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](https://github.com/rust-lang/rust/blob/f25e5abea229a6b6aa77b45e21cb784e785c6040/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.

2 files changed, 3 insertions, 3 deletions

diff --git a/compiler/rustc_lint/src/builtin.rs b/compiler/rustc_lint/src/builtin.rs
index ef8100da9e2..125fe9b3f16 100644
--- a/compiler/rustc_lint/src/builtin.rs
+++ b/compiler/rustc_lint/src/builtin.rs
@@ -2288,10 +2288,10 @@ impl EarlyLintPass for IncompleteInternalFeatures {
     fn check_crate(&mut self, cx: &EarlyContext<'_>, _: &ast::Crate) {
         let features = cx.builder.features();
         features
-            .declared_lang_features
+            .enabled_lang_features()
             .iter()
             .map(|(name, span, _)| (name, span))
-            .chain(features.declared_lib_features.iter().map(|(name, span)| (name, span)))
+            .chain(features.enabled_lib_features().iter().map(|(name, span)| (name, span)))
             .filter(|(&name, _)| features.incomplete(name) || features.internal(name))
             .for_each(|(&name, &span)| {
                 if features.incomplete(name) {
diff --git a/compiler/rustc_lint/src/levels.rs b/compiler/rustc_lint/src/levels.rs
index 95a8e7625ff..ff2ae69e1db 100644
--- a/compiler/rustc_lint/src/levels.rs
+++ b/compiler/rustc_lint/src/levels.rs
@@ -858,7 +858,7 @@ impl<'s, P: LintLevelsProvider> LintLevelsBuilder<'s, P> {
     #[track_caller]
     fn check_gated_lint(&self, lint_id: LintId, span: Span, lint_from_cli: bool) -> bool {
         let feature = if let Some(feature) = lint_id.lint.feature_gate
-            && !self.features.active(feature)
+            && !self.features.enabled(feature)
         {
             // Lint is behind a feature that is not enabled; eventually return false.
             feature