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<h1 class="menu-title">Rust Compiler Development Guide</h1>
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<h1 id="debugging-and-testing-dependencies"><a class="header" href="#debugging-and-testing-dependencies">Debugging and testing dependencies</a></h1>
<h2 id="testing-the-dependency-graph"><a class="header" href="#testing-the-dependency-graph">Testing the dependency graph</a></h2>
<p>There are various ways to write tests against the dependency graph. The
simplest mechanisms are the <code>#[rustc_if_this_changed]</code> and
<code>#[rustc_then_this_would_need]</code> annotations. These are used in <a href="tests/ui.html">ui</a> tests to test
whether the expected set of paths exist in the dependency graph.</p>
<p>As an example, see <a href="https://github.com/rust-lang/rust/blob/master/tests/ui/dep-graph/dep-graph-caller-callee.rs"><code>tests/ui/dep-graph/dep-graph-caller-callee.rs</code></a>, or the
tests below.</p>
<pre><code class="language-rust ignore">#[rustc_if_this_changed]
fn foo() { }
#[rustc_then_this_would_need(TypeckTables)] //~ ERROR OK
fn bar() { foo(); }</code></pre>
<p>This should be read as</p>
<blockquote>
<p>If this (<code>foo</code>) is changed, then this (i.e. <code>bar</code>)'s TypeckTables would need to be changed.</p>
</blockquote>
<p>Technically, what occurs is that the test is expected to emit the string "OK" on
stderr, associated to this line.</p>
<p>You could also add the lines</p>
<pre><code class="language-rust ignore">#[rustc_then_this_would_need(TypeckTables)] //~ ERROR no path
fn baz() { }</code></pre>
<p>Whose meaning is</p>
<blockquote>
<p>If <code>foo</code> is changed, then <code>baz</code>'s TypeckTables does not need to be changed.
The macro must emit an error, and the error message must contains "no path".</p>
</blockquote>
<p>Recall that the <code>//~ ERROR OK</code> is a comment from the point of view of the Rust
code we test, but is meaningful from the point of view of the test itself.</p>
<h2 id="debugging-the-dependency-graph"><a class="header" href="#debugging-the-dependency-graph">Debugging the dependency graph</a></h2>
<h3 id="dumping-the-graph"><a class="header" href="#dumping-the-graph">Dumping the graph</a></h3>
<p>The compiler is also capable of dumping the dependency graph for your
debugging pleasure. To do so, pass the <code>-Z dump-dep-graph</code> flag. The
graph will be dumped to <code>dep_graph.{txt,dot}</code> in the current
directory. You can override the filename with the <code>RUST_DEP_GRAPH</code>
environment variable.</p>
<p>Frequently, though, the full dep graph is quite overwhelming and not
particularly helpful. Therefore, the compiler also allows you to filter
the graph. You can filter in three ways:</p>
<ol>
<li>All edges originating in a particular set of nodes (usually a single node).</li>
<li>All edges reaching a particular set of nodes.</li>
<li>All edges that lie between given start and end nodes.</li>
</ol>
<p>To filter, use the <code>RUST_DEP_GRAPH_FILTER</code> environment variable, which should
look like one of the following:</p>
<pre><code class="language-text">source_filter // nodes originating from source_filter
-&gt; target_filter // nodes that can reach target_filter
source_filter -&gt; target_filter // nodes in between source_filter and target_filter
</code></pre>
<p><code>source_filter</code> and <code>target_filter</code> are a <code>&amp;</code>-separated list of strings.
A node is considered to match a filter if all of those strings appear in its
label. So, for example:</p>
<pre><code class="language-text">RUST_DEP_GRAPH_FILTER='-&gt; TypeckTables'
</code></pre>
<p>would select the predecessors of all <code>TypeckTables</code> nodes. Usually though you
want the <code>TypeckTables</code> node for some particular fn, so you might write:</p>
<pre><code class="language-text">RUST_DEP_GRAPH_FILTER='-&gt; TypeckTables &amp; bar'
</code></pre>
<p>This will select only the predecessors of <code>TypeckTables</code> nodes for functions
with <code>bar</code> in their name.</p>
<p>Perhaps you are finding that when you change <code>foo</code> you need to re-type-check
<code>bar</code>, but you don't think you should have to. In that case, you might do:</p>
<pre><code class="language-text">RUST_DEP_GRAPH_FILTER='Hir &amp; foo -&gt; TypeckTables &amp; bar'
</code></pre>
<p>This will dump out all the nodes that lead from <code>Hir(foo)</code> to
<code>TypeckTables(bar)</code>, from which you can (hopefully) see the source
of the erroneous edge.</p>
<h3 id="tracking-down-incorrect-edges"><a class="header" href="#tracking-down-incorrect-edges">Tracking down incorrect edges</a></h3>
<p>Sometimes, after you dump the dependency graph, you will find some
path that should not exist, but you will not be quite sure how it came
to be. <strong>When the compiler is built with debug assertions,</strong> it can
help you track that down. Simply set the <code>RUST_FORBID_DEP_GRAPH_EDGE</code>
environment variable to a filter. Every edge created in the dep-graph
will be tested against that filter – if it matches, a <code>bug!</code> is
reported, so you can easily see the backtrace (<code>RUST_BACKTRACE=1</code>).</p>
<p>The syntax for these filters is the same as described in the previous
section. However, note that this filter is applied to every <strong>edge</strong>
and doesn't handle longer paths in the graph, unlike the previous
section.</p>
<p>Example:</p>
<p>You find that there is a path from the <code>Hir</code> of <code>foo</code> to the type
check of <code>bar</code> and you don't think there should be. You dump the
dep-graph as described in the previous section and open <code>dep-graph.txt</code>
to see something like:</p>
<pre><code class="language-text">Hir(foo) -&gt; Collect(bar)
Collect(bar) -&gt; TypeckTables(bar)
</code></pre>
<p>That first edge looks suspicious to you. So you set
<code>RUST_FORBID_DEP_GRAPH_EDGE</code> to <code>Hir&amp;foo -&gt; Collect&amp;bar</code>, re-run, and
then observe the backtrace. Voila, bug fixed!</p>
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