Global allocators have to be defined in the root of the crate

At least until rust-lang/rust#44113 is fixed.

Author: fitzgen

src/lib.rs

@@ -3,8 +3,19 @@ extern crate wasm_bindgen;
 
 mod utils;
 
+use cfg_if::cfg_if;
 use wasm_bindgen::prelude::*;
 
+cfg_if! {
+    // When the `wee_alloc` feature is enabled, use `wee_alloc` as the global
+    // allocator.
+    if #[cfg(feature = "wee_alloc")] {
+        extern crate wee_alloc;
+        #[global_allocator]
+        static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;
+    }
+}
+
 #[wasm_bindgen]
 extern {
     fn alert(s: &str);

src/utils.rs

@@ -11,13 +11,3 @@ cfg_if! {
         pub fn set_panic_hook() {}
     }
 }
-
-cfg_if! {
-    // When the `wee_alloc` feature is enabled, use `wee_alloc` as the global
-    // allocator.
-    if #[cfg(feature = "wee_alloc")] {
-        extern crate wee_alloc;
-        #[global_allocator]
-        static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;
-    }
-}