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<h1 class='fqn'><span class='in-band'>Module <a href='../index.html'>syn</a>::<wbr><a class="mod" href=''>buffer</a></span><span class='out-of-band'><span id='render-detail'>
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[<span class='inner'>&#x2212;</span>]
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</span><a class='srclink' href='../../src/syn/buffer.rs.html#9-469' title='goto source code'>[src]</a></span></h1>
<div class='docblock'><p>A stably addressed token buffer supporting efficient traversal based on a
cheaply copyable cursor.</p>
<p>The <a href="../synom/trait.Synom.html"><code>Synom</code></a> trait is implemented for syntax tree types that can be parsed
from one of these token cursors.</p>
<p><em>This module is available if Syn is built with the <code>&quot;parsing&quot;</code> feature.</em></p>
<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
<p>This example shows a basic token parser for parsing a token stream without
using Syn's parser combinator macros.</p>
<pre class="rust rust-example-rendered">
<span class="attribute">#![<span class="ident">feature</span>(<span class="ident">proc_macro_diagnostic</span>)]</span>
<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">syn</span>;
<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">proc_macro</span>;
<span class="attribute">#[<span class="ident">macro_use</span>]</span>
<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">quote</span>;
<span class="kw">use</span> <span class="ident">syn</span>::{<span class="ident">token</span>, <span class="ident">ExprTuple</span>};
<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">buffer</span>::{<span class="ident">Cursor</span>, <span class="ident">TokenBuffer</span>};
<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">spanned</span>::<span class="ident">Spanned</span>;
<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">synom</span>::<span class="ident">Synom</span>;
<span class="kw">use</span> <span class="ident">proc_macro</span>::{<span class="ident">Diagnostic</span>, <span class="ident">Span</span>, <span class="ident">TokenStream</span>};
<span class="doccomment">/// A basic token parser for parsing a token stream without using Syn&#39;s</span>
<span class="doccomment">/// parser combinator macros.</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Parser</span><span class="op">&lt;</span><span class="lifetime">&#39;a</span><span class="op">&gt;</span> {
<span class="ident">cursor</span>: <span class="ident">Cursor</span><span class="op">&lt;</span><span class="lifetime">&#39;a</span><span class="op">&gt;</span>,
}
<span class="kw">impl</span><span class="op">&lt;</span><span class="lifetime">&#39;a</span><span class="op">&gt;</span> <span class="ident">Parser</span><span class="op">&lt;</span><span class="lifetime">&#39;a</span><span class="op">&gt;</span> {
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">new</span>(<span class="ident">cursor</span>: <span class="ident">Cursor</span><span class="op">&lt;</span><span class="lifetime">&#39;a</span><span class="op">&gt;</span>) <span class="op">-&gt;</span> <span class="self">Self</span> {
<span class="ident">Parser</span> { <span class="ident">cursor</span> }
}
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">current_span</span>(<span class="kw-2">&amp;</span><span class="self">self</span>) <span class="op">-&gt;</span> <span class="ident">Span</span> {
<span class="self">self</span>.<span class="ident">cursor</span>.<span class="ident">span</span>().<span class="ident">unstable</span>()
}
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">parse</span><span class="op">&lt;</span><span class="ident">T</span>: <span class="ident">Synom</span><span class="op">&gt;</span>(<span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-&gt;</span> <span class="prelude-ty">Result</span><span class="op">&lt;</span><span class="ident">T</span>, <span class="ident">Diagnostic</span><span class="op">&gt;</span> {
<span class="kw">let</span> (<span class="ident">val</span>, <span class="ident">rest</span>) <span class="op">=</span> <span class="ident">T</span>::<span class="ident">parse</span>(<span class="self">self</span>.<span class="ident">cursor</span>)
.<span class="ident">map_err</span>(<span class="op">|</span><span class="ident">e</span><span class="op">|</span> <span class="kw">match</span> <span class="ident">T</span>::<span class="ident">description</span>() {
<span class="prelude-val">Some</span>(<span class="ident">desc</span>) <span class="op">=&gt;</span> {
<span class="self">self</span>.<span class="ident">current_span</span>().<span class="ident">error</span>(<span class="macro">format</span><span class="macro">!</span>(<span class="string">&quot;{}: expected {}&quot;</span>, <span class="ident">e</span>, <span class="ident">desc</span>))
}
<span class="prelude-val">None</span> <span class="op">=&gt;</span> {
<span class="self">self</span>.<span class="ident">current_span</span>().<span class="ident">error</span>(<span class="ident">e</span>.<span class="ident">to_string</span>())
}
})<span class="question-mark">?</span>;
<span class="self">self</span>.<span class="ident">cursor</span> <span class="op">=</span> <span class="ident">rest</span>;
<span class="prelude-val">Ok</span>(<span class="ident">val</span>)
}
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">expect_eof</span>(<span class="kw-2">&amp;</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-&gt;</span> <span class="prelude-ty">Result</span><span class="op">&lt;</span>(), <span class="ident">Diagnostic</span><span class="op">&gt;</span> {
<span class="kw">if</span> <span class="op">!</span><span class="self">self</span>.<span class="ident">cursor</span>.<span class="ident">eof</span>() {
<span class="kw">return</span> <span class="prelude-val">Err</span>(<span class="self">self</span>.<span class="ident">current_span</span>().<span class="ident">error</span>(<span class="string">&quot;trailing characters; expected eof&quot;</span>));
}
<span class="prelude-val">Ok</span>(())
}
}
<span class="kw">fn</span> <span class="ident">eval</span>(<span class="ident">input</span>: <span class="ident">TokenStream</span>) <span class="op">-&gt;</span> <span class="prelude-ty">Result</span><span class="op">&lt;</span><span class="ident">TokenStream</span>, <span class="ident">Diagnostic</span><span class="op">&gt;</span> {
<span class="kw">let</span> <span class="ident">buffer</span> <span class="op">=</span> <span class="ident">TokenBuffer</span>::<span class="ident">new</span>(<span class="ident">input</span>);
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">parser</span> <span class="op">=</span> <span class="ident">Parser</span>::<span class="ident">new</span>(<span class="ident">buffer</span>.<span class="ident">begin</span>());
<span class="comment">// Parse some syntax tree types out of the input tokens. In this case we</span>
<span class="comment">// expect something like:</span>
<span class="comment">//</span>
<span class="comment">// (a, b, c) = (1, 2, 3)</span>
<span class="kw">let</span> <span class="ident">a</span> <span class="op">=</span> <span class="ident">parser</span>.<span class="ident">parse</span>::<span class="op">&lt;</span><span class="ident">ExprTuple</span><span class="op">&gt;</span>()<span class="question-mark">?</span>;
<span class="ident">parser</span>.<span class="ident">parse</span>::<span class="op">&lt;</span><span class="ident">token</span>::<span class="ident">Eq</span><span class="op">&gt;</span>()<span class="question-mark">?</span>;
<span class="kw">let</span> <span class="ident">b</span> <span class="op">=</span> <span class="ident">parser</span>.<span class="ident">parse</span>::<span class="op">&lt;</span><span class="ident">ExprTuple</span><span class="op">&gt;</span>()<span class="question-mark">?</span>;
<span class="ident">parser</span>.<span class="ident">expect_eof</span>()<span class="question-mark">?</span>;
<span class="comment">// Perform some validation and report errors.</span>
<span class="kw">let</span> (<span class="ident">a_len</span>, <span class="ident">b_len</span>) <span class="op">=</span> (<span class="ident">a</span>.<span class="ident">elems</span>.<span class="ident">len</span>(), <span class="ident">b</span>.<span class="ident">elems</span>.<span class="ident">len</span>());
<span class="kw">if</span> <span class="ident">a_len</span> <span class="op">!=</span> <span class="ident">b_len</span> {
<span class="kw">let</span> <span class="ident">diag</span> <span class="op">=</span> <span class="ident">b</span>.<span class="ident">span</span>().<span class="ident">unstable</span>()
.<span class="ident">error</span>(<span class="macro">format</span><span class="macro">!</span>(<span class="string">&quot;expected {} element(s), got {}&quot;</span>, <span class="ident">a_len</span>, <span class="ident">b_len</span>))
.<span class="ident">span_note</span>(<span class="ident">a</span>.<span class="ident">span</span>().<span class="ident">unstable</span>(), <span class="string">&quot;because of this&quot;</span>);
<span class="kw">return</span> <span class="prelude-val">Err</span>(<span class="ident">diag</span>);
}
<span class="comment">// Build the output tokens.</span>
<span class="kw">let</span> <span class="ident">out</span> <span class="op">=</span> <span class="macro">quote</span><span class="macro">!</span> {
<span class="macro">println</span><span class="macro">!</span>(<span class="string">&quot;All good! Received two tuples of size {}&quot;</span>, #<span class="ident">a_len</span>);
};
<span class="prelude-val">Ok</span>(<span class="ident">out</span>.<span class="ident">into</span>())
}</pre>
</div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
<table>
<tr class=' module-item'>
<td><a class="struct" href="struct.Cursor.html"
title='struct syn::buffer::Cursor'>Cursor</a></td>
<td class='docblock-short'>
<p>A cheaply copyable cursor into a <code>TokenBuffer</code>.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class="struct" href="struct.TokenBuffer.html"
title='struct syn::buffer::TokenBuffer'>TokenBuffer</a></td>
<td class='docblock-short'>
<p>A buffer that can be efficiently traversed multiple times, unlike
<code>TokenStream</code> which requires a deep copy in order to traverse more than
once.</p>
</td>
</tr></table></section>
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