Can't you simply use the docs? When I code in Rust I generally have the docs opened: https://doc.rust-lang.org/std/vec/struct.Vec.html (or more like likely the locally installed version).
Most crates have documentation available as well (generally linked directly from their entry on crates.io) and if it's not online for some reason you can just run "cargo doc" to generate it locally. Randomly taking the "image" crate as an example: https://docs.rs/image/0.17.0/image/
Beats grepping header files IMO.