## Customizing Builds with Release Profiles

In Rust, release profiles are predefined and customizable profiles with different configurations that allow a programmer to have more control over various options for compiling code. Each profile is configured independently of the others.

Cargo has two main profiles: the `dev` profile Cargo uses when you run `cargo build` and the `release` profile Cargo uses when you run `cargo build --release`. The `dev` profile is defined with good defaults for developing, and the `release` profile has good defaults for release builds.

These profile names might be familiar from the output of your builds, which shows the profile used in the build:

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

The `dev` and `release` shown in this build output indicate that the compiler is using different profiles.

Cargo has default settings for each of the profiles that apply when there aren’t any `[profile.*]` sections in the project’s Cargo.toml file. By adding `[profile.*]` sections for any profile we want to customize, we can override any subset of the default settings. For example, here are the default values for the `opt-level` setting for the `dev` and `release` profiles:

Filename: Cargo.toml

    [profile.dev]
    opt-level = 0

    [profile.release]
    opt-level = 3

The `opt-level` setting controls the number of optimizations Rust will apply to your code with a range of zero to three. Applying more optimizations extends compiling time, so if you’re in development and compiling your code often, you want faster compiling even at the expense of the resulting code running slower. That is the reason the default `opt-level` for `dev` is `0`. When you’re ready to release your code, it’s best to spend more time compiling. You’ll only compile in release mode once and run the compiled program many times, so release mode trades longer compile time for code that runs faster. That is the reason the default `opt-level` for the `release` profile is `3`.

We can override any default setting by adding a different value for it in Cargo.toml. For example, if we want to use optimization level 1 in the development profile, we can add these two lines to our project’s Cargo.toml file:

Filename: Cargo.toml

    [profile.dev]
    opt-level = 1

This code overrides the default setting of `0`. Now when we run `cargo` `build`, Cargo will use the defaults for the `dev` profile plus our customization to `opt-level`. Because we set `opt-level` to `1`, Cargo will apply more optimizations than the default, but not as many as a release build.

For the full list of configuration options and defaults for each profile, see Cargo’s documentation.