Auto merge of #131321 - RalfJung:feature-activation, r=nnethercote

terminology: #[feature] *enables* a feature (instead of "declaring" or "activating" it)

Mostly, we currently call a feature that has a corresponding `#[feature(name)]` attribute in the current crate a "declared" feature. I think that is confusing as it does not align with what "declaring" usually means. Furthermore, we *also* refer to `#[stable]`/`#[unstable]` as *declaring* a feature (e.g. in [these diagnostics](f25e5abea2/compiler/rustc_passes/messages.ftl (L297-L301))), which aligns better with what "declaring" usually means. To make things worse, the functions  `tcx.features().active(...)` and  `tcx.features().declared(...)` both exist and they are doing almost the same thing (testing whether a corresponding `#[feature(name)]`  exists) except that `active` would ICE if the feature is not an unstable lang feature. On top of this, the callback when a feature is activated/declared is called `set_enabled`, and many comments also talk about "enabling" a feature.

So really, our terminology is just a mess.

I would suggest we use "declaring a feature" for saying that something is/was guarded by a feature (e.g. `#[stable]`/`#[unstable]`), and "enabling a feature" for  `#[feature(name)]`. This PR implements that.

clippy_lints/src/manual_div_ceil.rs

@@ -111,11 +111,7 @@ fn check_int_ty_and_feature(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
     let expr_ty = cx.typeck_results().expr_ty(expr);
     match expr_ty.peel_refs().kind() {
         ty::Uint(_) => true,
-        ty::Int(_) => cx
-            .tcx
-            .features()
-            .declared_features
-            .contains(&Symbol::intern("int_roundings")),
+        ty::Int(_) => cx.tcx.features().enabled(Symbol::intern("int_roundings")),
 
         _ => false,
     }