Don't make rustc_insignificant_dtor feature gate

[syvb]

This isn't a feature gate, it's an attribute that is feature gated behind the rustc_attrs attribute. Closes #85680.

[rust-highfive]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

I don't think a test is necessary here, but it seems like it would be good to implement an assert that checks that all of the features listed as incomplete are also active? The runtime overhead is likely minimal; moving it to compile-time seems a little painful so I wouldn't bother if it's not trivial. We could make it part of the tidy feature checks, perhaps...

[Mark-Simulacrum]

Actually, I think a good way of doing this might be to add an incomplete attribute (doesn't really matter how) to the declare_features! macro -- that could then generate the active incomplete features into the const array already present. I'm not sure how easy that'll be to pull off in the macro rules though; maybe adding another boolean field to the Feature struct is a good way to do it.

[syvb]

@Mark-Simulacrum I've updated the syntax of the macro so that it's used like this:

// normal features (unchanged)
(active, abi_ptx, "1.15.0", Some(38788), None),
// new syntax for incomplete features
(incomplete, repr128, "1.16.0", Some(56071), None),

[Mark-Simulacrum]

r=me with last comment fixed

[Mark-Simulacrum]

@bors r+ rollup=never

[bors]

📌 Commit 7a51cf1 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 7a51cf1 with merge fecc65a...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing fecc65a to master...