Rollup merge of #134672 - Zalathar:revert-coverage-attr, r=wesleywiser

Revert stabilization of the `#[coverage(..)]` attribute

Due to a process mixup, the PR to stabilize the `#[coverage(..)]` attribute (#130766) was merged while there are still outstanding concerns. The default action in that situation is to revert, and the feature is not sufficiently urgent or uncontroversial to justify special treatment, so this PR reverts that stabilization.

---

- A key point that came up in offline discussions is that unlike most user-facing features, this one never had a proper RFC, so parts of the normal stabilization process that implicitly rely on an RFC break down in this case.
- As the implementor and de-facto owner of the feature in its current form, I would like to think that I made good choices in designing and implementing it, but I don't feel comfortable proceeding to stabilization without further scrutiny.
- There hasn't been a clear opportunity for T-compiler to weigh in or express concerns prior to stabilization.
- The stabilization PR cites a T-lang FCP that occurred in the tracking issue, but due to the messy design and implementation history (and lack of a clear RFC), it's unclear what that FCP approval actually represents in this case.
  - At the very least, we should not proceed without a clear statement from T-lang or the relevant members about the team's stance on this feature, especially in light of the other concerns listed here.
- The existing user-facing documentation doesn't clearly reflect which parts of the feature are stable commitments, and which parts are subject to change. And there doesn't appear to be a clear consensus anywhere about where that line is actually drawn, or whether the chosen boundary is acceptable to the relevant teams and individuals.
  - For example, the [stabilization report comment](rust-lang/rust#84605 (comment)) mentions that some aspects are subject to change, but that text isn't consistent with my earlier comments, and there doesn't appear to have been any explicit discussion or approval process.
  - [The current reference text](https://github.com/rust-lang/reference/blob/4dfaa4f/src/attributes/coverage-instrumentation.md) doesn't mention this distinction at all, and instead simply describes the current implementation behaviour.
- When the implementation was changed to its current form, the associated user-facing error messages were not updated, so they still refer to the attribute only being allowed on functions and closures.
  - On its own, this might have been reasonable to fix-forward in the absence of other concerns, but the fact that it never came up earlier highlights the breakdown in process that has occurred here.

---

Apologies to everyone who was excited for this stabilization to land, but unfortunately it simply isn't ready yet.

crates/hir-expand/src/inert_attr_macro.rs

@@ -237,7 +237,7 @@ pub const INERT_ATTRIBUTES: &[BuiltinAttribute] = &[
         template!(List: "address, kcfi, memory, thread"), DuplicatesOk,
         experimental!(no_sanitize)
     ),
-    ungated!(coverage, Normal, template!(Word, List: "on|off"), WarnFollowing),
+    gated!(coverage, Normal, template!(Word, List: "on|off"), WarnFollowing, coverage_attribute, experimental!(coverage)),
 
     ungated!(
         doc, Normal, template!(List: "hidden|inline|...", NameValueStr: "string"), DuplicatesOk