when

`#![feature] may not be used on the stable release channel`

try

cargo install nightly

then

rustup run nightly wasm-pack build --target web --out-name wasm --out-dir ./static

`--target web --out-name wasm --out-dir ./static` is optional

refs: https://github.com/rustwasm/wasm-pack/issues/629#issuecomment-488051800