wasm32/js: Clarify truncating conversions.

usize and u32 both implement `From<u16>` so use u16 for the length.

Author: briansmith

src/js.rs

@@ -9,7 +9,7 @@ use wasm_bindgen::{prelude::wasm_bindgen, JsCast, JsValue};
 
 // Size of our temporary Uint8Array buffer used with WebCrypto methods
 // Maximum is 65536 bytes see https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues
-const WEB_CRYPTO_BUFFER_SIZE: usize = 256;
+const WEB_CRYPTO_BUFFER_SIZE: u16 = 256;
 // Node.js's crypto.randomFillSync requires the size to be less than 2**31.
 const NODE_MAX_BUFFER_SIZE: usize = (1 << 31) - 1;
 
@@ -50,9 +50,10 @@ pub(crate) fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error>
             RngSource::Web(crypto, buf) => {
                 // getRandomValues does not work with all types of WASM memory,
                 // so we initially write to browser memory to avoid exceptions.
-                for chunk in dest.chunks_mut(WEB_CRYPTO_BUFFER_SIZE) {
+                for chunk in dest.chunks_mut(WEB_CRYPTO_BUFFER_SIZE.into()) {
                     // The chunk can be smaller than buf's length, so we call to
                     // JS to create a smaller view of buf without allocation.
+                    #[allow(clippy::cast_possible_truncation)]
                     let sub_buf = buf.subarray(0, chunk.len() as u32);
 
                     if crypto.get_random_values(&sub_buf).is_err() {
@@ -95,7 +96,7 @@ fn getrandom_init() -> Result<RngSource, Error> {
         },
     };
 
-    let buf = Uint8Array::new_with_length(WEB_CRYPTO_BUFFER_SIZE as u32);
+    let buf = Uint8Array::new_with_length(WEB_CRYPTO_BUFFER_SIZE.into());
     Ok(RngSource::Web(crypto, buf))
 }