Make from_waker, waker and from_raw unstably const

[BennoLossin]

Make

• Context::from_waker
• Context::waker
• Waker::from_raw

const.

Also added a small test.

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

Please see the contribution instructions for more information.

[BennoLossin]

@rustbot label +T-libs-api -T-libs

[Mark-Simulacrum]

r? @lcnr for wg-const-eval approval of new const-unstable APIs

(Closest reading of policy I could find suggests that wg-const-eval should approve, though I think these particular ones seem OK to my relatively uninformed opinion).

[lcnr]

these look fine for me, please add a tracking issue, then r=me

cc @rust-lang/wg-const-eval

[lcnr]

@bors r+ rollup

[bors]

📌 Commit 8e848dc has been approved by lcnr

It is now in the queue for this repository.