Feature request: Set cargo --features via environment variables

Consider a Cargo.toml including

[features]
default = []
unstable = [] # Enables nightly features

Simply put, you'd now be able to enable unstable by setting CARGO_SET_FEATURE_UNSTABLE to "truthy". Alternatively, you'd be able to set CARGO_SET_FEATURES to a comma separated string of the features you wish to enable. This of course works for any number of features by setting more env vars or adding more to the comma separated string.

Having this capability would make .travis.yml files capable of enabling features very succinctly without relying on travis-cargo or other methods. An example:

language: rust
sudo: false
dist: trusty
rust:
- 1.22.1
- stable
- beta

matrix:
  allow_failures:
    - rust: nightly
  include:
    - rust: nightly
      env: CARGO_SET_FEATURE_UNSTABLE

cache: cargo

The issue does not name files, tests, or an implementation entry point. Start by locating Cargo’s feature-selection and environment-variable handling, then clarify the truthy values and how per-feature variables interact with the comma-separated form. Done means the selected features are enabled reliably from both environment-variable styles and covered by tests.