How to specify a certain commit in dependencies in Cargo.toml?

As hinted in the Cargo.toml vs Cargo.lock section of the Cargo guide, you can use the rev property to specify a commit hash:

[...] If you build this package today, and then you send a copy to me, and I build this package tomorrow, something bad could happen. There could be more commits to rand in the meantime, and my build would include new commits while yours would not. Therefore, we would get different builds. This would be bad because we want reproducible builds.

We could fix this problem by putting a rev line in our Cargo.toml:

[dependencies]
rand = { git = "https://github.com/rust-lang-nursery/rand.git", rev = "9f35b8e" }

It is also mentioned in Specifying dependencies, although no example is given (emphasis mine):

Since we haven’t specified any other information, Cargo assumes that we intend to use the latest commit on the master branch to build our package. You can combine the git key with the rev, tag, or branch keys to specify something else. [...]

You can use the rev key to specify a commit hash. For example:

thelib = { git = "https://github.com/someguys/thelib", rev = "9f35b8e" }

It's briefly mentioned in this section of the Cargo book.