Fix compilation with -Z minimal versions

[konstin]

This pulls in the fix from dtolnay/indoc#29

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c6a5c05). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #470   +/-   ##
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  Coverage          ?   87.87%           
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  Files             ?       65           
  Lines             ?     3398           
  Branches          ?        0           
=========================================
  Hits              ?     2986           
  Misses            ?      412           
  Partials          ?        0

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