Auto merge of rust-lang#116437 - nnethercote:rustc_features, r=Nilstrieb

Clean up `rustc_features`

Plenty more to be done, but this is a decent start.

r? `@Nilstrieb`

clippy_lints/src/manual_float_methods.rs

@@ -85,7 +85,7 @@ impl<'tcx> LateLintPass<'tcx> for ManualFloatMethods {
         if !in_external_macro(cx.sess(), expr.span)
             && (
                 matches!(cx.tcx.constness(cx.tcx.hir().enclosing_body_owner(expr.hir_id)), Constness::NotConst)
-                    || cx.tcx.features().active(sym!(const_float_classify))
+                    || cx.tcx.features().declared(sym!(const_float_classify))
             ) && let ExprKind::Binary(kind, lhs, rhs) = expr.kind
             && let ExprKind::Binary(lhs_kind, lhs_lhs, lhs_rhs) = lhs.kind
             && let ExprKind::Binary(rhs_kind, rhs_lhs, rhs_rhs) = rhs.kind