Visitor glue regressed rustc performance by 8 seconds

[pcwalton]

Are we not generating it lazily?

[pcwalton]

cc @graydon

[graydon]

Lazily, yes, but it's generated for anything that has a full tydesc instantiated, which is a lot of them. Anything boxed, say.

[graydon]

I don't think there's a bug here to be fixed, since we're going to continue generating this code. If it turns out to be hot in profiles we'll spend time optimizing it like anything else, but otherwise the feature's not backing out, so.. closing. LMK if you think this is wrong.