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| <main> |
| <h1 id="using-tracing-to-debug-the-compiler"><a class="header" href="#using-tracing-to-debug-the-compiler">Using tracing to debug the compiler</a></h1> |
| <p>The compiler has a lot of <a href="https://docs.rs/tracing/0.1/tracing/macro.debug.html"><code>debug!</code></a> (or <code>trace!</code>) calls, which print out logging information |
| at many points. These are very useful to at least narrow down the location of |
| a bug if not to find it entirely, or just to orient yourself as to why the |
| compiler is doing a particular thing.</p> |
| <p>To see the logs, you need to set the <code>RUSTC_LOG</code> environment variable to your |
| log filter. The full syntax of the log filters can be found in the <a href="https://docs.rs/tracing-subscriber/0.2.24/tracing_subscriber/filter/struct.EnvFilter.html#directives">rustdoc |
| of <code>tracing-subscriber</code></a>.</p> |
| <h2 id="function-level-filters"><a class="header" href="#function-level-filters">Function level filters</a></h2> |
| <p>Lots of functions in rustc are annotated with</p> |
| <pre><code>#[instrument(level = "debug", skip(self))] |
| fn foo(&self, bar: Type) {} |
| </code></pre> |
| <p>which allows you to use</p> |
| <pre><code>RUSTC_LOG=[foo] |
| </code></pre> |
| <p>to do the following all at once</p> |
| <ul> |
| <li>log all function calls to <code>foo</code></li> |
| <li>log the arguments (except for those in the <code>skip</code> list)</li> |
| <li>log everything (from anywhere else in the compiler) until the function returns</li> |
| </ul> |
| <h3 id="i-dont-want-everything"><a class="header" href="#i-dont-want-everything">I don’t want everything</a></h3> |
| <p>Depending on the scope of the function, you may not want to log everything in its body. |
| As an example: the <code>do_mir_borrowck</code> function will dump hundreds of lines even for trivial |
| code being borrowchecked.</p> |
| <p>Since you can combine all filters, you can add a crate/module path, e.g.</p> |
| <pre><code>RUSTC_LOG=rustc_borrowck[do_mir_borrowck] |
| </code></pre> |
| <h3 id="i-dont-want-all-calls"><a class="header" href="#i-dont-want-all-calls">I don’t want all calls</a></h3> |
| <p>If you are compiling libcore, you likely don’t want <em>all</em> borrowck dumps, but only one |
| for a specific function. You can filter function calls by their arguments by regexing them.</p> |
| <pre><code>RUSTC_LOG=[do_mir_borrowck{id=\.\*from_utf8_unchecked\.\*}] |
| </code></pre> |
| <p>will only give you the logs of borrowchecking <code>from_utf8_unchecked</code>. Note that you will |
| still get a short message per ignored <code>do_mir_borrowck</code>, but none of the things inside those |
| calls. This helps you in looking through the calls that are happening and helps you adjust |
| your regex if you mistyped it.</p> |
| <h2 id="query-level-filters"><a class="header" href="#query-level-filters">Query level filters</a></h2> |
| <p>Every <a href="query.html">query</a> is automatically tagged with a logging span so that |
| you can display all log messages during the execution of the query. For |
| example, if you want to log everything during type checking:</p> |
| <pre><code>RUSTC_LOG=[typeck] |
| </code></pre> |
| <p>The query arguments are included as a tracing field which means that you can |
| filter on the debug display of the arguments. For example, the <code>typeck</code> query |
| has an argument <code>key: LocalDefId</code> of what is being checked. You can use a |
| regex to match on that <code>LocalDefId</code> to log type checking for a specific |
| function:</p> |
| <pre><code>RUSTC_LOG=[typeck{key=.*name_of_item.*}] |
| </code></pre> |
| <p>Different queries have different arguments. You can find a list of queries and |
| their arguments in |
| <a href="https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_middle/src/query/mod.rs#L18"><code>rustc_middle/src/query/mod.rs</code></a>.</p> |
| <h2 id="broad-module-level-filters"><a class="header" href="#broad-module-level-filters">Broad module level filters</a></h2> |
| <p>You can also use filters similar to the <code>log</code> crate’s filters, which will enable |
| everything within a specific module. This is often too verbose and too unstructured, |
| so it is recommended to use function level filters.</p> |
| <p>Your log filter can be just <code>debug</code> to get all <code>debug!</code> output and |
| higher (e.g., it will also include <code>info!</code>), or <code>path::to::module</code> to get <em>all</em> |
| output (which will include <code>trace!</code>) from a particular module, or |
| <code>path::to::module=debug</code> to get <code>debug!</code> output and higher from a particular |
| module.</p> |
| <p>For example, to get the <code>debug!</code> output and higher for a specific module, you |
| can run the compiler with <code>RUSTC_LOG=path::to::module=debug rustc my-file.rs</code>. |
| All <code>debug!</code> output will then appear in standard error.</p> |
| <p>Note that you can use a partial path and the filter will still work. For |
| example, if you want to see <code>info!</code> output from only |
| <code>rustdoc::passes::collect_intra_doc_links</code>, you could use |
| <code>RUSTDOC_LOG=rustdoc::passes::collect_intra_doc_links=info</code> <em>or</em> you could use |
| <code>RUSTDOC_LOG=rustdoc::passes::collect_intra=info</code>.</p> |
| <p>If you are developing rustdoc, use <code>RUSTDOC_LOG</code> instead. If you are developing |
| Miri, use <code>MIRI_LOG</code> instead. You get the idea :)</p> |
| <p>See the <a href="https://docs.rs/tracing"><code>tracing</code></a> crate’s docs, and specifically the docs for <a href="https://docs.rs/tracing/0.1/tracing/macro.debug.html"><code>debug!</code></a> to |
| see the full syntax you can use. (Note: unlike the compiler, the <a href="https://docs.rs/tracing"><code>tracing</code></a> |
| crate and its examples use the <code>RUSTC_LOG</code> environment variable. rustc, rustdoc, |
| and other tools set custom environment variables.)</p> |
| <p><strong>Note that unless you use a very strict filter, the logger will emit a lot of |
| output, so use the most specific module(s) you can (comma-separated if |
| multiple)</strong>. It’s typically a good idea to pipe standard error to a file and |
| look at the log output with a text editor.</p> |
| <p>So, to put it together:</p> |
| <pre><code class="language-bash"># This puts the output of all debug calls in `rustc_middle/src/traits` into |
| # standard error, which might fill your console backscroll. |
| $ RUSTC_LOG=rustc_middle::traits=debug rustc +stage1 my-file.rs |
| |
| # This puts the output of all debug calls in `rustc_middle/src/traits` in |
| # `traits-log`, so you can then see it with a text editor. |
| $ RUSTC_LOG=rustc_middle::traits=debug rustc +stage1 my-file.rs 2>traits-log |
| |
| # Not recommended! This will show the output of all `debug!` calls |
| # in the Rust compiler, and there are a *lot* of them, so it will be |
| # hard to find anything. |
| $ RUSTC_LOG=debug rustc +stage1 my-file.rs 2>all-log |
| |
| # This will show the output of all `info!` calls in `rustc_codegen_ssa`. |
| # |
| # There's an `info!` statement in `codegen_instance` that outputs |
| # every function that is codegen'd. This is useful to find out |
| # which function triggers an LLVM assertion, and this is an `info!` |
| # log rather than a `debug!` log so it will work on the official |
| # compilers. |
| $ RUSTC_LOG=rustc_codegen_ssa=info rustc +stage1 my-file.rs |
| |
| # This will show all logs in `rustc_codegen_ssa` and `rustc_resolve`. |
| $ RUSTC_LOG=rustc_codegen_ssa,rustc_resolve rustc +stage1 my-file.rs |
| |
| # This will show the output of all `info!` calls made by rustdoc |
| # or any rustc library it calls. |
| $ RUSTDOC_LOG=info rustdoc +stage1 my-file.rs |
| |
| # This will only show `debug!` calls made by rustdoc directly, |
| # not any `rustc*` crate. |
| $ RUSTDOC_LOG=rustdoc=debug rustdoc +stage1 my-file.rs |
| </code></pre> |
| <h2 id="log-colors"><a class="header" href="#log-colors">Log colors</a></h2> |
| <p>By default, rustc (and other tools, like rustdoc and Miri) will be smart about |
| when to use ANSI colors in the log output. If they are outputting to a terminal, |
| they will use colors, and if they are outputting to a file or being piped |
| somewhere else, they will not. However, it’s hard to read log output in your |
| terminal unless you have a very strict filter, so you may want to pipe the |
| output to a pager like <code>less</code>. But then there won’t be any colors, which makes |
| it hard to pick out what you’re looking for!</p> |
| <p>You can override whether to have colors in log output with the <code>RUSTC_LOG_COLOR</code> |
| environment variable (or <code>RUSTDOC_LOG_COLOR</code> for rustdoc, or <code>MIRI_LOG_COLOR</code> |
| for Miri, etc.). There are three options: <code>auto</code> (the default), <code>always</code>, and |
| <code>never</code>. So, if you want to enable colors when piping to <code>less</code>, use something |
| similar to this command:</p> |
| <pre><code class="language-bash"># The `-R` switch tells less to print ANSI colors without escaping them. |
| $ RUSTC_LOG=debug RUSTC_LOG_COLOR=always rustc +stage1 ... | less -R |
| </code></pre> |
| <p>Note that <code>MIRI_LOG_COLOR</code> will only color logs that come from Miri, not logs |
| from rustc functions that Miri calls. Use <code>RUSTC_LOG_COLOR</code> to color logs from |
| rustc.</p> |
| <h2 id="how-to-keep-or-remove-debug-and-trace-calls-from-the-resulting-binary"><a class="header" href="#how-to-keep-or-remove-debug-and-trace-calls-from-the-resulting-binary">How to keep or remove <code>debug!</code> and <code>trace!</code> calls from the resulting binary</a></h2> |
| <p>While calls to <code>error!</code>, <code>warn!</code> and <code>info!</code> are included in every build of the compiler, |
| calls to <code>debug!</code> and <code>trace!</code> are only included in the program if |
| <code>debug-logging=true</code> is turned on in bootstrap.toml (it is |
| turned off by default), so if you don’t see <code>DEBUG</code> logs, especially |
| if you run the compiler with <code>RUSTC_LOG=rustc rustc some.rs</code> and only see |
| <code>INFO</code> logs, make sure that <code>debug-logging=true</code> is turned on in your |
| bootstrap.toml.</p> |
| <h2 id="logging-etiquette-and-conventions"><a class="header" href="#logging-etiquette-and-conventions">Logging etiquette and conventions</a></h2> |
| <p>Because calls to <code>debug!</code> are removed by default, in most cases, don’t worry |
| about the performance of adding “unnecessary” calls to <code>debug!</code> and leaving them in code you |
| commit - they won’t slow down the performance of what we ship.</p> |
| <p>That said, there can also be excessive tracing calls, especially |
| when they are redundant with other calls nearby or in functions called from |
| here. There is no perfect balance to hit here, and is left to the reviewer’s |
| discretion to decide whether to let you leave <code>debug!</code> statements in or whether to ask |
| you to remove them before merging.</p> |
| <p>It may be preferable to use <code>trace!</code> over <code>debug!</code> for very noisy logs.</p> |
| <p>A loosely followed convention is to use <code>#[instrument(level = "debug")]</code> |
| (<a href="https://docs.rs/tracing-attributes/0.1.17/tracing_attributes/attr.instrument.html">also see the attribute’s documentation</a>) |
| in favour of <code>debug!("foo(...)")</code> at the start of a function <code>foo</code>. |
| Within functions, prefer <code>debug!(?variable.field)</code> over <code>debug!("xyz = {:?}", variable.field)</code> |
| and <code>debug!(bar = ?var.method(arg))</code> over <code>debug!("bar = {:?}", var.method(arg))</code>. |
| The documentation for this syntax can be found <a href="https://docs.rs/tracing/0.1.28/tracing/#recording-fields">here</a>.</p> |
| <p>One thing to be <strong>careful</strong> of is <strong>expensive</strong> operations in logs.</p> |
| <p>If in the module <code>rustc::foo</code> you have a statement</p> |
| <pre><code class="language-Rust">debug!(x = ?random_operation(tcx)); |
| </code></pre> |
| <p>Then if someone runs a debug <code>rustc</code> with <code>RUSTC_LOG=rustc::foo</code>, then |
| <code>random_operation()</code> will run. <code>RUSTC_LOG</code> filters that do not enable this |
| debug statement will not execute <code>random_operation</code>.</p> |
| <p>This means that you should not put anything too expensive or likely to crash |
| there - that would annoy anyone who wants to use logging for that module. |
| No-one will know it until someone tries to use logging to find <em>another</em> bug.</p> |
| |
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