Prevent unnecessary wasm-bindgen imports

Cargo.toml

@@ -315,7 +315,7 @@ once_cell = { version = "1.5.2", default-features = false, features=["std"], opt
 once_cell = { version = "1.5.2", default-features = false, features=["std"] }
 
 [target.'cfg(all(target_arch = "wasm32", target_vendor = "unknown", target_os = "unknown", target_env = ""))'.dependencies]
-web-sys = { version = "0.3.37", default-features = false, features = ["Crypto", "Window"] }
+web-sys = { version = "0.3.37", default-features = false, features = ["Crypto", "Window"], optional = true }
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = { version = "0.3.8", default-features = false, features = ["ntsecapi", "wtypesbase"] }
@@ -347,7 +347,7 @@ slow_tests = []
 std = ["alloc"]
 test_logging = []
 wasm32_c = []
-# wasm32_unknown_unknown_js = ["web-sys"]
+wasm32_unknown_unknown_js = ["web-sys"]
 size_optimized = []
 
 # XXX: debug = false because of https://github.com/rust-lang/rust/issues/34122

src/rand.rs

@@ -264,6 +264,7 @@ mod sysrand_chunk {
 }
 
 #[cfg(all(
+    feature = "wasm32_unknown_unknown_js",
     target_arch = "wasm32",
     target_vendor = "unknown",
     target_os = "unknown",
@@ -291,6 +292,21 @@ mod sysrand_chunk {
     }
 }
 
+#[cfg(all(
+    not(feature = "wasm32_unknown_unknown_js"),
+    target_arch = "wasm32",
+    target_vendor = "unknown",
+    target_os = "unknown",
+    target_env = "",
+))]
+mod sysrand_chunk {
+    use crate::error;
+
+    pub fn chunk(mut _dest: &mut [u8]) -> Result<usize, error::Unspecified> {
+        unimplemented!()
+    }
+}
+
 #[cfg(windows)]
 mod sysrand_chunk {
     use crate::{error, polyfill};