impl From<Cow> for Box, Rc, and Arc
These forward `Borrowed`/`Owned` values to existing `From` impls.
- `Box<T>` is a fundamental type, so it would be a breaking change to add a blanket impl. Therefore, `From<Cow>` is only implemented for `[T]`, `str`, `CStr`, `OsStr`, and `Path`.
- For `Rc<T>` and `Arc<T>`, `From<Cow>` is implemented for everything that implements `From` the borrowed and owned types separately.
* Add a new method `has_infer_types_or_consts` that's used instead most
of the time, since there's generally no reason to only consider types.
* Remove use of `has_closure_types`, because closures are no longer
implicitly linked to the `InferCtxt`.
No functional changes are made, and all APIs are moved to strictly less
restrictive bounds.
These APIs changed from the old bound listed to the new bound:
T: Hash + Eq -> T
* new
* with_capacity
T: Eq + Hash, S: BuildHasher -> T
* with_hasher
* with_capacity_and_hasher
* hasher
T: Eq + Hash + Debug -> T: Debug
S: BuildHasher -> S
<HashSet as Debug>
T: Eq + Hash -> T
S: BuildHasher + Default -> S: Default
<HashSet as Default>
- uses a never-stable core::array::LengthAtMost32 to bound the impls
- includes a custom error message to avoid mentioning LengthAtMost32 too often
- doesn't use macros for the slice implementations to avoid #62433
