Tracking Issue for profile-hint-mostly-unused

[joshtriplett]

Warning

If you are looking to submit an experience report, note that you should not be setting anything via RUSTFLAGS; you should be setting this via either the Cargo profile setting in your top-level crate for a specific dependency, or the Cargo [hints] table in a specific dependency.

Summary

Implementation: #15643, #15673
Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#profile-hint-mostly-unused-option

This feature provides a new option in the [profile] section to enable the rustc hint-mostly-unused option. This is primarily useful to enable for specific dependencies:

[profile.dev.package.huge-mostly-unused-dependency]
hint-mostly-unused = true

To enable this feature, pass -Zprofile-hint-mostly-unused. However, since this option is a hint, using it without passing -Zprofile-hint-mostly-unused will only warn and ignore the profile option. Versions of Cargo prior to the introduction of this feature will give an "unused manifest key" warning, but will otherwise function without erroring. This allows using the hint in a crate's Cargo.toml without mandating the use of a newer Cargo to build it.

Unresolved Issues

• Support in .cargo/config.toml
• cargo publish stripping hints section from Cargo.toml #15836

Future Extensions

In the future, we may also want to provide a way for a crate to hint that it should have this option turned on by default, unless overridden by the top-level crate.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

[biruburu]

Tested on a top-level crate with 644 dependencies, including axum / aws-sdk / aws-lc-sys, and got the following results:

RUSTFLAGS="-Zhint-mostly-unused" cargo +nightly build
run 1: 1m 24s
run 2: 1m 30s
run 3: 1m 31s

cargo +nightly build
run 1: 1m 15s
run 2: 1m 16s
run 3: 1m 17s

I ran cargo +nightly clean between tests for a fair comparison. The standard build was consistently faster than -Zhint-mostly-unused, with some variation due to the changes in system temperature. Surprisingly, standard builds consumed roughly 40% more disk space (4.7GB vs 3.3GB hint-mostly-unused.)

cargo +nightly --version returns cargo 1.90.0-nightly (eabb4cd 2025-07-09) and the host system is MacOS M3 Pro with 36GB RAM.

For comparison, rustc 1.88.0 compiles the same codebase in 1m 15s, i.e. no practical difference from the 1.90.0 nightly.

[papow65]

Naively applying this to all 406 dependencies (including Bevy, with its dynamic_linking feature) of my project required a lot of memory for a full dev build with opt-level = 3.
Top level dependencies

# With bevy/dynamic_linking *enabled*, requiring 11.8GB ram:
cargo +nightly -Zprofile-hint-mostly-unused build
real    10m40,525s
user    88m17,878s
sys     1m49,396s

# With bevy/dynamic_linking *disabled*, requiring 11.0GB ram (0.8GB less):
cargo +nightly -Zprofile-hint-mostly-unused build --no-default-features
real    10m36,934s
user    88m35,152s
sys     2m2,587s

cargo +nightly build (corrected timings)
real    3m24,027s
user    43m56,553s
sys     1m21,596s

(In my earlier tests and versions of this comment, the high memory usage triggered earlyoom.)

[Kobzol]

Just noting, in case it wasn't clear from the blog post, applying this flag to all your dependencies will most likely make your compilation times be terrible. It is designed to be only applied to a handful of selected large dependencies whose code is mostly unused in your crate graph. You should pass the flag to these dependencies using Cargo (https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused/#plumbing-this-through-cargo-with-profiles).

[Arnavion]

To save everyone else the time, latest nightly as of now (cargo 1.90.0-nightly (eabb4cd92 2025-07-09) does not support [hints] mostly-unused = true that is mentioned in the blog post. That feature was added in 1921d09 which is newer than that cargo release.

Latest nightly, with cargo 1.90.0-nightly (6833aa715 2025-07-13), now supports [hints] mostly-unused = true.

[ze]

Tried on a crate of mine. I specifically only tried it on aws-sdk-s3.
cargo 1.90.0-nightly (eabb4cd92 2025-07-09)

Each run had a cargo clean ran. There were two runs done.

Before:

run1: 4m 20s
run2: 4m 06s

After:

run1: 3m 32s
run2: 3m 18s

To get a baseline on cargo 1.88.0 (873a06493 2025-05-10):

run1: 4m 12s
run2: 4m 14s

This looks great! 💯.

[cemoktra]

cargo 1.90.0-nightly (930b4f6 2025-06-28)

For my big workspace that depends on aws crates for example:

• no hint => 1m 00s (same as stable 1.88)
• hint-mostly-unused => 1m 15s

[workingjubilee]

@cemoktra How did you enable it?

Did you (incorrectly) add it to RUSTFLAGS as in #15644 (comment) or?

Per the blog post:

Please report any performance improvements, or unexpected performance issues, or especially any failures you observe, to the #15644. When reporting, please tell us:

• What hints or profile settings you added
• What crates you added them to
• What top-level crate you're building
• What features you set when building
• What build profile you're using (e.g. the default dev profile, or the release profile)
• Whether you did a clean build or an incremental rebuild
• What performance numbers you got with and without the option you added

[cemoktra]

@workingjubilee yes i did RUSTFLAGS="-Zhint-mostly-unused" cargo +nightly build

[omid]

I also natively run this on all deps of a project. Numbers below are for debug mode and fresh compile.

When I run it on the main binary, it changes from 85s to 60s. (140% faster 🎉 )
Just the binary compilation (last linking step), changes from 1s to 22s! (Slowed down)

But when I run it on all binaries (21 binaries, in my case) in the same code base, it slows down.
So for the complete package, it changes from 87s to 107s.

As you see, before having the flag, it took 2 extra seconds to compile all binaries in comparison to one.
But after including the flag, it took about 50 extra seconds (almost doubled).

The command I run was:
RUSTFLAGS="-Zhint-mostly-unused" cargo +nightly build --timings and RUSTFLAGS="-Zhint-mostly-unused" cargo +nightly build --timings --bin X

[JarredAllen]

On a decently-large project at my work (a bit over 1k crates including transitive dependencies, a bit over 100 binaries), I tested using cargo clean && time cargo build --workspace, on cargo 1.90.0-nightly (eabb4cd92 2025-07-09).

Without this flag:

real    4m27.345s
user    45m39.633s
sys     5m59.555s

(a second try had a user time of 46m26s and a system time of 6m5s, so there's some variation, but small enough that I believe the results below are real differences)

When I applied the flag to tokio:

real    4m14.523s                                           
user    44m39.785s                                          
sys     5m49.740s                                           

When I applied it to all of my dependencies:

real    4m22.295s
user    47m22.738s
sys     6m4.903s

It looks like this setting makes things take longer when applied indiscriminately, but it saves time when applied to the right crates. Just applying it to tokio was a 3% speedup on the entire build process, and I'd guess there are other, equally-sized wins in my dependency tree if I looked closer. (I think user and system times are the right values to look at, since I ran it on my laptop that has various background tasks that could interfere with the real time)

As a note, I applied it to all dependencies by putting this in my workspace-level Cargo.toml file:

[profile.dev.package."*"]
hint-mostly-unused = true

And I applied it just to tokio by doing the same thing but with tokio instead of "*".

[workingjubilee]

@omid @cemoktra Your reports are invalid. The RUSTFLAGS command is not the correct way to handle this.