constify From impls for the Cow::Borrowed variant

[npmccallum]

This issue is a follow-up PR from #146067 with the controversial impls removed.

This is useful in at least two cases:

1. Generic interfaces which may be const.
2. Conditional compilation (&str vs Cow<str>).

[rustbot]

r? @ibraheemdev

rustbot has assigned @ibraheemdev.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[tgross35]

Anybody reviewing this, please take a look at #146067 (comment)

[ibraheemdev]

This issue is a follow-up PR from #146067 with the controversial impls removed.

Which controversial parts were removed? If I understand @tgross35's comments correctly, we want to avoid introducing more rustc_const_unstable attributes into alloc to keep the API surface small before stabilization.

[npmccallum]

@ibraheemdev The original objection was const constructors for types like String. @tgross35's comments came later.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[clarfonthey]

Poking through old PRs: this seems to not have conflicts and be okay with the latest library constification policies.

@bors r+ rollup

[rust-bors]

📌 Commit c6cb702 has been approved by clarfonthey

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

Reason for tree closure: spurious failures

[jhpratt]

@bors r- #159941 (comment)

fmt check
error: expected a trait, found type
##[error] --> /checkout/library/alloc/src/vec/cow.rs:6:20
  |
6 | impl<'a, T: Clone> const From<&'a [T]> for Cow<'a, [T]> {
  |                    ^^^^^^^^^^^^^^^^^^^

error: expected a trait, found type
##[error]    --> /checkout/library/alloc/src/string.rs:3245:10
     |
3245 | impl<'a> const From<&'a str> for Cow<'a, str> {
     |          ^^^^^^^^^^^^^^^^^^^

error: expected a trait, found type
##[error]   --> /checkout/library/alloc/src/bstr.rs:598:10
    |
598 | impl<'a> const From<&'a ByteStr> for Cow<'a, ByteStr> {
    |          ^^^^^^^^^^^^^^^^^^^^^^^

error: expected a trait, found type
##[error]   --> /checkout/library/alloc/src/ffi/c_str.rs:885:10
    |
885 | impl<'a> const From<&'a CStr> for Cow<'a, CStr> {
    |          ^^^^^^^^^^^^^^^^^^^^

fmt: checked 7052 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Currently active steps:

[rust-bors]

This pull request was unapproved.

View changes since this unapproval

[clarfonthey]

Right, the syntax has changed since this was written. My bad.

[jhpratt]

Off the top of my head I believe it's now const impl Foo, but I could be wrong on that.