Following rust-toolchain?

My project uses rust-toolchain to specify and lock a rustc version. So now Travis will install Rust two times, 1 installing latest nightly or beta or stable, 1 installing the version specified by rust-toolchain file. It would be nice that Travis can be smarter!

This issue is previously reported on GitHub as well: https://github.com/travis-ci/travis-ci/issues/10002

https://github.com/travis-ci/travis-build/pull/1513 attempts to solve the issue, but not all items in the Override Precedence is considered in that PR.