`alloc`: clean and ensure `no_global_oom_handling` builds are warning-free
Rust 1.62.0 introduced a couple new `unused_imports` warnings
in `no_global_oom_handling` builds, making a total of 5 warnings.
<details>
```txt
warning: unused import: `Unsize`
--> library/alloc/src/boxed/thin.rs:6:33
|
6 | use core::marker::{PhantomData, Unsize};
| ^^^^^^
|
= note: `#[warn(unused_imports)]` on by default
warning: unused import: `from_fn`
--> library/alloc/src/string.rs:51:18
|
51 | use core::iter::{from_fn, FusedIterator};
| ^^^^^^^
warning: unused import: `core::ops::Deref`
--> library/alloc/src/vec/into_iter.rs:12:5
|
12 | use core::ops::Deref;
| ^^^^^^^^^^^^^^^^
warning: associated function `shrink` is never used
--> library/alloc/src/raw_vec.rs:424:8
|
424 | fn shrink(&mut self, cap: usize) -> Result<(), TryReserveError> {
| ^^^^^^
|
= note: `#[warn(dead_code)]` on by default
warning: associated function `forget_remaining_elements` is never used
--> library/alloc/src/vec/into_iter.rs:126:19
|
126 | pub(crate) fn forget_remaining_elements(&mut self) {
| ^^^^^^^^^^^^^^^^^^^^^^^^^
```
</details>
This PR cleans them and ensures no new ones are introduced
so that projects compiling `alloc` without infallible allocations
do not see them (and may want to enable `-Dwarnings`).
The couple `dead_code` ones may be reverted when some fallible
allocation support starts using them.
7dedec7be7