Remove ptr-int transmute in std::sync::mpsc

[saethlin]

Since #95340 landed, Miri with -Zmiri-check-number-validity produces an error on the test suites of some crates which implement concurrency tools^*, because it seems like such crates tend to use std::sync::mpsc in their tests. This fixes the problem by storing pointer bytes in a pointer.

^* I have so far seen errors in the test suites of once_cell, parking_lot, and crossbeam-utils.
(just updating the list for fun, idk)
Also threadpool, async-lock, futures-timer, fragile, scoped_threadpool, procfs, slog-async, scheduled-thread-pool, tokio-threadpool, mac, futures-cpupool, ntest, actix, zbus, jsonrpc-client-transports, fail, libp2p-gossipsub, parity-send-wrapper, async-broadcast, libp2p-relay, http-client, mockito, simple-mutex, surf, pollster, and pulse. Then I turned the bot off.

[rust-highfive]

r? @yaahc

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[saethlin]

The previous CI run failed due to a 360 minute timeout. Interesting.

[RalfJung]

The patch LGTM (it's mostly a straight-forward replacement of usize by a pointer type), and I checked that mpsc/tests.rs after this PR passes in Miri with strict provenance. So, I think we can land this.

@bors r+

[bors]

📌 Commit dec73f5 has been approved by RalfJung

[bors]

⌛ Testing commit dec73f5 with merge 7af9329...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing 7af9329 to master...

[rust-timer]

Finished benchmarking commit (7af9329): comparison url.

Summary:

• Primary benchmarks: 😿 relevant regression found
• Secondary benchmarks: 😿 relevant regressions found

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count1	1	6	0	0	1
mean2	0.5%	0.4%	N/A	N/A	0.5%
max	0.5%	0.5%	N/A	N/A	0.5%

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression

Footnotes

1. number of relevant changes ↩

2. the arithmetic mean of the percent change ↩

[Mark-Simulacrum]

Highly likely a real regression, but it's small enough that I'm not too worried, and seems to largely be due to increases in trait-related queries. This is probably due to increased complexity of standard library types making it a little more expensive to prove things like Sized, Send, etc.

May also be noise and paired with #95253 / https://perf.rust-lang.org/compare.html?start=7af93292c27cd8b4a14f0f35bcb4c7e7ca9c287a&end=32c26302620b2dbbe0a2291f1969bc0b1622ae59&stat=instructions:u, which reverted most of the regression here.

@rustbot label: +perf-regression-triaged