Crate Directory

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A draft RFC for solidifying the library usage mechanisms for non-Cargo Rust projects.

  • Feature Name: lib_dir
  • Start Date: 2018-03-22
  • RFC PR: (leave this empty)
  • Rust Issue: (leave this empty)


Add a standardized folder in the Rust project directory structure for external libraries, in source or compiled form, for ease of use in projects that do not use Cargo.


While Cargo is the official Rust build tool and most common human-facing interface to the compiler, it is not universally used or usable. Cargo currently is the primary mechanism for passing dependencies into a Rust project, which makes it harder to build dependency-consuming Rust projects without it.

This RFC proposal will make the process of managing and using external dependencies easier for projects that do not use Cargo, and invoke the Rust compiler manually or in the scripts of another build tool.

This proposal will also ease the process of vendoring dependencies into a project.

Both of these are concerns of use cases in environments such as corporate or otherwise strictly controlled organizations where a foreign build tool or network-fetched dependencies are unacceptable.

The expected outcome is that it will be very easy to invoke a rustc command line to compile a project that has external crate dependencies without using Cargo to manage the dependency storage and rustc command line generation.

Guide-level explanation

When creating a Rust project in an environment that cannot use Cargo, make a project directory my_new_project/ with child directories src/ and lib/. If you have external dependencies available, say from a company repository, place them inside lib/.

Precompiled artifacts (.rlib, .dll, .a, .so, for example) do not need their own folder; they would be stored as my_new_project/lib/foo.rlib and my_new_project/lib/bar.dll.

If you have source dependencies, these are placed in a subfolder, such as my_new_project/lib/baz/. The lib/baz subfolder is the project root of the baz project, and when baz is compiled, it will prefix its artifacts with baz and place them directly under lib/.

When it comes time to compile your Rust project, whose root file is by default src/ or src/, you will execute a rustc command line from the project root directory.

$ pwd

$ tree
├── lib
│   ├── regex.rlib
│   └──
└── src

$ head -6 src/
extern crate regex;

#[link = "zip"]
extern "C" fn compress(ptr: *const u8, len: usize) -> i32;

fn main() {
rustc src/ --crate-name my_new_project

By default, rustc knows to read any extern crate lines in your src/ files and immediately look in your lib/ folder for them. If you have a line extern crate regex;, then the artifact lib/libregex.rlib will be linked into your project. If lib/regex.rlib does not exist, then rustc will continue searching in its set of known library directories, and if unsuccessful, exit with an error.

rustc also knows to use this folder for dependencies that are not Rust libraries, so any extern fn declarations that have a #[link = "zip"] will cause the compiler to search for a lib/libzip with any of the appropriate library file extensions before searching in its other known library directories.

If you wish to call your library folder something else, or to not have it stored inside your project dir, then you can instruct rustc to search elsewhere with the -L /path/to/libraries flag. Any paths given with this flag take precedence over builtin library search paths.

Reference-level explanation

Add a ./lib at the front of the library search path set in the compiler, and search extern crate-declared libraries in the library search path without requiring an --extern /path/to/crate flag in the compiler invocation.


I can’t really think of any.

Rationale and alternatives

  • Why is this design the best in the space of possible designs?

The project-level lib/ directory is a reasonably common idiom for projects that must carry their dependencies with them. Making the compiler aware of an in-tree directory to search for dependency artifacts, both rlib and not, makes a more ergonomic experience for using Rust without the Cargo tool.

  • What other designs have been considered and what is the rationale for not choosing them?

None that I know, hence why this RFC document is on my website and not a pull request.

TODO: Remove before requesting pull.

  • What is the impact of not doing this?

It will be slightly less ergonomic to invoke rustc by hand or by foreign build system to use vendored dependencies, aka status quo.

Prior art

Discuss prior art, both the good and the bad, in relation to this proposal. A few examples of what this can include are:

  • For language, library, cargo, tools, and compiler proposals: Does this feature exists in other programming languages and what experience have their community had?
  • For community proposals: Is this done by some other community and what were their experiences with it?
  • For other teams: What lessons can we learn from what other communities have done here?
  • Papers: Are there any published papers or great posts that discuss this? If you have some relevant papers to refer to, this can serve as a more detailed theoretical background.

This section is intended to encourage you as an author to think about the lessons from other languages, provide readers of your RFC with a fuller picture. If there is no prior art, that is fine - your ideas are interesting to us whether they are brand new or if it is an adaptation from other languages.

Note that while precedent set by other languages is some motivation, it does not on its own motivate an RFC. Please also take into consideration that rust sometimes intentionally diverges from common language features.

Unresolved questions

  • What parts of the design do you expect to resolve through the RFC process before this gets merged?
  • What parts of the design do you expect to resolve through the implementation of this feature before stabilization?
  • What related issues do you consider out of scope for this RFC that could be addressed in the future independently of the solution that comes out of this RFC?