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<h1>
Lightweight Linting
</h1>
<div class="post-text">
<p><a
href="https://tree-sitter.github.io/tree-sitter/using-parsers#pattern-matching-with-queries">Tree-sitter</a>
queries allow you to search for patterns in syntax trees, much like a
regex would, in text. Combine that with some Rust glue to write simple,
custom linters.</p>
<h3 id="tree-sitter-syntax-trees">Tree-sitter syntax trees</h3>
<p>Here is a quick crash course on syntax trees generated by
tree-sitter. Syntax trees produced by tree-sitter are represented by
S-expressions. The generated S-expression for the following Rust
code,</p>
<div class="sourceCode" id="cb1"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">fn</span> main() <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> x <span class="op">=</span> <span class="dv">2</span><span class="op">;</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>would be:</p>
<div class="sourceCode" id="cb2"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a>(source_file</span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a> (function_item</span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a> name: (identifier)</span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a> parameters: (parameters)</span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a> body: </span>
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a> (block</span>
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a> (let_declaration </span>
<span id="cb2-8"><a href="#cb2-8" aria-hidden="true" tabindex="-1"></a> pattern: (identifier)</span>
<span id="cb2-9"><a href="#cb2-9" aria-hidden="true" tabindex="-1"></a> value: (integer_literal)))))</span></code></pre></div>
<p>Syntax trees generated by tree-sitter have a couple of other cool
properties: they are <em>lossless</em> syntax trees. Given a lossless
syntax tree, you can regenerate the original source code in its
entirety. Consider the following addition to our example:</p>
<div class="sourceCode" id="cb3"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a> <span class="kw">fn</span> main() <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="op">+</span> <span class="co">// a comment goes here</span></span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> x <span class="op">=</span> <span class="dv">2</span><span class="op">;</span></span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a> <span class="op">}</span></span></code></pre></div>
<p>The tree-sitter syntax tree preserves the comment, while the typical
abstract syntax tree wouldn’t:</p>
<div class="sourceCode" id="cb4"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a> (source_file</span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a> (function_item</span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a> name: (identifier)</span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a> parameters: (parameters)</span>
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a> body:</span>
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a> (block</span>
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a><span class="op">+</span> (line_comment)</span>
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a> (let_declaration</span>
<span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a> pattern: (identifier)</span>
<span id="cb4-10"><a href="#cb4-10" aria-hidden="true" tabindex="-1"></a> value: (integer_literal)))))</span></code></pre></div>
<h3 id="tree-sitter-queries">Tree-sitter queries</h3>
<p>Tree-sitter provides a DSL to match over CSTs. These queries resemble
our S-expression syntax trees, here is a query to match all line
comments in a Rust CST:</p>
<div class="sourceCode" id="cb5"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>(line_comment)</span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a><span class="co">; matches the following rust code</span></span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a><span class="co">; // a comment goes here</span></span></code></pre></div>
<p>Neat, eh? But don’t take my word for it, give it a go on the <a
href="https://tree-sitter.github.io/tree-sitter/playground">tree-sitter
playground</a>. Type in a query like so:</p>
<div class="sourceCode" id="cb6"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="co">; the web playground requires you to specify a "capture"</span></span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a><span class="co">; you will notice the capture and the nodes it captured</span></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a><span class="co">; turn blue</span></span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a>(line_comment) @capture</span></code></pre></div>
<p>Here’s another to match <code>let</code> expressions that bind an
integer to an identifier:</p>
<div class="sourceCode" id="cb7"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a>(let_declaration</span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a> pattern: (identifier)</span>
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a> value: (integer_literal))</span>
<span id="cb7-4"><a href="#cb7-4" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb7-5"><a href="#cb7-5" aria-hidden="true" tabindex="-1"></a><span class="co">; matches:</span></span>
<span id="cb7-6"><a href="#cb7-6" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = 2;</span></span></code></pre></div>
<p>We can <em>capture</em> nodes into variables:</p>
<div class="sourceCode" id="cb8"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a>(let_declaration </span>
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a> pattern: (identifier) @my-capture</span>
<span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a> value: (integer_literal))</span>
<span id="cb8-4"><a href="#cb8-4" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb8-5"><a href="#cb8-5" aria-hidden="true" tabindex="-1"></a><span class="co">; matches:</span></span>
<span id="cb8-6"><a href="#cb8-6" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = 2;</span></span>
<span id="cb8-7"><a href="#cb8-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb8-8"><a href="#cb8-8" aria-hidden="true" tabindex="-1"></a><span class="co">; captures:</span></span>
<span id="cb8-9"><a href="#cb8-9" aria-hidden="true" tabindex="-1"></a><span class="co">; foo</span></span></code></pre></div>
<p>And apply certain <em>predicates</em> to captures:</p>
<div class="sourceCode" id="cb9"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a>((let_declaration</span>
<span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a> pattern: (identifier) @my-capture</span>
<span id="cb9-3"><a href="#cb9-3" aria-hidden="true" tabindex="-1"></a> value: (integer_literal))</span>
<span id="cb9-4"><a href="#cb9-4" aria-hidden="true" tabindex="-1"></a> (<span class="sc">#e</span>q? @my-capture <span class="st">"foo"</span>))</span>
<span id="cb9-5"><a href="#cb9-5" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb9-6"><a href="#cb9-6" aria-hidden="true" tabindex="-1"></a><span class="co">; matches:</span></span>
<span id="cb9-7"><a href="#cb9-7" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = 2;</span></span>
<span id="cb9-8"><a href="#cb9-8" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb9-9"><a href="#cb9-9" aria-hidden="true" tabindex="-1"></a><span class="co">; and not:</span></span>
<span id="cb9-10"><a href="#cb9-10" aria-hidden="true" tabindex="-1"></a><span class="co">; let bar = 2;</span></span></code></pre></div>
<p>The <code>#match?</code> predicate checks if a capture matches a
regex:</p>
<div class="sourceCode" id="cb10"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a>((let_declaration</span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a> pattern: (identifier) @my-capture</span>
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a> value: (integer_literal))</span>
<span id="cb10-4"><a href="#cb10-4" aria-hidden="true" tabindex="-1"></a> (#match? @my-capture <span class="st">"foo|bar"</span>))</span>
<span id="cb10-5"><a href="#cb10-5" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb10-6"><a href="#cb10-6" aria-hidden="true" tabindex="-1"></a><span class="co">; matches both `foo` and `bar`:</span></span>
<span id="cb10-7"><a href="#cb10-7" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = 2;</span></span>
<span id="cb10-8"><a href="#cb10-8" aria-hidden="true" tabindex="-1"></a><span class="co">; let bar = 2;</span></span></code></pre></div>
<p>Exhibit indifference, as a stoic programmer would, with the
<em>wildcard</em> pattern:</p>
<div class="sourceCode" id="cb11"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a>(let_declaration</span>
<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a> pattern: (identifier)</span>
<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a> value: (<span class="op">_</span>))</span>
<span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a> </span>
<span id="cb11-5"><a href="#cb11-5" aria-hidden="true" tabindex="-1"></a><span class="co">; matches:</span></span>
<span id="cb11-6"><a href="#cb11-6" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = "foo";</span></span>
<span id="cb11-7"><a href="#cb11-7" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = 42;</span></span>
<span id="cb11-8"><a href="#cb11-8" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = bar;</span></span></code></pre></div>
<p><a
href="https://tree-sitter.github.io/tree-sitter/using-parsers#pattern-matching-with-queries">The
documentation</a> does the tree-sitter query DSL more justice, but we
now know enough to write our first lint.</p>
<h3 id="write-you-a-tree-sitter-lint">Write you a tree-sitter lint</h3>
<p>Strings in <code>std::env</code> functions are error prone:</p>
<div class="sourceCode" id="cb12"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb12-1"><a href="#cb12-1" aria-hidden="true" tabindex="-1"></a><span class="pp">std::env::</span>remove_var(<span class="st">"RUST_BACKTACE"</span>)<span class="op">;</span></span>
<span id="cb12-2"><a href="#cb12-2" aria-hidden="true" tabindex="-1"></a> <span class="co">// ^^^^ "TACE" instead of "TRACE"</span></span></code></pre></div>
<p>I prefer this instead:</p>
<div class="sourceCode" id="cb13"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb13-1"><a href="#cb13-1" aria-hidden="true" tabindex="-1"></a><span class="co">// somewhere in a module that is well spellchecked</span></span>
<span id="cb13-2"><a href="#cb13-2" aria-hidden="true" tabindex="-1"></a><span class="kw">static</span> BACKTRACE<span class="op">:</span> <span class="op">&</span><span class="dt">str</span> <span class="op">=</span> <span class="st">"RUST_BACKTRACE"</span><span class="op">;</span></span>
<span id="cb13-3"><a href="#cb13-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb13-4"><a href="#cb13-4" aria-hidden="true" tabindex="-1"></a><span class="co">// rest of the codebase</span></span>
<span id="cb13-5"><a href="#cb13-5" aria-hidden="true" tabindex="-1"></a><span class="pp">std::env::</span>remove_var(BACKTRACE)<span class="op">;</span></span></code></pre></div>
<p>Let’s write a lint to find <code>std::env</code> functions that use
strings. Put aside the effectiveness of this lint for the moment, and
take a stab at writing a tree-sitter query. For reference, a function
call like so:</p>
<div class="sourceCode" id="cb14"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb14-1"><a href="#cb14-1" aria-hidden="true" tabindex="-1"></a>remove_var(<span class="st">"RUST_BACKTRACE"</span>)</span></code></pre></div>
<p>Produces the following S-expression:</p>
<div class="sourceCode" id="cb15"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb15-1"><a href="#cb15-1" aria-hidden="true" tabindex="-1"></a>(call_expression</span>
<span id="cb15-2"><a href="#cb15-2" aria-hidden="true" tabindex="-1"></a> function: (identifier)</span>
<span id="cb15-3"><a href="#cb15-3" aria-hidden="true" tabindex="-1"></a> arguments: (arguments (string_literal)))</span></code></pre></div>
<p>We are definitely looking for a <code>call_expression</code>:</p>
<div class="sourceCode" id="cb16"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb16-1"><a href="#cb16-1" aria-hidden="true" tabindex="-1"></a>(call_expression) @raise</span></code></pre></div>
<p>Whose function name matches <code>std::env::var</code> or
<code>std::env::remove_var</code> at the very least (I know, I know,
this isn’t the most optimal regex):</p>
<div class="sourceCode" id="cb17"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb17-1"><a href="#cb17-1" aria-hidden="true" tabindex="-1"></a>((call_expression</span>
<span id="cb17-2"><a href="#cb17-2" aria-hidden="true" tabindex="-1"></a> function: (<span class="op">_</span>) @fn-name) @raise</span>
<span id="cb17-3"><a href="#cb17-3" aria-hidden="true" tabindex="-1"></a> (#match? @fn-name <span class="st">"std::env::(var|remove_var)"</span>))</span></code></pre></div>
<p>Let’s turn that <code>std::</code> prefix optional:</p>
<div class="sourceCode" id="cb18"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb18-1"><a href="#cb18-1" aria-hidden="true" tabindex="-1"></a>((call_expression</span>
<span id="cb18-2"><a href="#cb18-2" aria-hidden="true" tabindex="-1"></a> function: (<span class="op">_</span>) @fn-name) @raise</span>
<span id="cb18-3"><a href="#cb18-3" aria-hidden="true" tabindex="-1"></a> (#match? @fn-name <span class="st">"(std::|)env::(var|remove_var)"</span>))</span></code></pre></div>
<p>And ensure that <code>arguments</code> is a string:</p>
<div class="sourceCode" id="cb19"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb19-1"><a href="#cb19-1" aria-hidden="true" tabindex="-1"></a>((call_expression</span>
<span id="cb19-2"><a href="#cb19-2" aria-hidden="true" tabindex="-1"></a> function: (<span class="op">_</span>) @fn-name</span>
<span id="cb19-3"><a href="#cb19-3" aria-hidden="true" tabindex="-1"></a> arguments: (arguments (string_literal)))</span>
<span id="cb19-4"><a href="#cb19-4" aria-hidden="true" tabindex="-1"></a> (#match? @fn-name <span class="st">"(std::|)env::(var|remove_var)"</span>))</span></code></pre></div>
<h3 id="running-our-linter">Running our linter</h3>
<p>We could always plug our query into the web playground, but let’s go
a step further:</p>
<div class="sourceCode" id="cb20"><pre
class="sourceCode bash"><code class="sourceCode bash"><span id="cb20-1"><a href="#cb20-1" aria-hidden="true" tabindex="-1"></a><span class="ex">cargo</span> new <span class="at">--bin</span> toy-lint</span></code></pre></div>
<p>Add <code>tree-sitter</code> and <code>tree-sitter-rust</code> to
your dependencies:</p>
<div class="sourceCode" id="cb21"><pre
class="sourceCode toml"><code class="sourceCode toml"><span id="cb21-1"><a href="#cb21-1" aria-hidden="true" tabindex="-1"></a><span class="co"># within Cargo.toml</span></span>
<span id="cb21-2"><a href="#cb21-2" aria-hidden="true" tabindex="-1"></a><span class="kw">[dependencies]</span></span>
<span id="cb21-3"><a href="#cb21-3" aria-hidden="true" tabindex="-1"></a><span class="dt">tree-sitter</span> <span class="op">=</span> <span class="st">"0.20"</span></span>
<span id="cb21-4"><a href="#cb21-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb21-5"><a href="#cb21-5" aria-hidden="true" tabindex="-1"></a><span class="kw">[dependencies.tree-sitter-rust]</span></span>
<span id="cb21-6"><a href="#cb21-6" aria-hidden="true" tabindex="-1"></a><span class="dt">git</span> <span class="op">=</span> <span class="st">"https://github.com/tree-sitter/tree-sitter-rust"</span></span></code></pre></div>
<p>Let’s load in some Rust code to work with. As <a
href="https://en.wikipedia.org/wiki/Self-reference">an ode to Gödel</a>
(G<code>ode</code>l?), why not load in our linter itself:</p>
<div class="sourceCode" id="cb22"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb22-1"><a href="#cb22-1" aria-hidden="true" tabindex="-1"></a><span class="kw">fn</span> main() <span class="op">{</span></span>
<span id="cb22-2"><a href="#cb22-2" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> src <span class="op">=</span> <span class="pp">include_str!</span>(<span class="st">"main.rs"</span>)<span class="op">;</span></span>
<span id="cb22-3"><a href="#cb22-3" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>Most tree-sitter APIs require a reference to a <code>Language</code>
struct, we will be working with Rust if you haven’t already guessed:</p>
<div class="sourceCode" id="cb23"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb23-1"><a href="#cb23-1" aria-hidden="true" tabindex="-1"></a><span class="kw">use</span> <span class="pp">tree_sitter::</span>Language<span class="op">;</span></span>
<span id="cb23-2"><a href="#cb23-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb23-3"><a href="#cb23-3" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> rust_lang<span class="op">:</span> Language <span class="op">=</span> <span class="pp">tree_sitter_rust::</span>language()<span class="op">;</span></span></code></pre></div>
<p>Enough scaffolding, let’s parse some Rust:</p>
<div class="sourceCode" id="cb24"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb24-1"><a href="#cb24-1" aria-hidden="true" tabindex="-1"></a><span class="kw">use</span> <span class="pp">tree_sitter::</span>Parser<span class="op">;</span></span>
<span id="cb24-2"><a href="#cb24-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb24-3"><a href="#cb24-3" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> <span class="kw">mut</span> parser <span class="op">=</span> <span class="pp">Parser::</span>new()<span class="op">;</span></span>
<span id="cb24-4"><a href="#cb24-4" aria-hidden="true" tabindex="-1"></a>parser<span class="op">.</span>set_language(rust_lang)<span class="op">.</span>unwrap()<span class="op">;</span></span>
<span id="cb24-5"><a href="#cb24-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb24-6"><a href="#cb24-6" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> parse_tree <span class="op">=</span> parser<span class="op">.</span>parse(<span class="op">&</span>src<span class="op">,</span> <span class="cn">None</span>)<span class="op">.</span>unwrap()<span class="op">;</span></span></code></pre></div>
<p>The second argument to <code>Parser::parse</code> may be of interest.
Tree-sitter has this cool feature that allows for quick reparsing of
existing parse trees if they contain edits. If you do happen to want to
reparse a source file, you can pass in the old tree:</p>
<div class="sourceCode" id="cb25"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb25-1"><a href="#cb25-1" aria-hidden="true" tabindex="-1"></a><span class="co">// if you wish to reparse instead of parse</span></span>
<span id="cb25-2"><a href="#cb25-2" aria-hidden="true" tabindex="-1"></a>old_tree<span class="op">.</span>edit(<span class="co">/* redacted */</span>)<span class="op">;</span></span>
<span id="cb25-3"><a href="#cb25-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb25-4"><a href="#cb25-4" aria-hidden="true" tabindex="-1"></a><span class="co">// generate shiny new reparsed tree</span></span>
<span id="cb25-5"><a href="#cb25-5" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> new_tree <span class="op">=</span> parser<span class="op">.</span>parse(<span class="op">&</span>src<span class="op">,</span> <span class="cn">Some</span>(old_tree))<span class="op">.</span>unwrap()</span></code></pre></div>
<p>Anyhow (<a href="http://github.com/dtolnay/anyhow">hah!</a>), now
that we have a parse tree, we can inspect it:</p>
<div class="sourceCode" id="cb26"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb26-1"><a href="#cb26-1" aria-hidden="true" tabindex="-1"></a><span class="pp">println!</span>(<span class="st">"{}"</span><span class="op">,</span> parse_tree<span class="op">.</span>root_node()<span class="op">.</span>to_sexp())<span class="op">;</span></span></code></pre></div>
<p>Or better yet, run a query on it:</p>
<div class="sourceCode" id="cb27"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb27-1"><a href="#cb27-1" aria-hidden="true" tabindex="-1"></a><span class="kw">use</span> <span class="pp">tree_sitter::</span>Query<span class="op">;</span></span>
<span id="cb27-2"><a href="#cb27-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb27-3"><a href="#cb27-3" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> query <span class="op">=</span> <span class="pp">Query::</span>new(</span>
<span id="cb27-4"><a href="#cb27-4" aria-hidden="true" tabindex="-1"></a> rust_lang<span class="op">,</span></span>
<span id="cb27-5"><a href="#cb27-5" aria-hidden="true" tabindex="-1"></a> <span class="st">r#"</span></span>
<span id="cb27-6"><a href="#cb27-6" aria-hidden="true" tabindex="-1"></a><span class="st"> ((call_expression</span></span>
<span id="cb27-7"><a href="#cb27-7" aria-hidden="true" tabindex="-1"></a><span class="st"> function: (_) @fn-name</span></span>
<span id="cb27-8"><a href="#cb27-8" aria-hidden="true" tabindex="-1"></a><span class="st"> arguments: (arguments (string_literal))) @raise</span></span>
<span id="cb27-9"><a href="#cb27-9" aria-hidden="true" tabindex="-1"></a><span class="st"> (#match? @fn-name "(std::|)env::(var|remove_var)"))</span></span>
<span id="cb27-10"><a href="#cb27-10" aria-hidden="true" tabindex="-1"></a><span class="st"> "#</span></span>
<span id="cb27-11"><a href="#cb27-11" aria-hidden="true" tabindex="-1"></a>)</span>
<span id="cb27-12"><a href="#cb27-12" aria-hidden="true" tabindex="-1"></a><span class="op">.</span>unwrap()<span class="op">;</span></span></code></pre></div>
<p>A <code>QueryCursor</code> is tree-sitter’s way of maintaining state
as we iterate through the matches or captures produced by running a
query on the parse tree. Observe:</p>
<div class="sourceCode" id="cb28"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb28-1"><a href="#cb28-1" aria-hidden="true" tabindex="-1"></a><span class="kw">use</span> <span class="pp">tree_sitter::</span>QueryCursor<span class="op">;</span></span>
<span id="cb28-2"><a href="#cb28-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb28-3"><a href="#cb28-3" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> <span class="kw">mut</span> query_cursor <span class="op">=</span> <span class="pp">QueryCursor::</span>new()<span class="op">;</span></span>
<span id="cb28-4"><a href="#cb28-4" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> all_matches <span class="op">=</span> query_cursor<span class="op">.</span>matches(</span>
<span id="cb28-5"><a href="#cb28-5" aria-hidden="true" tabindex="-1"></a> <span class="op">&</span>query<span class="op">,</span></span>
<span id="cb28-6"><a href="#cb28-6" aria-hidden="true" tabindex="-1"></a> parse_tree<span class="op">.</span>root_node()<span class="op">,</span></span>
<span id="cb28-7"><a href="#cb28-7" aria-hidden="true" tabindex="-1"></a> src<span class="op">.</span>as_bytes()<span class="op">,</span></span>
<span id="cb28-8"><a href="#cb28-8" aria-hidden="true" tabindex="-1"></a>)<span class="op">;</span></span></code></pre></div>
<p>We begin by passing our query to the cursor, followed by the “root
node”, which is another way of saying, “start from the top”, and lastly,
the source itself. If you have already taken a look at the C API, you
will notice that the last argument, the source (known as the
<code>TextProvider</code>), is not required. The Rust bindings seem to
require this argument to provide predicate functionality such as
<code>#match?</code> and <code>#eq?</code>.</p>
<p>Do something with the matches:</p>
<div class="sourceCode" id="cb29"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb29-1"><a href="#cb29-1" aria-hidden="true" tabindex="-1"></a><span class="co">// get the index of the capture named "raise"</span></span>
<span id="cb29-2"><a href="#cb29-2" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> raise_idx <span class="op">=</span> query<span class="op">.</span>capture_index_for_name(<span class="st">"raise"</span>)<span class="op">.</span>unwrap()<span class="op">;</span></span>
<span id="cb29-3"><a href="#cb29-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb29-4"><a href="#cb29-4" aria-hidden="true" tabindex="-1"></a><span class="cf">for</span> each_match <span class="kw">in</span> all_matches <span class="op">{</span></span>
<span id="cb29-5"><a href="#cb29-5" aria-hidden="true" tabindex="-1"></a> <span class="co">// iterate over all captures called "raise"</span></span>
<span id="cb29-6"><a href="#cb29-6" aria-hidden="true" tabindex="-1"></a> <span class="co">// ignore captures such as "fn-name"</span></span>
<span id="cb29-7"><a href="#cb29-7" aria-hidden="true" tabindex="-1"></a> <span class="cf">for</span> capture <span class="kw">in</span> each_match</span>
<span id="cb29-8"><a href="#cb29-8" aria-hidden="true" tabindex="-1"></a> <span class="op">.</span>captures</span>
<span id="cb29-9"><a href="#cb29-9" aria-hidden="true" tabindex="-1"></a> <span class="op">.</span>iter()</span>
<span id="cb29-10"><a href="#cb29-10" aria-hidden="true" tabindex="-1"></a> <span class="op">.</span>filter(<span class="op">|</span>c<span class="op">|</span> c<span class="op">.</span>idx <span class="op">==</span> raise_idx)</span>
<span id="cb29-11"><a href="#cb29-11" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span></span>
<span id="cb29-12"><a href="#cb29-12" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> range <span class="op">=</span> capture<span class="op">.</span>node<span class="op">.</span>range()<span class="op">;</span></span>
<span id="cb29-13"><a href="#cb29-13" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> text <span class="op">=</span> <span class="op">&</span>src[range<span class="op">.</span>start_byte<span class="op">..</span>range<span class="op">.</span>end_byte]<span class="op">;</span></span>
<span id="cb29-14"><a href="#cb29-14" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> line <span class="op">=</span> range<span class="op">.</span>start_point<span class="op">.</span>row<span class="op">;</span></span>
<span id="cb29-15"><a href="#cb29-15" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> col <span class="op">=</span> range<span class="op">.</span>start_point<span class="op">.</span>column<span class="op">;</span></span>
<span id="cb29-16"><a href="#cb29-16" aria-hidden="true" tabindex="-1"></a> <span class="pp">println!</span>(</span>
<span id="cb29-17"><a href="#cb29-17" aria-hidden="true" tabindex="-1"></a> <span class="st">"[Line: {}, Col: {}] Offending source code: `{}`"</span><span class="op">,</span></span>
<span id="cb29-18"><a href="#cb29-18" aria-hidden="true" tabindex="-1"></a> line<span class="op">,</span> col<span class="op">,</span> text</span>
<span id="cb29-19"><a href="#cb29-19" aria-hidden="true" tabindex="-1"></a> )<span class="op">;</span></span>
<span id="cb29-20"><a href="#cb29-20" aria-hidden="true" tabindex="-1"></a> <span class="op">}</span></span>
<span id="cb29-21"><a href="#cb29-21" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>Lastly, add the following line to your source code, to get the linter
to catch something:</p>
<div class="sourceCode" id="cb30"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb30-1"><a href="#cb30-1" aria-hidden="true" tabindex="-1"></a><span class="pp">env::</span>remove_var(<span class="st">"RUST_BACKTRACE"</span>)<span class="op">;</span></span></code></pre></div>
<p>And <code>cargo run</code>:</p>
<pre class="shell"><code>λ cargo run
Compiling toy-lint v0.1.0 (/redacted/path/to/toy-lint)
Finished dev [unoptimized + debuginfo] target(s) in 0.74s
Running `target/debug/toy-lint`
[Line: 40, Col: 4] Offending source code: `env::remove_var("RUST_BACKTRACE")`</code></pre>
<p>Thank you tree-sitter!</p>
<h3 id="bonus">Bonus</h3>
<p>Keen readers will notice that I avoided
<code>std::env::set_var</code>. Because <code>set_var</code> is called
with two arguments, a “key” and a “value”, unlike <code>env::var</code>
and <code>env::remove_var</code>. As a result, it requires more
juggling:</p>
<div class="sourceCode" id="cb32"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb32-1"><a href="#cb32-1" aria-hidden="true" tabindex="-1"></a>((call_expression</span>
<span id="cb32-2"><a href="#cb32-2" aria-hidden="true" tabindex="-1"></a> function: (<span class="op">_</span>) @fn-name</span>
<span id="cb32-3"><a href="#cb32-3" aria-hidden="true" tabindex="-1"></a> arguments: (arguments <span class="op">.</span> (string_literal)<span class="op">?</span> <span class="op">.</span> (string_literal) <span class="op">.</span>)) @raise</span>
<span id="cb32-4"><a href="#cb32-4" aria-hidden="true" tabindex="-1"></a> (#match? @fn-name <span class="st">"(std::|)env::(var|remove_var|set_var)"</span>))</span></code></pre></div>
<p>The interesting part of this query is the humble <code>.</code>, the
<em>anchor</em> operator. Anchors help constrain child nodes in certain
ways. In this case, it ensures that we match exactly two
<code>string_literal</code>s who are siblings or exactly one
<code>string_literal</code> with no siblings. Unfortunately, this query
also matches the following invalid Rust code:</p>
<div class="sourceCode" id="cb33"><pre
class="sourceCode rust"><code class="sourceCode rust"><span id="cb33-1"><a href="#cb33-1" aria-hidden="true" tabindex="-1"></a><span class="co">// remove_var accepts only 1 arg!</span></span>
<span id="cb33-2"><a href="#cb33-2" aria-hidden="true" tabindex="-1"></a><span class="pp">std::env::</span>remove_var(<span class="st">"RUST_BACKTRACE"</span><span class="op">,</span> <span class="st">"1"</span>)<span class="op">;</span></span></code></pre></div>
<h3 id="notes">Notes</h3>
<p>All-in-all, the query DSL does a great job in lowering the bar to
writing language tools. The knowledge gained from mastering the query
DSL can be applied to other languages that have tree-sitter grammars
too. This query detects <code>to_json</code> methods that do not accept
additional arguments, in Ruby:</p>
<div class="sourceCode" id="cb34"><pre
class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb34-1"><a href="#cb34-1" aria-hidden="true" tabindex="-1"></a>((method</span>
<span id="cb34-2"><a href="#cb34-2" aria-hidden="true" tabindex="-1"></a> name: (identifier) @fn</span>
<span id="cb34-3"><a href="#cb34-3" aria-hidden="true" tabindex="-1"></a> !parameters)</span>
<span id="cb34-4"><a href="#cb34-4" aria-hidden="true" tabindex="-1"></a> (<span class="sc">#i</span>s? @fn <span class="st">"to_json"</span>))</span></code></pre></div>
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<div class="intro">
Hi.
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<p>I'm Akshay, programmer and pixel-artist.
I write <a href="https://git.peppe.rs">open-source stuff</a>.
I also design fonts:
<a href="https://git.peppe.rs/fonts/scientifica/about">scientifica</a>,
<a href="https://git.peppe.rs/fonts/curie/about">curie</a>.
</p>
<p>Reach out at [email protected].</p>
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