Enable unreachable_pub lint in core

[Urgau]

This PR enables the unreachable_pub as warn in core, rtstartup and panic_unwind.

The motivation is similar to the compiler MCP: Enable deny(unreachable_pub) on rustc_* crates :

"Where is this thing used?" is a question I ask all the time when reading unfamiliar code. Because of this, I generally find it annoying when things are marked with a more permissive visibility than necessary. "This thing marked pub, which other crates is it used in? Oh, it's not used in any other crates."

Another motivation is to help to lint by utilizing it in-tree and seeing it's limitation in more complex scenarios.

The diff was mostly generated with ./x.py fix --stage 1 library/core/ -- --broken-code, as well as manual edits for code in macros, generated code and other targets.

r? libs

[ibraheemdev]

This seems reasonable to me, does it need libs signoff?

[Urgau]

I don't know T-libs customs around enabling new lints. Given that T-compiler as done an MCP I guess it doesn't hurt to at least nominate it.

@rustbot labels -S-waiting-on-review +S-waiting-on-team +I-libs-nominated

[Amanieu]

We discussed this in the libs meeting today, overall several people were in favor of enabling the lint and nobody objected.

[ibraheemdev]

@bors r+

[bors]

📌 Commit 2fa6dbc has been approved by ibraheemdev

It is now in the queue for this repository.

[rust-timer]

Finished benchmarking commit (a42d5ec): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (secondary -2.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 764.999s -> 765.476s (0.06%)
Artifact size: 326.01 MiB -> 326.00 MiB (-0.00%)