Is it possible to have multiple coexisting Rust installations?

Sure. In the development version, use the --prefix option to ./configure, e.g. --prefix=~/opt/rust-dev, and then its installed files will be contained entirely inside that directory.

The current solution is to use rustup. Once installed, you can install multiple toolchains:

rustup install nightly
rustup install stable
rustup install 1.7

If you have a local build of Rust, you can link it as a toolchain

rustup toolchain link my-development /path/to/rust/code

You can pick a default toolchain

rustup default stable

Or add an override toolchain for a specific directory on your machine only via rustup

cd /my/cool/project
rustup override set nightly

Or add an override toolchain that lives with a specific directory, like a repository, via a rust-toolchain file

cd /my/cool/project
echo "nightly" > rust-toolchain

If you want to just use a different toolchain temporarily, you can use the "plus syntax":

rustc +1.7 --help
cargo +nightly build

In other cases you can use rustup run to run any arbitrary command in a specific toolchain:

rustup run nightly any command you want here 

See also:

• How to execute cargo test using the nightly channel?