rustc: Forbid `-Z` flags on stable/beta channels

[alexcrichton]

First deprecated in rustc 1.8.0 the intention was to never allow -Z flags make
their way to the stable channel (or unstable options). After a year of warnings
we've seen one of the main use cases, -Z no-trans, stabilized as cargo check. Otherwise while other use cases remain the sentiment is that now's the
time to start forbidding -Z by default on stable/beta.

Closes #31847

[rust-highfive]

r? @eddyb

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[kennytm]

so... what about #41743 ("Firefox requires -Zdebug-macros")?

[alexcrichton]

That's a feature that should be stabilized in time, not block this.

[Mark-Simulacrum]

Travis failed legitimately, I think. I'm somewhat concerned that we'd break Firefox's debugging process by landing this, though, especially with no current clear path forward for debug-macros.

[00:05:47] error: unused import: `early_warn`
[00:05:47]     --> /checkout/src/librustc/session/config.rs:1664:32
[00:05:47]      |
[00:05:47] 1664 |     use session::{early_error, early_warn};
[00:05:47]      |                                ^^^^^^^^^^
[00:05:47]      |
[00:05:47]      = note: #[deny(unused_imports)] implied by #[deny(warnings)]
[00:05:47] note: lint level defined here
[00:05:47]     --> /checkout/src/librustc/lib.rs:24:9
[00:05:47]      |
[00:05:47] 24   | #![deny(warnings)]
[00:05:47]      |         ^^^^^^^^
[00:05:47] 
[00:05:48] error: aborting due to previous error

[alexcrichton]

@Mark-Simulacrum we've been printing for over a year that this will become a warning, debugging with these features will continue to work on nightly, as is intended.

[eddyb]

@bors r+

[bors]

📌 Commit ccbcc72 has been approved by eddyb

[bors]

⌛️ Testing commit ccbcc72 with merge 50b9858...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing 50b9858 to master...

[jsgf]

I think this is a mistake: https://internals.rust-lang.org/t/disabling-z-for-stable-builds-is-a-mistake/5221