* 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 "doing something".
* 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)
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