When building Rust packages that provide libraries but no executable
binaries, `cargo install` doesn't do anything; you need to use `cargo
build` and install any libraries manually. See e.g.
rust-lang/cargo#8294.
Unfortunately, Homebrew's Rubocop "use cargo install *std_cargo_args"
rule, as currently written, blocks all invocations of `cargo build`.
This commit changes that rule to exclude invocations of `cargo build`
that use the `--lib` argument (`--lib` specifies to Cargo that a
package's library targets should be built). This will enable library
packages to be built while retaining the "use cargo install
*std_cargo_args" message for the more common case when a Rust package
provides executable binaries.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.
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