It's both mentioned in the blog posting being discussed and from there a link to the standard Cargo documentation: http://doc.crates.io/config.html

If I set

    [build]
    target = "x86_64-unknown-linux-musl"

in Cargo.toml and run cargo build --release

    unused manifest key: build.target

What am I doing wrong?

.cargo/config is different from Cargo.toml

Oops, you're right.