@warn_unused in syntax is now called @no_unused - this is because
"warning about unused" is a C thing (or rather, an extension to C)
and various languages might want to use stricter behavior for this.
Its associated API does the reverse now - it lets you query whether
being unused is allowed at all. This is to match future behavior
of Eolian (once it supports versioning) that will likely reverse it.
@feature
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