Trait-not-implemented diagnostics consider all compiler-internal traits to be unstable

[Zalathar]

#151036 improved trait-not-implemented diagnostics in some cases, by explicitly mentioning that the trait in question is unstable:

rust/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

Line 5616 in 1396514

"nightly-only, unstable "

An unfortunate side-effect of that change is that when a trait-not-implemented error occurs while working on the compiler itself, for a trait defined in the compiler source (whether pub or not), that extra text always appears.

This is technically “correct” because rustc crates are built with -Zforce-unstable-if-unmarked. But it's unhelpful and confusing for compiler contributors.

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We can probably carve out some kind of narrow exemption to suppress the extra text, perhaps even just automatically hiding it when -Zforce-unstable-if-unmarked is set (or another -Z flag that correlates with working on the compiler).