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| <h1 id="design-principles"><a class="header" href="#design-principles">Design Principles</a></h1> |
| <p>The purpose of Cargo is to formalize a canonical Rust workflow, by automating |
| the standard tasks associated with distributing software. Cargo simplifies |
| structuring a new project, adding dependencies, writing and running unit |
| tests, and more.</p> |
| <p>Cargo is not intended to be a general-purpose build tool. Ideally, it should |
| be easy to integrate it within another build tool, though admittedly that is |
| not as seamless as desired.</p> |
| <h2 id="stability-and-compatibility"><a class="header" href="#stability-and-compatibility">Stability and compatibility</a></h2> |
| <h3 id="backwards-compatibility"><a class="header" href="#backwards-compatibility">Backwards compatibility</a></h3> |
| <p>Cargo strives to remain backwards compatible with projects created in previous |
| versions. The CLI interface also strives to remain backwards compatible, such |
| that the commands and options behave the same. That being said, changes in |
| behavior, and even outright breakage are sometimes done in limited situations. |
| The following outlines some situations where backwards-incompatible changes are |
| made:</p> |
| <ul> |
| <li>Anything that addresses a security concern.</li> |
| <li>Dropping support for older platforms and tooling. Cargo follows the Rust |
| <a href="https://doc.rust-lang.org/nightly/rustc/platform-support.html">tiered platform support</a>.</li> |
| <li>Changes to resolve possibly unsafe or unreliable behavior.</li> |
| </ul> |
| <p>None of these changes should be taken lightly, and should be avoided if |
| possible, or possibly with some transition period to alert the user of the |
| potential change.</p> |
| <p>Behavior is sometimes changed in ways that have a high confidence that it |
| won’t break existing workflows. Almost every change carries this risk, so it |
| is often a judgment call balancing the benefit of the change with the |
| perceived possibility of its negative consequences.</p> |
| <p>At times, some changes fall in the gray area, where the current behavior is |
| undocumented, or not working as intended. These are more difficult judgment |
| calls. The general preference is to balance towards avoiding breaking existing |
| workflows.</p> |
| <p>Support for older registry APIs and index formats may be dropped, if there is |
| high confidence that there aren’t any active registries that may be affected. |
| This has never (to my knowledge) happened so far, and is unlikely to happen in |
| the future, but remains a possibility.</p> |
| <p>In all of the above, a transition period may be employed if a change is known |
| to cause breakage. A warning can be issued to alert the user that something |
| will change, and provide them with an alternative to resolve the issue |
| (preferably in a way that is compatible across versions if possible).</p> |
| <p>Cargo is only expected to work with the version of the related Rust tools |
| (<code>rustc</code>, <code>rustdoc</code>, etc.) that it is released with. As a matter of choice, |
| the latest nightly works with the most recent stable release, but that is |
| mostly to accommodate development of Cargo itself, and should not be expected |
| by users.</p> |
| <h3 id="forwards-compatibility"><a class="header" href="#forwards-compatibility">Forwards compatibility</a></h3> |
| <p>Additionally, Cargo strives a limited degree of <em>forwards compatibility</em>. |
| Changes should not egregiously prevent older versions from working. This is |
| mostly relevant for persistent data, such as on-disk files and the registry |
| interface and index. It also applies to a lesser degree to the registry API.</p> |
| <p>Changes to <code>Cargo.lock</code> require a transition time, where the new format is not |
| automatically written when the lock file is updated. The transition time |
| should not be less than 6 months, though preferably longer. New projects may |
| use the new format in a shorter time frame.</p> |
| <p>Changes to <code>Cargo.toml</code> can be made in any release. This is because the user |
| must manually modify the file, and opt-in to any new changes. Additionally, |
| Cargo will usually only issue a warning about new fields it doesn’t |
| understand, but otherwise continue to function.</p> |
| <p>Changes to cache files (such as artifacts in the <code>target</code> directory, or cached |
| data in Cargo’s home directory) should not <em>prevent</em> older versions from |
| running, but they may cause older versions to recreate the cache, which may |
| result in a performance impact.</p> |
| <p>Changes to the registry index should not prevent older versions from working. |
| Generally, older versions ignore new fields, so the format should be easily |
| extensible. Changes to the format or interpretation of existing fields should |
| be done very carefully to avoid preventing older versions of Cargo from |
| working. In some cases, this may mean that older versions of Cargo will not be |
| able to <em>select</em> a newly published crate, but it shouldn’t prevent them from |
| working at all. This level of compatibility may not last forever, but the |
| exact time frame for such a change has not yet been decided.</p> |
| <p>The registry API may be changed in such a way to prevent older versions of |
| Cargo from working. Generally, compatibility should be retained for as long as |
| possible, but the exact length of time is not specified.</p> |
| <h2 id="simplicity-and-layers"><a class="header" href="#simplicity-and-layers">Simplicity and layers</a></h2> |
| <p>Standard workflows should be easy and consistent. Each knob that is added has |
| a high cost, regardless if it is intended for a small audience. Layering and |
| defaults can help avoid the surface area that the user needs to be concerned |
| with. Try to avoid small functionalities that may have complex interactions |
| with one another.</p> |
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