Auto merge of #26843 - servo:dependabot/cargo/syn-1.0.31, r=jdm

build(deps): bump syn from 1.0.27 to 1.0.31

Bumps [syn](https://github.com/dtolnay/syn) from 1.0.27 to 1.0.31.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a href="https://github.com/dtolnay/syn/releases">syn's releases</a>.</em></p>
<blockquote>
<h2>1.0.31</h2>
<ul>
<li>
<p>Add <a href="https://docs.rs/syn/1.0.31/syn/enum.Expr.html#method.parse_without_eager_brace"><code>Expr::parse_without_eager_brace</code></a> to parse expressions in ambiguous syntactic position.</p>
<p>Rust grammar has an ambiguity where braces sometimes turn a path expression into a struct initialization and sometimes do not. In the following code, the expression <code>S {}</code> is one expression. Presumably there is an empty struct <code>struct S {}</code> defined somewhere which it is instantiating.</p>
<pre lang="rust"><code>let _ = *S {};
<p>// parsed by rustc as: <code>*(S {})</code>
</code></pre></p>
<p>We would want to parse the above using <code>Expr::parse</code> after the <code>=</code> token.</p>
<p>But in the following, <code>S {}</code> is <em>not</em> a struct init expression.</p>
<pre lang="rust"><code>if *S {} {}
<p>// parsed by rustc as:
//
//    if (<em>S) {
//        /</em> empty block <em>/
//    }
//    {
//        /</em> another empty block */
//    }
</code></pre></p>
<p>For that reason we would want to parse if-conditions using <code>Expr::parse_without_eager_brace</code> after the <code>if</code> token. Same for similar syntactic positions such as the condition expr after a <code>while</code> token or the expr at the top of a <code>match</code>.</p>
<p>The Rust grammar's choices around which way this ambiguity is resolved at various syntactic positions is fairly arbitrary. Really either parse behavior could work in most positions, and language designers just decide each case based on which is more likely to be what the programmer had in mind most of the time.</p>
<pre lang="rust"><code>if return S {} {}
<p>// parsed by rustc as:
//
//    if (return (S {})) {
//    }
//
// but could equally well have been this other arbitrary choice:
//
//    if (return S) {
//    }
//    {}
</code></pre></p>
<p>Note the grammar ambiguity on trailing braces is distinct from precedence and is not captured by assigning a precedence level to the braced struct init expr in relation to other operators. This can be illustrated by <code>return 0..S {}</code> vs <code>match 0..S {}</code>. The former parses as <code>return (0..(S {}))</code> implying tighter precedence for struct init than <code>..</code>, while the latter parses as <code>match (0..S) {}</code> implying tighter precedence for <code>..</code> than struct init, a contradiction.</p>
</li>
</ul>
<h2>1.0.30</h2>
</tr></table> ... (truncated)
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/dtolnay/syn/commit/ec205a83fd1a343e93e061d00078b9b88ed32acb"><code>ec205a8</code></a> Release 1.0.31</li>
<li><a href="https://github.com/dtolnay/syn/commit/a3b67e42129eefa0c596a84172cf40018cc5c6d8"><code>a3b67e4</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/dtolnay/syn/issues/849">#849</a> from dtolnay/eparse</li>
<li><a href="https://github.com/dtolnay/syn/commit/a03ec1212225331b848660b27ac7edbce4e48392"><code>a03ec12</code></a> Rewrite parse_without_eager_brace doc with better examples</li>
<li><a href="https://github.com/dtolnay/syn/commit/e2a5120572c690bee7354e9cc91260740145c6a9"><code>e2a5120</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/dtolnay/syn/issues/848">#848</a> from udoprog/parse-without-eager-brace</li>
<li><a href="https://github.com/dtolnay/syn/commit/2a9200dfaf7261ea1af1c97a3f5e18bff02b7831"><code>2a9200d</code></a> Make expr_no_struct public as Expr::parse_without_eager_brace</li>
<li><a href="https://github.com/dtolnay/syn/commit/cf76afd1141877692d1ebe651d24cc8e28dbf5db"><code>cf76afd</code></a> Update test suite to nightly-2020-06-01</li>
<li><a href="https://github.com/dtolnay/syn/commit/89658fc242ee17041a4e2dd74b455f8d89d5de31"><code>89658fc</code></a> Release 1.0.30</li>
<li><a href="https://github.com/dtolnay/syn/commit/dd68696574a7ba96f70b0d475f653a2b723037ce"><code>dd68696</code></a> Merge pull request 843 from Aaron1011/feature/multiple-empty</li>
<li><a href="https://github.com/dtolnay/syn/commit/405f1bb90c1443ecb6e98e2005370a91c8df2268"><code>405f1bb</code></a> Handle multiple nested <code>None</code>-delimited groups</li>
<li><a href="https://github.com/dtolnay/syn/commit/7a81558977eddd61515c6a8c368246ee6655ebf4"><code>7a81558</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/dtolnay/syn/issues/842">#842</a> from dtolnay/group</li>
<li>Additional commits viewable in <a href="https://github.com/dtolnay/syn/compare/1.0.27...1.0.31">compare view</a></li>
</ul>
</details>
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