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Can't reasonably handle a metavariable with an optional argument:

I'm trying to replicate the derive_builder functionality where you can say #[builder(setter(skip))] as short for #[builder(setter(skip=true))].

I've written this blob:

    ${define T_B_SETTER_SKIP
      ${if all(tmeta(builder(setter(skip))),
              approx_equal({${tmeta(builder(setter(skip)))}}, {})) { // (1)
          true
      } else if tmeta(builder(setter(skip))) {
          ${tmeta(builder(setter(skip))) as tokens}
      } else {
          false
      }}
    }

But that doesn't actually accept #[builder(setter(skip))]: the first expansion (marked with (1)) fails because there is no value for skip.

Possible solution:

  • Split the Xmeta condition in to Xmeta_present, Xmeta_empty, and Xmeta_has_value.
  • Add a new Xmeta expansion decorator that accepts empty conditions, as in: ${Xmeta(foo) empty_ok}. This would probably need to interact with anything we do for #40.