Feature-gate #[derive_*] even when produced by macro expansion.

[eddyb]

Fixes#32655 by marking #[derive_Trait] attributes produced by the compiler from the
user-friendly #[derive(Trait)] syntax to allow unstable features, and gating on that.

[rust-highfive]

r? @nikomatsakis

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[eddyb]

I couldn't get this to work, seems like the span_allows_unstable check isn't enough.
cc @pnkfelix@nrc

[durka]

Wouldn't you need to also turn on the feature gate for #[derive_X] syntax? Is there even such a gate? namely #![feature(custom_derive)]

[eddyb]

There is, custom_derive, but allow_internal_unstable is supposed to bypass the actual feature gating (i.e. any feature can be used inside an #[allow_internal_unstable] macro).
This works with, e.g. #[thread_local] static FOO: usize = 0;, but not #[derive_Clone].

[durka]

Ah, nevermind then, I thought it looked at the actual features enabled in the macro-declaring crate.

[alexcrichton]

Crater report

[nikomatsakis]

@eddyb based on the conversation with @erickt, if we can make this issue a warning -- even better, it'd be great if we could suggest a cargo update when see syntex in use -- that'd be good.

[bors]

☔ The latest upstream changes (presumably #32791) made this pull request unmergeable. Please resolve the merge conflicts.