Profiles

Profiles provide a way to alter the compiler settings, influencing things like optimizations and debugging symbols.

Cargo has 4 built-in profiles: dev, release, test, and bench. It automatically chooses the profile based on which command is being run, the package and target that is being built, and command-line flags like --release. The selection process is described below.

Profile settings can be changed in Cargo.toml with the [profile] table. Within each named profile, individual settings can be changed with key/value pairs like this:

[profile.dev]
opt-level = 1               # Use slightly better optimizations.
overflow-checks = false     # Disable integer overflow checks.

Cargo only looks at the profile settings in the Cargo.toml manifest at the root of the workspace. Profile settings defined in dependencies will be ignored.

Profile settings

The following is a list of settings that can be controlled in a profile.

opt-level

The opt-level setting controls the -C opt-level flag which controls the level of optimization. Higher optimization levels may produce faster runtime code at the expense of longer compiler times. Higher levels may also change and rearrange the compiled code which may make it harder to use with a debugger.

The valid options are:

  • 0: no optimizations, also turns on cfg(debug_assertions).
  • 1: basic optimizations
  • 2: some optimizations
  • 3: all optimizations
  • "s": optimize for binary size
  • "z": optimize for binary size, but also turn off loop vectorization.

It is recommended to experiment with different levels to find the right balance for your project. There may be surprising results, such as level 3 being slower than 2, or the "s" and "z" levels not being necessarily smaller. You may also want to reevaluate your settings over time as newer versions of rustc changes optimization behavior.

See also Profile Guided Optimization for more advanced optimization techniques.

debug

The debug setting controls the -C debuginfo flag which controls the amount of debug information included in the compiled binary.

The valid options are:

  • 0 or false: no debug info at all
  • 1: line tables only
  • 2 or true: full debug info

debug-assertions

The debug-assertions setting controls the -C debug-assertions flag which turns cfg(debug_assertions) conditional compilation on or off. Debug assertions are intended to include runtime validation which is only available in debug/development builds. These may be things that are too expensive or otherwise undesirable in a release build. Debug assertions enables the debug_assert! macro in the standard library.

The valid options are:

  • true: enabled
  • false: disabled

overflow-checks

The overflow-checks setting controls the -C overflow-checks flag which controls the behavior of runtime integer overflow. When overflow-checks are enabled, a panic will occur on overflow.

The valid options are:

  • true: enabled
  • false: disabled

lto

The lto setting controls the -C lto flag which controls LLVM's link time optimizations. LTO can produce better optimized code, using whole-program analysis, at the cost of longer linking time.

The valid options are:

  • false: Performs "thin local LTO" which performs "thin" LTO on the local crate only across its codegen units. No LTO is performed if codegen units is 1 or opt-level is 0.
  • true or "fat": Performs "fat" LTO which attempts to perform optimizations across all crates within the dependency graph.
  • "thin": Performs "thin" LTO. This is similar to "fat", but takes substantially less time to run while still achieving performance gains similar to "fat".
  • "off": Disables LTO.

See also the -C linker-plugin-lto rustc flag for cross-language LTO.

panic

The panic setting controls the -C panic flag which controls which panic strategy to use.

The valid options are:

  • "unwind": Unwind the stack upon panic.
  • "abort": Terminate the process upon panic.

When set to "unwind", the actual value depends on the default of the target platform. For example, the NVPTX platform does not support unwinding, so it always uses "abort".

Tests, benchmarks, build scripts, and proc macros ignore the panic setting. The rustc test harness currently requires unwind behavior. See the panic-abort-tests unstable flag which enables abort behavior.

Additionally, when using the abort strategy and building a test, all of the dependencies will also be forced to built with the unwind strategy.

incremental

The incremental setting controls the -C incremental flag which controls whether or not incremental compilation is enabled. Incremental compilation causes rustc to to save additional information to disk which will be reused when recompiling the crate, improving re-compile times. The additional information is stored in the target directory.

The valid options are:

  • true: enabled
  • false: disabled

Incremental compilation is only used for workspace members and "path" dependencies.

The incremental value can be overridden globally with the CARGO_INCREMENTAL environment variable or the build.incremental config variable.

codegen-units

The codegen-units setting controls the -C codegen-units flag which controls how many "code generation units" a crate will be split into. More code generation units allows more of a crate to be processed in parallel possibly reducing compile time, but may produce slower code.

This option takes an integer greater than 0.

This option is ignored if incremental is enabled, in which case rustc uses an internal heuristic to split the crate.

rpath

The rpath setting controlls the -C rpath flag which controls whether or not rpath is enabled.

Default profiles

dev

The dev profile is used for normal development and debugging. It is the default for build commands like cargo build.

The default settings for the dev profile are:

[profile.dev]
opt-level = 0
debug = true
debug-assertions = true
overflow-checks = true
lto = false
panic = 'unwind'
incremental = true
codegen-units = 16  # Note: ignored because `incremental` is enabled.
rpath = false

release

The release profile is intended for optimized artifacts used for releases and in production. This profile is used when the --release flag is used, and is the default for cargo install.

The default settings for the release profile are:

[profile.release]
opt-level = 3
debug = false
debug-assertions = false
overflow-checks = false
lto = false
panic = 'unwind'
incremental = false
codegen-units = 16
rpath = false

test

The test profile is used for building tests, or when benchmarks are built in debug mode with cargo build.

The default settings for the test profile are:

[profile.test]
opt-level = 0
debug = 2
debug-assertions = true
overflow-checks = true
lto = false
panic = 'unwind'    # This setting is always ignored.
incremental = true
codegen-units = 16  # Note: ignored because `incremental` is enabled.
rpath = false

bench

The bench profile is used for building benchmarks, or when tests are built with the --release flag.

The default settings for the bench profile are:

[profile.bench]
opt-level = 3
debug = false
debug-assertions = false
overflow-checks = false
lto = false
panic = 'unwind'    # This setting is always ignored.
incremental = false
codegen-units = 16
rpath = false

Profile selection

The profile used depends on the command, the package, the Cargo target, and command-line flags like --release.

Build commands like cargo build, cargo rustc, cargo check, and cargo run default to using the dev profile. The --release flag may be used to switch to the release profile.

The cargo install command defaults to the release profile, and may use the --debug flag to switch to the dev profile.

Test targets are built with the test profile by default. The --release flag switches tests to the bench profile.

Bench targets are built with the bench profile by default. The cargo build command can be used to build a bench target with the test profile to enable debugging.

Note that when using the cargo test and cargo bench commands, the test/bench profiles only apply to the final test executable. Dependencies will continue to use the dev/release profiles. Also note that when a library is built for unit tests, then the library is built with the test profile. However, when building an integration test target, the library target is built with the dev profile and linked into the integration test executable.

Overrides

Profile settings can be overridden for specific packages and build-time crates. To override the settings for a specific package, use the package table to change the settings for the named package:

# The `foo` package will use the -Copt-level=3 flag.
[profile.dev.package.foo]
opt-level = 3

The package name is actually a Package ID Spec, so you can target individual versions of a package with syntax such as [profile.dev.package."foo:2.1.0"].

To override the settings for all dependencies (but not any workspace member), use the "*" package name:

# Set the default for dependencies.
[profile.dev.package."*"]
opt-level = 2

To override the settings for build scripts, proc macros, and their dependencies, use the build-override table:

# Set the settings for build scripts and proc-macros.
[profile.dev.build-override]
opt-level = 3

Note: When a dependency is both a normal dependency and a build dependency, Cargo will try to only build it once when --target is not specified. When using build-override, the dependency may need to be built twice, once as a normal dependency and once with the overridden build settings. This may increase initial build times.

The precedence for which value is used is done in the following order (first match wins):

  1. [profile.dev.package.name] — A named package.
  2. [profile.dev.package."*"] — For any non-workspace member.
  3. [profile.dev.build-override] — Only for build scripts, proc macros, and their dependencies.
  4. [profile.dev] — Settings in Cargo.toml.
  5. Default values built-in to Cargo.

Overrides cannot specify the panic, lto, or rpath settings.

Overrides and generics

The location where generic code is instantiated will influence the optimization settings used for that generic code. This can cause subtle interactions when using profile overrides to change the optimization level of a specific crate. If you attempt to raise the optimization level of a dependency which defines generic functions, those generic functions may not be optimized when used in your local crate. This is because the code may be generated in the crate where it is instantiated, and thus may use the optimization settings of that crate.

For example, nalgebra is a library which defines vectors and matrices making heavy use of generic parameters. If your local code defines concrete nalgebra types like Vector4<f64> and uses their methods, the corresponding nalgebra code will be instantiated and built within your crate. Thus, if you attempt to increase the optimization level of nalgebra using a profile override, it may not result in faster performance.

Further complicating the issue, rustc has some optimizations where it will attempt to share monomorphized generics between crates. If the opt-level is 2 or 3, then a crate will not use monomorphized generics from other crates, nor will it export locally defined monomorphized items to be shared with other crates. When experimenting with optimizing dependencies for development, consider trying opt-level 1, which will apply some optimizations while still allowing monomorphized items to be shared.