## 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 development,
and the `release` profile has good defaults for release builds.

These profile names might be familiar from the output of your builds:

```text
$ 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 you want to customize, you 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:

<span class="filename">Filename: Cargo.toml</span>

```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 0 to 3. Applying more optimizations extends
compiling time, so if you’re in development and compiling your code often,
you’ll want faster compiling even if the resulting code runs 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, but you’ll run the compiled program many times, so
release mode trades longer compile time for code that runs faster. That is why
the default `opt-level` for the `release` profile is `3`.

You 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:

<span class="filename">Filename: Cargo.toml</span>

```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 in a release build.

For the full list of configuration options and defaults for each profile, see
[Cargo’s documentation](https://doc.rust-lang.org/cargo/reference/manifest.html#the-profile-sections).
