turn some long-deprecated -C options into errors

[RalfJung]

• -Car has been documented to do nothing for 8 years (691ab6c) and causes a warning for more than a year (mark deprecated option as deprecated in rustc_session to remove copypasta and small refactor #135126).
• -Cno-stack-check has been made a NOP and deprecated with a warning 10 years ago (c670293)
• -Cinline-threshold has been made a NOP and deprecated with a warning almost 2 years ago (Deprecate no-op codegen option -Cinline-threshold=... #124712).

With them being ignored there's always a risk someone thinks they'll do something and they don't notice the warning. I think these have been deprecated for long enough that we can turn them into hard errors.

Also change the type of these fields to () so there's no information here that the rest of the compiler could use. This supersedes the rustc_lint_opt_deny_field_access trick.

MCP: rust-lang/compiler-team#978

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[ehuss]

Can you please also update https://github.com/rust-lang/rust/blob/main/src/doc/rustc/src/codegen-options/index.md?

[Mark-Simulacrum]

A GitHub search finds a bunch of cases of inline-threshold, from a quick skim it looks like it's set in two relatively popular fuzzing/coverage related libraries:

• https://github.com/kennytm/cov/blob/c75fbc5c849ffec5376893d967796fa9caae26cb/cargo-cov/src/shim.rs#L91
• https://github.com/rust-fuzz/honggfuzz-rs/blob/243e95f584bfb25db64a6113ebfc81a3f8b8d422/src/bin/cargo-hfuzz.rs#L260

-Cno-stack-check appears in:

• https://github.com/rust-lang/cc-rs/blob/bf119a15810308271fe9087db0480ef0ac77fc98/src/flags.rs#L523

-Car is much harder to search for, I didn't find any cases, but I think we should cut PRs and get them merged at least against the above cases. Doing a rg against crates.io would probably also be good (cc https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Put.20all.20crates.20on.20dev.20desktops/with/581193254...)

[RalfJung]

Thanks for the search!

• I filed an issue for honggfuzz
• cov seems to be unmaintained (last update 8 years ago)
• The occurrence in cc-rs is just a list of all flags to aid in parsing command-line invocations

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.