Add assert_impl_one! macro
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The current implementation doesn't entirely work because traits after the first aren't checked against each other with `assert_not_impl_all!`.
Fixes the issue where subsequent traits were only checked against the first trait and not to one another. This solution was made possible by @SimonSapin demonstrating to me how to expand the input sequence into combinations of inputs. Closes #20.
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This adds
assert_impl_one!, which ensures that exactly one in a set of traits is implemented.This current implementation doesn't entirely work because traits after the first aren't checked against each other withassert_not_impl_all!.Thanks to @SimonSapin's elegant solution, this macro is made possible. Subsequent traits were previously only checked against the first and not to one another. A correct implementation requires checking
assert_not_impl_all!against all trait pairs. Also, it turns out that the "Mutually Exclusive Trait Implementations" docs before also wasn't implemented correctly for this reason.