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Auto merge of #113169 - oli-obk:tait_must_be_constrained_if_in_sig, r…
…=lcnr Tait must be constrained if in sig r? `@compiler-errors` kind of reverts #62423, but that PR only removed cycles in cases that we want to forbid now anyway. see https://rust-lang.zulipchat.com/#narrow/stream/315482-t-compiler.2Fetc.2Fopaque-types/topic/lcnr.20oli.20meeting/near/370712246 for related discussion and motivating example a TAIT showing up in a signature doesn't just mean it can register a hidden type, but that it must. This is the [design the types team decided upon](https://hackmd.io/QOsEaEJtQK-XDS_xN4UyQA#Proposal-preferred) for the following reasons * avoids a hypothetical situation where getting smarter in trait solving could cause new cycle errors or new inference errors to show up, where today something is treated as opaque. * avoids having the situation where a (minor?) change to a function body causes an error, because its TAIT usage suddenly isn't "opaque enough" anymore. * avoids having to explain why in some cases you need to put a Tait into a tiny module together with its defining usages. Now you basically gotta always do this, the moment a Tait is in a signature that isn't in the defining scope. * avoids false-cycle errors * anything diverging from this pattern needs to either * move the TAIT declaration and defining function into their own helper sub module * use the attributes we'll provide in the future to explicitly opt in or out of being in the defining scope fixes #117861 reverts #125362 cc `@Nilstrieb` `@joboet`
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