrop::new(JsValue { idx: js })
+        ManuallyDrop::new(JsValue { idx: NonZeroU32::new_unchecked(js) })
     }
 }
 

src/lib.rs

@@ -5,12 +5,13 @@
 //! this crate and this crate also provides JS bindings through the `JsValue`
 //! interface.
 
-#![feature(use_extern_macros, wasm_import_module, try_reserve, unsize)]
+#![feature(use_extern_macros, wasm_import_module, try_reserve, unsize, nonzero)]
 #![no_std]
 
 extern crate wasm_bindgen_macro;
 
 use core::cell::UnsafeCell;
+use core::num::NonZeroU32;
 use core::ops::Deref;
 use core::ptr;
 
@@ -52,7 +53,7 @@ if_std! {
 /// will transfer into wasm directly and this will likely become more efficient,
 /// but for now it may be slightly slow.
 pub struct JsValue {
-    idx: u32,
+    idx: NonZeroU32,
 }
 
 impl JsValue {
@@ -62,7 +63,7 @@ impl JsValue {
     /// be owned by the JS garbage collector.
     pub fn from_str(s: &str) -> JsValue {
         unsafe {
-            JsValue { idx: __wbindgen_string_new(s.as_ptr(), s.len()) }
+            JsValue { idx: NonZeroU32::new_unchecked(__wbindgen_string_new(s.as_ptr(), s.len())) }
         }
     }
 
@@ -72,7 +73,7 @@ impl JsValue {
     /// allocated number) and returns a handle to the JS version of it.
     pub fn from_f64(n: f64) -> JsValue {
         unsafe {
-            JsValue { idx: __wbindgen_number_new(n) }
+            JsValue { idx: NonZeroU32::new_unchecked(__wbindgen_number_new(n)) }
         }
     }
 
@@ -82,21 +83,21 @@ impl JsValue {
     /// allocated boolean) and returns a handle to the JS version of it.
     pub fn from_bool(b: bool) -> JsValue {
         unsafe {
-            JsValue { idx: __wbindgen_boolean_new(b as u32) }
+            JsValue { idx: NonZeroU32::new_unchecked(__wbindgen_boolean_new(b as u32)) }
         }
     }
 
     /// Creates a new JS value representing `undefined`.
     pub fn undefined() -> JsValue {
         unsafe {
-            JsValue { idx: __wbindgen_undefined_new() }
+            JsValue { idx: NonZeroU32::new_unchecked(__wbindgen_undefined_new()) }
         }
     }
 
     /// Creates a new JS value representing `null`.
     pub fn null() -> JsValue {
         unsafe {
-            JsValue { idx: __wbindgen_null_new() }
+            JsValue { idx: NonZeroU32::new_unchecked(__wbindgen_null_new()) }
         }
     }
 
@@ -108,7 +109,7 @@ impl JsValue {
         unsafe {
             let ptr = description.map(|s| s.as_ptr()).unwrap_or(ptr::null());
             let len = description.map(|s| s.len()).unwrap_or(0);
-            JsValue { idx: __wbindgen_symbol_new(ptr, len) }
+            JsValue { idx: NonZeroU32::new_unchecked(__wbindgen_symbol_new(ptr, len)) }
         }
     }
 
@@ -137,7 +138,7 @@ impl JsValue {
     pub fn as_f64(&self) -> Option<f64> {
         let mut invalid = 0;
         unsafe {
-            let ret = __wbindgen_number_get(self.idx, &mut invalid);
+            let ret = __wbindgen_number_get(self.idx.get(), &mut invalid);
             if invalid == 1 {
                 None
             } else {
@@ -155,7 +156,7 @@ impl JsValue {
     pub fn as_string(&self) -> Option<String> {
         unsafe {
             let mut len = 0;
-            let ptr = __wbindgen_string_get(self.idx, &mut len);
+            let ptr = __wbindgen_string_get(self.idx.get(), &mut len);
             if ptr.is_null() {
                 None
             } else {
@@ -172,7 +173,7 @@ impl JsValue {
     /// `None`.
     pub fn as_bool(&self) -> Option<bool> {
         unsafe {
-            match __wbindgen_boolean_get(self.idx) {
+            match __wbindgen_boolean_get(self.idx.get()) {
                 0 => Some(false),
                 1 => Some(true),
                 _ => None,
@@ -183,21 +184,21 @@ impl JsValue {
     /// Tests whether this JS value is `null`
     pub fn is_null(&self) -> bool {
         unsafe {
-            __wbindgen_is_null(self.idx) == 1
+            __wbindgen_is_null(self.idx.get()) == 1
         }
     }
 
     /// Tests whether this JS value is `undefined`
     pub fn is_undefined(&self) -> bool {
         unsafe {
-            __wbindgen_is_undefined(self.idx) == 1
+            __wbindgen_is_undefined(self.idx.get()) == 1
         }
     }
 
     /// Tests whether the type of this JS value is `symbol`
     pub fn is_symbol(&self) -> bool {
         unsafe {
-            __wbindgen_is_symbol(self.idx) == 1
+            __wbindgen_is_symbol(self.idx.get()) == 1
         }
     }
 }
@@ -269,16 +270,16 @@ extern {
 impl Clone for JsValue {
     fn clone(&self) -> JsValue {
         unsafe {
-            let idx = __wbindgen_object_clone_ref(self.idx);
-            JsValue { idx }
+            let idx = __wbindgen_object_clone_ref(self.idx.get());
+            JsValue { idx: NonZeroU32::new_unchecked(idx) }
         }
     }
 }
 
 impl Drop for JsValue {
     fn drop(&mut self) {
         unsafe {
-            __wbindgen_object_drop_ref(self.idx);
+            __wbindgen_object_drop_ref(self.idx.get());
         }
     }
 }