## Release profiles

Cargo supports a notion of release profiles. These profiles control various options for compiling your code and let you configure each profile independently of the others. You've seen a hint of this feature in the output of your builds:

    $ cargo build
        Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
    $ cargo build --release
        Finished release [optimized] target(s) in 0.0 secs

The "debug" and "release" notifications here indicate that the compiler is using different profiles. Cargo supports four profiles:

- `dev`: used for `cargo build`
- `release` used for `cargo build --release`
- `test` used for `cargo test`
- `doc` used for `cargo doc`

We can customize our `Cargo.toml` file with `[profile.*]` sections to tweak various compiler options for these profiles. For example, here's one of the default options for the `dev` and `release` profiles:

    [profile.dev]
    opt-level = 0

    [profile.release]
    opt-level = 3

The `opt-level` setting controls how many optimizations Rust will apply to your code. The setting goes from zero to three. Applying more optimizations takes more time. When you're compiling very often in development, you'd usually want compiling to be fast at the expense of the resulting code running slower. When you're ready to release, it's better to spend more time compiling the one time that you build your code to trade off for code that will run faster every time you use that compiled code.

We could override these defaults by changing them in `Cargo.toml`. For example, if we wanted to use optimization level 1 in development:

    [profile.dev]
    opt-level = 1

This overrides the default setting of `0`, and now our development builds will use more optimizations. Not as much as a release build, but a little bit more.

For the full list of settings and the defaults for each profile, see Cargo's documentation.