Rollup merge of #152746 - cyrgani:stable-features, r=petrochenkov

remove `#![allow(stable_features)]` from most tests

The only remaining usages are tests that specifically deal with feature gates.
This also deletes the very weird `#![feature(issue_5723_bootstrap)]`, a 13 year old "temporary fix" (rust-lang/rust#5723).

compiler/rustc_feature/src/accepted.rs

@@ -25,9 +25,6 @@ declare_features! (
     // feature-group-start: for testing purposes
     // -------------------------------------------------------------------------
 
-    /// A temporary feature gate used to enable parser extensions needed
-    /// to bootstrap fix for #5723.
-    (accepted, issue_5723_bootstrap, "1.0.0", None),
     /// These are used to test this portion of the compiler,
     /// they don't actually mean anything.
     (accepted, test_accepted_feature, "1.0.0", None),

compiler/rustc_feature/src/removed.rs

@@ -172,6 +172,9 @@ declare_features! (
     /// Allow anonymous constants from an inline `const` block in pattern position
     (removed, inline_const_pat, "1.88.0", Some(76001),
      Some("removed due to implementation concerns as it requires significant refactorings"), 138492),
+    /// A temporary feature gate used to enable parser extensions needed
+    /// to bootstrap fix for #5723.
+    (removed, issue_5723_bootstrap, "CURRENT_RUSTC_VERSION", None, None),
     /// Lazily evaluate constants. This allows constants to depend on type parameters.
     (removed, lazy_normalization_consts, "1.56.0", Some(72219), Some("superseded by `generic_const_exprs`"), 88369),
     /// Changes `impl Trait` to capture all lifetimes in scope.