Call for Testing: Speeding up compilation with hint-mostly-unused

Author: joshtriplett

content/inside-rust/call-for-testing-hint-mostly-unused.md

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+path = "inside-rust/2025/07/14/call-for-testing-hint-mostly-unused"
+title = "Call for Testing: Speeding up compilation with `hint-mostly-unused`"
+authors = ["Josh Triplett"]
+aliases = ["inside-rust/2025/07/14/call-for-testing-hint-mostly-unused.html"]
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+
+I'm pleased to announce, and call for testing of, the nightly-only `rustc`
+`-Zhint-mostly-unused` option, and the corresponding nightly Cargo features
+`profile.hint-mostly-unused` and `hints.mostly-unused`. These options can help
+accelerate your Rust compile time in some cases, by avoiding compilation of
+items from your dependencies that you aren't using. Your feedback will help
+evaluate these features and make progress towards stabilizing them in the
+future.
+
+## Background
+
+Some crates provide comprehensive APIs with a very large surface area, yet many
+of their users need only a few entry points. In such cases, the compiler
+currently spends time generating code for the entire crate, and then ends up
+throwing most of that code away.
+
+This can waste a substantial amount of compile time. Some large crates can take
+minutes to compile, and when you use these large crates as dependencies, they
+can take a disproportionate amount of the entire compilation time of your
+top-level crate.
+
+In some cases, crates add feature flags to control compilation of their API
+surface. This can improve compile time, but adds complexity for users, who now
+need to determine which features they need for the APIs they use. Features also
+constitute a stable interface of a crate, and changing feature flags can be a
+breaking change. And even with feature flags, not every enabled function will
+be needed; there is a balance between granularity and ease of use.
+
+## Deferring code generation with `-Zhint-mostly-unused`
+
+The latest nightly `rustc` compiler now supports an option
+`-Zhint-mostly-unused`, which tells `rustc` that the crate's API surface will
+mostly go unused. This is a hint, and `rustc` doesn't make guarantees about its
+exact behavior (so that we can extend or improve it in the future), but
+currently it causes the compiler to defer as much of code generation as
+possible. (The compiler does this using the same machinery as
+`#[inline(always)]`.)
+
+Applying this option to key crates you depend on (and use only a small subset
+of) can provide a substantial reduction in compile time, for debug builds and
+especially for release builds.
+
+## How does this perform?
+
+| **Dependency Crate** | **Before** | **`hint-mostly-unused`** | **Delta** |
+| :- | -: | -: | -: |
+| `windows`, all Graphics/UI features | 18.3s | 10.7s | -42% |
+| `windows`, all features | 3m 48s | 2m 55s | -23% |
+| `rustix`, `all-apis` feature | 5.9s | 4.3s | -27% |
+| `x11rb` and `x11rb-protocol` | 5.3s | 2.6s | -51% |
+| `aws-sdk-ec2` | 4m 07s | 2m 04s | -50% |
+
+This performance improvement comes from deferring code generation. For
+instance, the `windows` crate in the first example goes from building in 15.1s
+of which 49% is codegen, to building in 7.5s of which 1% is codegen.
+
+Note that this option does not provide a universal performance improvement for
+every crate. Using it for crates whose API surface is mostly used, and/or used
+in multiple different crates or binaries (e.g. multiple test binaries that each
+test a substantial swath of the API), may result in redoing code generation for
+the same items repeatedly.
+
+## Plumbing this through Cargo with profiles
+
+In order to support compiling specific dependencies with this option, Cargo
+supports a [profile option
+`hint-mostly-unused`](https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#profile-hint-mostly-unused-option)
+to mark a crate with this hint:
+
+```toml
+[profile.dev.package.huge-mostly-unused-dependency]
+hint-mostly-unused = true
+
+[profile.release.package.huge-mostly-unused-dependency]
+hint-mostly-unused = true
+```
+
+Note that if you build in multiple profiles (e.g. the default dev profile and
+the `-r` release profile), you'll want to set this flag for both, as shown
+above.
+
+Because this option is still nightly-only, and depends on a nightly-only
+`rustc` option as well, enabling it requires passing
+`-Zprofile-hint-mostly-unused` on the `cargo` command line. Without this
+option, cargo will ignore this with a warning (but not an error, as it's still
+just a hint). Note that as with any profile option, it only takes effect when
+set in the top-level crate you're building.
+
+You should not, in general, set this flag for all your dependencies, or for
+your own crate; you should set it selectively and test to make sure it provides
+an improvement. Using the [cargo `--timings`
+option](https://doc.rust-lang.org/nightly/cargo/reference/timings.html) can
+help to identify crates that might benefit from this hint. And when testing
+this hint, `--timings` can help detect whether the build time of *other* crates
+in the dependency tree went up.
+
+## Making this automatic: Cargo `[hints]`
+
+A profile hint still requires the top-level crate to configure the hint for
+some of its dependencies. However, some crates know that almost all of their
+users will want this hint enabled. For such crates, we've introduced a new
+`hints` mechanism in Cargo. Unlike profiles, which only apply when set in the
+top-level crate you build, hints are set within individual crates in your
+dependency graph. Hints provide a default behavior that you can still override.
+
+A crate that knows most of its users will not use most of its API surface can
+set this hint in its `Cargo.toml` manifest:
+
+```toml
+[hints]
+mostly-unused = true
+```
+
+Note that setting a hint does *not* increase the Minimum Supported Rust Version
+(MSRV) of your crate. Hints are always ignored if not understood. So, you can
+safely set this hint immediately, without waiting for this feature to be
+stabilized, and users of nightly will immediately benefit (if they pass
+`-Zprofile-hint-mostly-unused` to cargo to enable the feature).
+
+### Future hints
+
+The `hints` mechanism in Cargo is a general feature, and we plan to make use of
+it for other purposes in the future. For instance, we may offer a
+`min-opt-level` option, for crates that are so performance-sensitive (e.g.
+numerics code) that most users will want to build them with optimization even
+in development mode. As with other hints, such a mechanism would still always
+allow the top-level crate to override.
+
+## How do I help?
+
+We'd love for you to test out this feature on the latest Rust nightly compiler[^nightly].
+
+[^nightly]: Make sure to run `rustup update nightly` (or however you manage your Rust releases).
+
+If you maintain a crate that has a large API surface, and you expect that the
+typical user might use only a fraction of it, try setting `hints.mostly-unused`
+in your `Cargo.toml`:
+
+```toml
+[hints]
+mostly-unused = true
+```
+
+You can test the effect of this by building a *typical* crate that depends on
+your crate, with and without this hint set, using nightly Cargo:
+`cargo +nightly -Zprofile-hint-mostly-unused build -r`. If this provides a
+noticeable performance improvement, consider setting it in your published
+crate.
+
+Please report any performance improvements, or unexpected performance issues,
+or *especially* any failures you observe, to the [tracking issue for
+profile-hint-mostly-unused](https://github.com/rust-lang/cargo/issues/15644).
+We'll take this feedback into account to fix any issues with either the rustc
+compiler feature or the Cargo features, and to evaluate when those features
+have seen enough testing to be ready to stabilize.
+
+## Acknowledgements
+
+Much appreciation to:
+- [Ben Kimock](https://github.com/saethlin), whose work towards MIR-only rlibs
+  provided inspiration and infrastructure for this work.
+- The [Rust All Hands](https://rustweek.org/all-hands/) and its organizers, for
+  providing a forum to discuss and progress this work.