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<main>
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<h1 id="error-codes"><a class="header" href="#error-codes">Error codes</a></h1>
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<p>We generally try to assign each error message a unique code like <code>E0123</code>. These
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codes are defined in the compiler in the <code>diagnostics.rs</code> files found in each
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crate, which basically consist of macros. All error codes have an associated
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explanation: new error codes must include them. Note that not all <em>historical</em>
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(no longer emitted) error codes have explanations.</p>
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<h2 id="error-explanations"><a class="header" href="#error-explanations">Error explanations</a></h2>
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<p>The explanations are written in Markdown (see the <a href="https://spec.commonmark.org/current/">CommonMark Spec</a> for
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specifics around syntax), and all of them are linked in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_error_codes/index.html"><code>rustc_error_codes</code></a>
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crate. Please read <a href="https://github.com/rust-lang/rfcs/blob/master/text/1567-long-error-codes-explanation-normalization.md">RFC 1567</a> for details on how to format and write long error
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codes. As of <!-- date-check --> February 2023, there is an
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effort<sup class="footnote-reference" id="fr-new-explanations-1"><a href="#footnote-new-explanations">1</a></sup> to replace this largely outdated RFC with a new more
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flexible standard.</p>
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<p>Error explanations should expand on the error message and provide details about
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<em>why</em> the error occurs. It is not helpful for users to copy-paste a quick fix;
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explanations should help users understand why their code cannot be accepted by
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the compiler. Rust prides itself on helpful error messages and long-form
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explanations are no exception. However, before error explanations are
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overhauled<sup class="footnote-reference" id="fr-new-explanations-2"><a href="#footnote-new-explanations">1</a></sup> it is a bit open as to how exactly they should be
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written, as always: ask your reviewer or ask around on the Rust Discord or Zulip.</p>
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<h2 id="allocating-a-fresh-code"><a class="header" href="#allocating-a-fresh-code">Allocating a fresh code</a></h2>
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<p>Error codes are stored in <code>compiler/rustc_error_codes</code>.</p>
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<p>To create a new error, you first need to find the next available
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code. You can find it with <code>tidy</code>:</p>
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<pre><code>./x test tidy
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</code></pre>
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<p>This will invoke the tidy script, which generally checks that your code obeys
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our coding conventions. Some of these jobs check error codes and ensure that
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there aren't duplicates, etc (the tidy check is defined in
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<code>src/tools/tidy/src/error_codes.rs</code>). Once it is finished with that, tidy will
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print out the highest used error code:</p>
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<pre><code>...
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tidy check
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Found 505 error codes
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Highest error code: `E0591`
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...
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</code></pre>
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<p>Here we see the highest error code in use is <code>E0591</code>, so we <em>probably</em> want
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<code>E0592</code>. To be sure, run <code>rg E0592</code> and check, you should see no references.</p>
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<p>You will have to write an extended description for your error,
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which will go in <code>rustc_error_codes/src/error_codes/E0592.md</code>.
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To register the error, open <code>rustc_error_codes/src/error_codes.rs</code> and add the
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code (in its proper numerical order) into<code> register_diagnostics!</code> macro, like
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this:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>register_diagnostics! {
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...
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E0592: include_str!("./error_codes/E0592.md"),
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}
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<span class="boring">}</span></code></pre></pre>
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<p>To actually issue the error, you can use the <code>struct_span_code_err!</code> macro:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>struct_span_code_err!(self.dcx(), // some path to the `DiagCtxt` here
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span, // whatever span in the source you want
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E0592, // your new error code
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fluent::example::an_error_message)
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.emit() // actually issue the error
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<span class="boring">}</span></code></pre></pre>
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<p>If you want to add notes or other snippets, you can invoke methods before you
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call <code>.emit()</code>:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>struct_span_code_err!(...)
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.span_label(another_span, fluent::example::example_label)
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.span_note(another_span, fluent::example::separate_note)
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.emit()
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<span class="boring">}</span></code></pre></pre>
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<p>For an example of a PR adding an error code, see <a href="https://github.com/rust-lang/rust/pull/76143">#76143</a>.</p>
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<h2 id="running-error-code-doctests"><a class="header" href="#running-error-code-doctests">Running error code doctests</a></h2>
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<p>To test the examples added in <code>rustc_error_codes/src/error_codes</code>, run the
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error index generator using:</p>
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<pre><code>./x test ./src/tools/error_index_generator
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</code></pre>
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<hr>
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<ol class="footnote-definition"><li id="footnote-new-explanations">
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<p>See the draft RFC <a href="https://github.com/rust-lang/rfcs/pull/3370">here</a>. <a href="#fr-new-explanations-1">↩</a> <a href="#fr-new-explanations-2">↩2</a></p>
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