core: no-std panic::AssertUnwindSafe (etc.)

[brodycj]

STATUS:

I think the following PRs should be merged first to resolve the CI build issues:

• resolve unnecessary map_or Clippy issue #2904 - resolve clippy failure
• Bump general MSRV to 1.63 #2907 - bump main / general MSRV to resolve MSRV failure
• Bump MSRV of core crates to 1.56.0 #2906 - bump core MSRV as needed for this PR

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WHAT:

• use panic::AssertUnwindSafe from Rust core lib in futures-core
• remove extern crate std not needed from futures-core & futures-sink

I would be happy to split these updates into separate PRs, if needed. I figured we may as well do these updates together.

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CI testing TODO(s):

• core MSRV failure (expected) - requires core MSRV bump to 1.41 - already resolved by PR: Bump MSRV of core crates to 1.56.0 #2906
• clippy failure - already resolved by PR: resolve unnecessary map_or Clippy issue #2904
• general msrv failure - as resolved by PR: Bump general MSRV to 1.63 #2907

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other TODO(s):

• remove std option from futures-core & futures-sink - this would be another breaking change - I would be happy to do this here or in a separate PR

[taiki-e]

This is necessary to generate the appropriate error if the user accidentally enables the std feature.

Otherwise, we may run into trouble with users who accidentally rely on the fact that currently we are not using std and get angry that their builds are broken during future patch releases.

[taiki-e]

Thanks for the PR.

Is this PR based on a specific use case? Or is this a “do it because it's possible” style of PR?
catch_unwind is not in core, so I don't think using this in core is that common.

(You can use catch_unwind from third-party no-std crate such as unwinding crate, but it doesn't need UnwindSafe.)