cargo-profiles.md

Summary

Add the ability to create custom profiles in Cargo.toml, to provide further control over how the project is built. Allow overriding profile keys for certain dependency trees.

Motivation

Currently the "stable" way to tweak build parameters like "debug symbols", "debug assertions", and "optimization level" is to edit Cargo.toml.

This file is typically checked in tree, so for many projects overriding things involves making temporary changes to this, which feels hacky. On top of this, if Cargo is being called by an encompassing build system as what happens in Firefox, these changes can seem surprising. There are currently two main profiles in Cargo ("dev" and "release"), and we're forced to fit everything we need into these two categories. This isn't really enough.

Furthermore, this doesn't allow for much customization. For example, when trying to optimize for compilation speed by building in debug mode, build scripts will get built in debug mode as well. In case of complex build-time dependencies like bindgen, this can end up significantly slowing down compilation. It would be nice to be able to say "build in debug mode, but build build dependencies in release". Also, your program may have large dependencies that it doesn't use in critical paths, being able to ask for just these dependencies to be run in debug mode would be nice.

Guide-level explanation

Currently, the Cargo guide has a section on this.

We amend this to add that you can define custom profiles with the profile.foo key syntax. These can be invoked via cargo build --profile foo. The dev/doc/bench/etc profiles remain special. Each custom profile, aside from the "special" ones, gets a folder in target/, named after the profile. "dev" and "debug" are considered to be aliases

Profile keys can be "overridden":

[profile.dev]
opt-level = 0
debug = true

# the `image` crate will be compiled with -Copt-level=3
[profile.dev.overrides.image]
opt-level = 3

# Dependencies semver-matching any entry in the space separated list
# will be compiled without debuginfo
[profile.dev.overrides."image=0.2 piston>5.0"]
debug=false

# All dependencies (but not this crate itself) will be compiled
# with -Copt-level=2 . This includes build dependencies.
[profile.dev.overrides."*"]
opt-level = 2

# Build scripts and their dependencies will be compiled with -Copt-level=3
# By default, build scripts use the same rules as the rest of the profile
[profile.dev.build_override]
opt-level = 3

Custom profiles can be listed in a .cargo/config, however the user is responsible for clearing up build directories if the profile changes.

Reference-level explanation

In case of overlapping rules, the last mentioned rule will be applied. This applies to build scripts as well; if, for example, you have the following profile:

[profile.dev]
opt-level = 0
[profile.dev.build_override]
opt-level = 3

and the image crate is both a build dependency and a regular dependency; it will be compiled as per the build_override rule. If you wish it to be compiled as per the original rule, use a normal override rule:

[profile.dev]
opt-level = 0
[profile.dev.build_override]
opt-level = 3
[profile.dev.overrides.image]
opt-level = 0

It is not possible to have the same crate compiled in different modes as a build dependency and a regular dependency within the same profile.

build_override is itself

cargo build --target foo will fail to run if foo clashes with the name of a profile; so avoid giving profiles the same name as possible build targets.

Drawbacks

This complicates cargo.

Rationale and alternatives

There are really two or three concerns here:

• A stable interface for setting various profile keys (cargo rustc -- -Clto is not good, for example, and doesn't integrate into Cargo's target directories)
• The ability to use a different profile for build scripts (usually, the ability to flip optimization modes; I don't think folks care as much about -g in build scripts)
• The ability to use a different profile for specific dependencies

The first one can be resolved partially by stabilizing cargo arguments for overriding these. It doesn't fix the target directory issue, but that might not be a major concern.

The second one can be fixed with a specific build-scripts = release key for profiles.

The third can't be as easily fixed, however it's not clear if that's a major need.

The nice thing about this proposal is that it is able to handle all three of these concerns. However, separate RFCs for separate features could be introduced as well.

In general there are plans for Cargo to support other build systems by making it more modular (so that you can ask it for a build plan and then execute it yourself). Such build systems would be able to provide the ability to override profiles themselves instead. It's unclear if the general Rust community needs the ability to override profiles.

Unresolved questions

• Bikeshedding the naming of the keys
• The priority order when doing resolution
• Should build_override itself take an overrides.foo key?