After desugaring, `let` expressions should only appear inside `if` expressions
or `match` guards, possibly nested within a let-chain. In both cases they are
specifically handled by the lowerings of those expressions, so this case is
currently unreachable.

…enkov

Refactor pre-getopts command line argument handling

Rebased version of rust-lang#111658. I've also fixed the Windows CI failure (although I don't have access to Windows to test it myself).

Use the OS thread name by default if `THREAD_INFO` has not been initialized

Currently if `THREAD_INFO` hasn't been initialized then the name will be set to `None`.  This PR changes it to use the OS thread name by default. This mostly affects foreign threads at the moment but we could expand this to make more use of the OS thread name in the future.

Note: I've only implemented `Thread::get_name` for windows, linux and macos (and macos adjacent) targets. The rest just return `None`.

…risDenton

Move thread local implementation to `sys`

Part of rust-lang#117276.

attempt to further clarify addr_of docs

style library/core/src/error.rs

Add an extra blank line for clarity in distinguishing implementations.

The ordinary lowering of `thir::ExprKind::Let` is unreachable

After desugaring, `let` expressions should only appear inside `if` expressions or `match` guards, possibly nested within a let-chain. In both cases they are specifically handled by the lowerings of those expressions, so this case is currently unreachable.

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Context: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Lowering.20of.20.60thir.3A.3AExprKind.3A.3ALet.60.20is.20unreachable

My conclusions are partly based on the observation that stubbing out this match arm doesn't cause any test failures. So either this really is unreachable, or it can be reached in some obscure circumstances that our test suite doesn't cover.

If we end up needing this code (or something like it) for an implementation of rust-lang/rfcs#3573, it should be easy enough to pull it back out of version control history.

I looked into having the `if`/`match` lowerings call back into `expr_into_dest`, but from what I can tell that won't work well, because there are extra scoping considerations that require some awareness of the enclosing if/match.

r? ``@Nadrieril``