Add `-Z hint-mostly-unused` to tell rustc that most of a crate will go unused

This hint allows the compiler to optimize its operation based on this
assumption, in order to compile faster. This is a hint, and does not
guarantee any particular behavior.

This option can substantially speed up compilation if applied to a large
dependency where the majority of the dependency does not get used. This flag
may slow down compilation in other cases.

Currently, this option makes the compiler defer as much code generation as
possible from functions in the crate, until later crates invoke those
functions. Functions that never get invoked will never have code generated for
them. For instance, if a crate provides thousands of functions, but only a few
of them will get called, this flag will result in the compiler only doing code
generation for the called functions. (This uses the same mechanisms as
cross-crate inlining of functions.) This does not affect `extern` functions, or
functions marked as `#[inline(never)]`.

Some performance numbers, based on a crate with many dependencies having
just *one* large dependency set to `-Z hint-mostly-unused` (using
Cargo's `profile-rustflags` option):

A release build went from 4m07s to 2m04s.

A non-release build went from 2m26s to 1m28s.
4 files changed
tree: cce3fb2305ccc406cc8aa828b69a428cded4a1de
  1. .github/
  2. compiler/
  3. library/
  4. LICENSES/
  5. src/
  6. tests/
  7. .clang-format
  8. .editorconfig
  9. .git-blame-ignore-revs
  10. .gitattributes
  11. .gitignore
  12. .gitmodules
  13. .ignore
  14. .mailmap
  15. bootstrap.example.toml
  16. Cargo.lock
  17. Cargo.toml
  18. CODE_OF_CONDUCT.md
  19. configure
  20. CONTRIBUTING.md
  21. COPYRIGHT
  22. INSTALL.md
  23. LICENSE-APACHE
  24. license-metadata.json
  25. LICENSE-MIT
  26. README.md
  27. RELEASES.md
  28. REUSE.toml
  29. rust-bors.toml
  30. rustfmt.toml
  31. triagebot.toml
  32. x
  33. x.ps1
  34. x.py
README.md

Website | Getting started | Learn | Documentation | Contributing

This is the main source code repository for Rust. It contains the compiler, standard library, and documentation.

Why Rust?

  • Performance: Fast and memory-efficient, suitable for critical services, embedded devices, and easily integrated with other languages.

  • Reliability: Our rich type system and ownership model ensure memory and thread safety, reducing bugs at compile-time.

  • Productivity: Comprehensive documentation, a compiler committed to providing great diagnostics, and advanced tooling including package manager and build tool (Cargo), auto-formatter (rustfmt), linter (Clippy) and editor support (rust-analyzer).

Quick Start

Read “Installation” from The Book.

Installing from Source

If you really want to install from source (though this is not recommended), see INSTALL.md.

Getting Help

See https://www.rust-lang.org/community for a list of chat platforms and forums.

Contributing

See CONTRIBUTING.md.

License

Rust is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0), with portions covered by various BSD-like licenses.

See LICENSE-APACHE, LICENSE-MIT, and COPYRIGHT for details.

Trademark

The Rust Foundation owns and protects the Rust and Cargo trademarks and logos (the “Rust Trademarks”).

If you want to use these names or brands, please read the Rust language trademark policy.

Third-party logos may be subject to third-party copyrights and trademarks. See Licenses for details.