Customizing Builds with Release Profiles (2024)

In Rust, release profiles are predefined and customizable profiles withdifferent configurations that allow a programmer to have more control overvarious options for compiling code. Each profile is configured independently ofthe 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, andthe release profile has good defaults for release builds.

These profile names might be familiar from the output of your builds, whichshows 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 compileris using different profiles.

Cargo has default settings for each of the profiles that apply when therearen’t any [profile.*] sections in the project’s Cargo.toml file. By adding[profile.*] sections for any profile we want to customize, we can overrideany subset of the default settings. For example, here are the default valuesfor 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 toyour code with a range of zero to three. Applying more optimizations extendscompiling time, so if you’re in development and compiling your code often, youwant 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 readyto release your code, it’s best to spend more time compiling. You’ll onlycompile in release mode once and run the compiled program many times, sorelease mode trades longer compile time for code that runs faster. That is thereason the default opt-level for the release profile is 3.

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

Filename: Cargo.toml

[profile.dev]opt-level = 1

This code overrides the default setting of 0. Now when we run cargobuild, Cargo will use the defaults for the dev profile plus ourcustomization to opt-level. Because we set opt-level to 1, Cargo willapply 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, seeCargo’s documentation.