[WIP] unbound bettering #142693
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[WIP] unbound bettering #142693
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Only relevant to the internal feature `more_maybe_bounds`.
We no longer move trait predicates where the self ty is a ty param to "the bounds of a ty param".
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* The phrasing "only does something for" made sense back when this diagnostic was a (hard) warning. Now however, it's simply a hard error and thus completely rules out. * The primary message was way too long * The new wording more closely mirrors the wording we use for applying other bound modifiers (like `const` and `async`) to incompatible traits. * "all other traits are not bound by default" is no longer accurate under Sized Hierarchy. E.g., traits and assoc tys are (currently) bounded by `MetaSized` by default but can't be relaxed using `?MetaSized` (instead, you relax it by adding `PointeeSized`). * I've decided against adding any diagnositic notes or suggestions for now like "trait `Trait` can't be relaxed as it's not bound by default" /which would be incorrect for `MetaSized` and assoc tys as mentioned above) or "consider changing `?MetaSized` to `PointeeSized`" as the Sized Hierarchy impl is still WIP)
Having multiple relaxed bounds like `?Sized + ?Iterator` is actually *fine*. We actually want to reject *duplicate* relaxed bounds like `?Sized + ?Sized` because these most certainly represent a user error. Note that this doesn't mean that we accept more code because a bound like `?Iterator` is still invalid as it's not relaxing a *default* trait and the only way to define / use more default bounds is under the experimental and internal feature `more_maybe_bounds` plus `lang_items` plus unstable flag `-Zexperimental-default-bounds`. Ultimately, this simply *reframes* the diagnostic. The scope of `more_maybe_bounds` / `-Zexperimental-default-bounds` remains unchanged as well.
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Scaffolding for #135229 and #136944.
CC #135331, #135841 (review)