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| <h1 id="procedures-for-breaking-changes"><a class="header" href="#procedures-for-breaking-changes">Procedures for breaking changes</a></h1> |
| <p>This page defines the best practices procedure for making bug fixes or soundness |
| corrections in the compiler that can cause existing code to stop compiling. This |
| text is based on |
| <a href="https://github.com/rust-lang/rfcs/blob/master/text/1589-rustc-bug-fix-procedure.md">RFC 1589</a>.</p> |
| <h1 id="motivation"><a class="header" href="#motivation">Motivation</a></h1> |
| <p>From time to time, we encounter the need to make a bug fix, soundness |
| correction, or other change in the compiler which will cause existing code to |
| stop compiling. When this happens, it is important that we handle the change in |
| a way that gives users of Rust a smooth transition. What we want to avoid is |
| that existing programs suddenly stop compiling with opaque error messages: we |
| would prefer to have a gradual period of warnings, with clear guidance as to |
| what the problem is, how to fix it, and why the change was made. This RFC |
| describes the procedure that we have been developing for handling breaking |
| changes that aims to achieve that kind of smooth transition.</p> |
| <p>One of the key points of this policy is that (a) warnings should be issued |
| initially rather than hard errors if at all possible and (b) every change that |
| causes existing code to stop compiling will have an associated tracking issue. |
| This issue provides a point to collect feedback on the results of that change. |
| Sometimes changes have unexpectedly large consequences or there may be a way to |
| avoid the change that was not considered. In those cases, we may decide to |
| change course and roll back the change, or find another solution (if warnings |
| are being used, this is particularly easy to do).</p> |
| <h3 id="what-qualifies-as-a-bug-fix"><a class="header" href="#what-qualifies-as-a-bug-fix">What qualifies as a bug fix?</a></h3> |
| <p>Note that this RFC does not try to define when a breaking change is permitted. |
| That is already covered under <a href="https://github.com/rust-lang/rfcs/blob/master/text/1122-language-semver.md">RFC 1122</a>. This document assumes that the |
| change being made is in accordance with those policies. Here is a summary of the |
| conditions from RFC 1122:</p> |
| <ul> |
| <li><strong>Soundness changes:</strong> Fixes to holes uncovered in the type system.</li> |
| <li><strong>Compiler bugs:</strong> Places where the compiler is not implementing the specified |
| semantics found in an RFC or lang-team decision.</li> |
| <li><strong>Underspecified language semantics:</strong> Clarifications to grey areas where the |
| compiler behaves inconsistently and no formal behavior had been previously |
| decided.</li> |
| </ul> |
| <p>Please see <a href="https://github.com/rust-lang/rfcs/blob/master/text/1122-language-semver.md">the RFC</a> for full details!</p> |
| <h1 id="detailed-design"><a class="header" href="#detailed-design">Detailed design</a></h1> |
| <p>The procedure for making a breaking change is as follows (each of these steps is |
| described in more detail below):</p> |
| <ol> |
| <li>Do a <strong>crater run</strong> to assess the impact of the change.</li> |
| <li>Make a <strong>special tracking issue</strong> dedicated to the change.</li> |
| <li>Do not report an error right away. Instead, <strong>issue forwards-compatibility |
| lint warnings</strong>. |
| <ul> |
| <li>Sometimes this is not straightforward. See the text below for suggestions |
| on different techniques we have employed in the past.</li> |
| <li>For cases where warnings are infeasible: |
| <ul> |
| <li>Report errors, but make every effort to give a targeted error message |
| that directs users to the tracking issue</li> |
| <li>Submit PRs to all known affected crates that fix the issue |
| <ul> |
| <li>or, at minimum, alert the owners of those crates to the problem and |
| direct them to the tracking issue</li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| <li>Once the change has been in the wild for at least one cycle, we can |
| <strong>stabilize the change</strong>, converting those warnings into errors.</li> |
| </ol> |
| <p>Finally, for changes to <code>rustc_ast</code> that will affect plugins, the general policy |
| is to batch these changes. That is discussed below in more detail.</p> |
| <h3 id="tracking-issue"><a class="header" href="#tracking-issue">Tracking issue</a></h3> |
| <p>Every breaking change should be accompanied by a <strong>dedicated tracking issue</strong> |
| for that change. The main text of this issue should describe the change being |
| made, with a focus on what users must do to fix their code. The issue should be |
| approachable and practical; it may make sense to direct users to an RFC or some |
| other issue for the full details. The issue also serves as a place where users |
| can comment with questions or other concerns.</p> |
| <p>A template for these breaking-change tracking issues can be found |
| <a href="https://github.com/rust-lang/rust/issues/new?template=tracking_issue_future.md">here</a>. An example of how such an issue should look can be <a href="https://gist.github.com/nikomatsakis/631ec8b4af9a18b5d062d9d9b7d3d967">found |
| here</a>.</p> |
| <h3 id="issuing-future-compatibility-warnings"><a class="header" href="#issuing-future-compatibility-warnings">Issuing future compatibility warnings</a></h3> |
| <p>The best way to handle a breaking change is to begin by issuing |
| future-compatibility warnings. These are a special category of lint warning. |
| Adding a new future-compatibility warning can be done as follows.</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>// 1. Define the lint in `compiler/rustc_lint/src/builtin.rs` and |
| // add the metadata for the future incompatibility: |
| declare_lint! { |
| pub YOUR_LINT_HERE, |
| Warn, |
| "illegal use of foo bar baz" |
| @future_incompatible = FutureIncompatibleInfo { |
| reason: fcw!(FutureReleaseError #1234) // your tracking issue here! |
| }, |
| } |
| |
| // 2. Add a decidacted lint pass for it. |
| // This step can be skipped if you emit the lint as part of an existing pass. |
| |
| #[derive(Default)] |
| pub struct MyLintPass { |
| ... |
| } |
| |
| impl {Early,Late}LintPass for MyLintPass { |
| ... |
| } |
| |
| impl_lint_pass!(MyLintPass => [YOUR_LINT_HERE]); |
| |
| // 3. emit the lint somewhere in your lint pass: |
| cx.emit_span_lint( |
| YOUR_LINT_HERE, |
| pat.span, |
| // some diagnostic struct |
| MyDiagnostic { |
| ... |
| }, |
| ); |
| |
| <span class="boring">}</span></code></pre> |
| <p>Finally, register the lint in <code>compiler/rustc_lint/src/lib.rs</code>. |
| There are many examples in that file that already show how to do so.</p> |
| <h4 id="helpful-techniques"><a class="header" href="#helpful-techniques">Helpful techniques</a></h4> |
| <p>It can often be challenging to filter out new warnings from older, pre-existing |
| errors. One technique that has been used in the past is to run the older code |
| unchanged and collect the errors it would have reported. You can then issue |
| warnings for any errors you would give which do not appear in that original set. |
| Another option is to abort compilation after the original code completes if |
| errors are reported: then you know that your new code will only execute when |
| there were no errors before.</p> |
| <h4 id="crater-and-cratesio"><a class="header" href="#crater-and-cratesio">Crater and crates.io</a></h4> |
| <p><a href="./tests/crater.html">Crater</a> is a bot that will compile all crates.io crates and many |
| public github repos with the compiler with your changes. A report will then be |
| generated with crates that ceased to compile with or began to compile with your |
| changes. Crater runs can take a few days to complete.</p> |
| <p>We should always do a crater run to assess impact. It is polite and considerate |
| to at least notify the authors of affected crates the breaking change. If we can |
| submit PRs to fix the problem, so much the better.</p> |
| <h4 id="is-it-ever-acceptable-to-go-directly-to-issuing-errors"><a class="header" href="#is-it-ever-acceptable-to-go-directly-to-issuing-errors">Is it ever acceptable to go directly to issuing errors?</a></h4> |
| <p>Changes that are believed to have negligible impact can go directly to issuing |
| an error. One rule of thumb would be to check against <code>crates.io</code>: if fewer than |
| 10 <strong>total</strong> affected projects are found (<strong>not</strong> root errors), we can move |
| straight to an error. In such cases, we should still make the “breaking change” |
| page as before, and we should ensure that the error directs users to this page. |
| In other words, everything should be the same except that users are getting an |
| error, and not a warning. Moreover, we should submit PRs to the affected |
| projects (ideally before the PR implementing the change lands in rustc).</p> |
| <p>If the impact is not believed to be negligible (e.g., more than 10 crates are |
| affected), then warnings are required (unless the compiler team agrees to grant |
| a special exemption in some particular case). If implementing warnings is not |
| feasible, then we should make an aggressive strategy of migrating crates before |
| we land the change so as to lower the number of affected crates. Here are some |
| techniques for approaching this scenario:</p> |
| <ol> |
| <li>Issue warnings for subparts of the problem, and reserve the new errors for |
| the smallest set of cases you can.</li> |
| <li>Try to give a very precise error message that suggests how to fix the problem |
| and directs users to the tracking issue.</li> |
| <li>It may also make sense to layer the fix: |
| <ul> |
| <li>First, add warnings where possible and let those land before proceeding to |
| issue errors.</li> |
| <li>Work with authors of affected crates to ensure that corrected versions are |
| available <em>before</em> the fix lands, so that downstream users can use them.</li> |
| </ul> |
| </li> |
| </ol> |
| <h3 id="stabilization"><a class="header" href="#stabilization">Stabilization</a></h3> |
| <p>After a change is made, we will <strong>stabilize</strong> the change using the same process |
| that we use for unstable features:</p> |
| <ul> |
| <li> |
| <p>After a new release is made, we will go through the outstanding tracking |
| issues corresponding to breaking changes and nominate some of them for <strong>final |
| comment period</strong> (FCP).</p> |
| </li> |
| <li> |
| <p>The FCP for such issues lasts for one cycle. In the final week or two of the |
| cycle, we will review comments and make a final determination:</p> |
| <ul> |
| <li>Convert to error: the change should be made into a hard error.</li> |
| <li>Revert: we should remove the warning and continue to allow the older code to |
| compile.</li> |
| <li>Defer: can’t decide yet, wait longer, or try other strategies.</li> |
| </ul> |
| </li> |
| </ul> |
| <p>Ideally, breaking changes should have landed on the <strong>stable branch</strong> of the |
| compiler before they are finalized.</p> |
| <p><a id="guide"></a></p> |
| <h3 id="removing-a-lint"><a class="header" href="#removing-a-lint">Removing a lint</a></h3> |
| <p>Once we have decided to make a “future warning” into a hard error, we need a PR |
| that removes the custom lint. As an example, here are the steps required to |
| remove the <code>overlapping_inherent_impls</code> compatibility lint. First, convert the |
| name of the lint to uppercase (<code>OVERLAPPING_INHERENT_IMPLS</code>) ripgrep through the |
| source for that string. We will basically by converting each place where this |
| lint name is mentioned (in the compiler, we use the upper-case name, and a macro |
| automatically generates the lower-case string; so searching for |
| <code>overlapping_inherent_impls</code> would not find much).</p> |
| <blockquote> |
| <p>NOTE: these exact files don’t exist anymore, but the procedure is still the same.</p> |
| </blockquote> |
| <h4 id="remove-the-lint"><a class="header" href="#remove-the-lint">Remove the lint.</a></h4> |
| <p>The first reference you will likely find is the lint definition <a href="https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc/lint/builtin.rs#L171-L175">in |
| <code>rustc_session/src/lint/builtin.rs</code> that resembles this</a>:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>declare_lint! { |
| pub OVERLAPPING_INHERENT_IMPLS, |
| Deny, // this may also say Warning |
| "two overlapping inherent impls define an item with the same name were erroneously allowed", |
| @future_incompatible = FutureIncompatibleInfo { |
| reason: fcw!(FutureReleaseError #1234), // your tracking issue here! |
| }, |
| } |
| <span class="boring">}</span></code></pre> |
| <p>This <code>declare_lint!</code> macro creates the relevant data structures. Remove it. You |
| will also find that there is a mention of <code>OVERLAPPING_INHERENT_IMPLS</code> later in |
| the file as <a href="https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc/lint/builtin.rs#L252-L290">part of a <code>lint_array!</code></a>; remove it too.</p> |
| <h4 id="add-the-lint-to-the-list-of-removed-lints"><a class="header" href="#add-the-lint-to-the-list-of-removed-lints">Add the lint to the list of removed lints.</a></h4> |
| <p>In <code>compiler/rustc_lint/src/lib.rs</code> there is a list of “renamed and removed lints”. |
| You can add this lint to the list:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>store.register_removed("overlapping_inherent_impls", "converted into hard error, see #36889"); |
| <span class="boring">}</span></code></pre> |
| <p>where <code>#36889</code> is the tracking issue for your lint.</p> |
| <h4 id="update-the-places-that-issue-the-lint"><a class="header" href="#update-the-places-that-issue-the-lint">Update the places that issue the lint</a></h4> |
| <p>Finally, the last class of references you will see are the places that actually |
| <strong>trigger</strong> the lint itself (i.e., what causes the warnings to appear). These |
| you do not want to delete. Instead, you want to convert them into errors. In |
| this case, the <a href="https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc_typeck/coherence/inherent.rs#L300-L303"><code>add_lint</code> call</a> looks like this:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>self.tcx.sess.add_lint(lint::builtin::OVERLAPPING_INHERENT_IMPLS, |
| node_id, |
| self.tcx.span_of_impl(item1).unwrap(), |
| msg); |
| <span class="boring">}</span></code></pre> |
| <p>You’ll also often find <code>node_span_lint</code> used for this.</p> |
| <p>We want to convert this into an error. In some cases, there may be an |
| existing error for this scenario. In others, we will need to allocate a |
| fresh diagnostic code. <a href="./diagnostics/error-codes.html">Instructions for allocating a fresh diagnostic |
| code can be found here.</a> You may want |
| to mention in the extended description that the compiler behavior |
| changed on this point, and include a reference to the tracking issue for |
| the change.</p> |
| <p>Let’s say that we’ve adopted <code>E0592</code> as our code. Then we can change the |
| <code>add_lint()</code> call above to something like:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>struct_span_code_err!(self.dcx(), self.tcx.span_of_impl(item1).unwrap(), E0592, msg) |
| .emit(); |
| <span class="boring">}</span></code></pre> |
| <p>Or better: a structured diagnostic like this:</p> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span><span class="boring">fn main() { |
| </span>#[derive(Diagnostic)] |
| struct MyDiagnostic { |
| #[label] |
| span: Span, |
| ... |
| } |
| <span class="boring">}</span></code></pre> |
| <h4 id="update-tests"><a class="header" href="#update-tests">Update tests</a></h4> |
| <p>Finally, run the test suite. These should be some tests that used to reference |
| the <code>overlapping_inherent_impls</code> lint, those will need to be updated. In |
| general, if the test used to have <code>#[deny(overlapping_inherent_impls)]</code>, that |
| can just be removed.</p> |
| <pre><code>./x test |
| </code></pre> |
| <h4 id="all-done"><a class="header" href="#all-done">All done!</a></h4> |
| <p>Open a PR. =)</p> |
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