Bumps the known_good_semver group with 1 update:
[syn](https://github.com/dtolnay/syn).

Updates `syn` from 2.0.38 to 2.0.39
<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>2.0.39</h2>
<ul>
<li>Fix parsing of return expression in match guards (<a
href="https://redirect.github.com/dtolnay/syn/issues/1528">#1528</a>)</li>
<li>Improve error message on labeled loop as value expression for break
(<a
href="https://redirect.github.com/dtolnay/syn/issues/1531">#1531</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/dtolnay/syn/commit/924217c1734c085ee6cdb5e198d36d610c234313"><code>924217c</code></a>
Release 2.0.39</li>
<li><a
href="https://github.com/dtolnay/syn/commit/95aeeb559866368d8b910d388c8e04b96083666a"><code>95aeeb5</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1531">#1531</a>
from dtolnay/breaklabel</li>
<li><a
href="https://github.com/dtolnay/syn/commit/b88f86fae5c1ac284179181bf7a34549e6d96d7c"><code>b88f86f</code></a>
Ignore single_element_loop clippy lint in test</li>
<li><a
href="https://github.com/dtolnay/syn/commit/a876185366557cb0e0e175c3be48721c8cc0d8e6"><code>a876185</code></a>
Improve error on break followed by labeled loop</li>
<li><a
href="https://github.com/dtolnay/syn/commit/32ab9794d6b75248c1946ae18baab12a46137eb0"><code>32ab979</code></a>
Add test of ambiguous label parsing in break</li>
<li><a
href="https://github.com/dtolnay/syn/commit/6f658f88a213e25b7ac7ec2dc77c537de679b745"><code>6f658f8</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1530">#1530</a>
from dtolnay/canbeginexpr</li>
<li><a
href="https://github.com/dtolnay/syn/commit/20497e1a555812cfe05769b4d8c117c4e0e4edbd"><code>20497e1</code></a>
More precise decision to parse expression after return keyword</li>
<li><a
href="https://github.com/dtolnay/syn/commit/c6a651aa12447d50c22db3189ef38cff90fc267b"><code>c6a651a</code></a>
Update name of ExprReturn parse function to match variant name</li>
<li><a
href="https://github.com/dtolnay/syn/commit/c2745901009f7d65e7b708cd80031642038f765e"><code>c274590</code></a>
Indicate that peek argument refers to syn::Ident</li>
<li><a
href="https://github.com/dtolnay/syn/commit/3e67cb0c660fc906770d212465652bfeb7a75727"><code>3e67cb0</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1529">#1529</a>
from dtolnay/up</li>
<li>Additional commits viewable in <a
href="https://github.com/dtolnay/syn/compare/2.0.38...2.0.39">compare
view</a></li>
</ul>
</details>
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