I'm not working on Swift these days, but the baseline rule is "no, using something underscored means your compiled program might already be broken" and that extends to "might work now but break in the future". There are some exceptions to this, with @_exported and @_cdecl being among them—these are attributes that individuals who work on the compiler have publicly made some guarantees about—but I'm not sure if @_implements is in that bucket. It probably works the way you describe, but I don't know myself, and so without a guarantee I'd be hesitant to use it.
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