The wasm_bindgen_webidl crate (used by web-sys's build.rs) uses env_logger for logging, which can be enabled by setting the RUST_LOG=wasm_bindgen_webidl environment variable while building the web-sys crate.

Make sure to enable "very verbose" output during cargo build to see these logs within web-sys's build script output.

cd crates/web-sys
RUST_LOG=wasm_bindgen_webidl cargo build -vv

If wasm_bindgen_webidl encounters WebIDL constructs that it doesn't know how to translate into wasm-bindgen AST items, it will emit warn-level logs.

WARN 2018-07-06T18:21:49Z: wasm_bindgen_webidl: Unsupported WebIDL interface: ...