Remove rustc_feature::State.

`State` is used to distinguish active vs accepted vs removed features.
However, these can also be distinguished by their location, in
`ACTIVE_FEATURES`, `ACCEPTED_FEATURES`, and `REMOVED_FEATURES`.

So this commit removes `State` and moves the internals of its variants
next to the `Feature` in each element of `*_FEATURES`, introducing new
types `ActiveFeature` and `RemovedFeature`. (There is no need for
`AcceptedFeature` because `State::Accepted` had no fields.)

This is a tighter type representation, avoids the need for some runtime
checks, and makes the code a bit shorter.

compiler/rustc_feature/src/accepted.rs

@@ -1,6 +1,6 @@
 //! List of the accepted feature gates.
 
-use super::{to_nonzero, Feature, State};
+use super::{to_nonzero, Feature};
 use rustc_span::symbol::sym;
 
 macro_rules! declare_features {
@@ -9,15 +9,12 @@ macro_rules! declare_features {
     )+) => {
         /// Those language feature has since been Accepted (it was once Active)
         pub const ACCEPTED_FEATURES: &[Feature] = &[
-            $(
-                Feature {
-                    state: State::Accepted,
-                    name: sym::$feature,
-                    since: $ver,
-                    issue: to_nonzero($issue),
-                    edition: None,
-                }
-            ),+
+            $(Feature {
+                name: sym::$feature,
+                since: $ver,
+                issue: to_nonzero($issue),
+                edition: None,
+            }),+
         ];
     }
 }

compiler/rustc_feature/src/active.rs

@@ -1,12 +1,17 @@
 //! List of the active feature gates.
 
-use super::{to_nonzero, Feature, State};
+use super::{to_nonzero, Feature};
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_span::edition::Edition;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::Span;
 
+pub struct ActiveFeature {
+    pub feature: Feature,
+    pub set_enabled: fn(&mut Features),
+}
+
 #[derive(PartialEq)]
 enum FeatureStatus {
     Default,
@@ -32,21 +37,18 @@ macro_rules! declare_features {
     )+) => {
         /// Represents active features that are currently being implemented or
         /// currently being considered for addition/removal.
-        pub const ACTIVE_FEATURES:
-            &[Feature] =
-            &[$(
-                // (sym::$feature, $ver, $issue, $edition, set!($feature))
-                Feature {
-                    state: State::Active {
-                        // Sets this feature's corresponding bool within `features`.
-                        set: |features| features.$feature = true,
-                    },
+        pub const ACTIVE_FEATURES: &[ActiveFeature] = &[
+            $(ActiveFeature {
+                feature: Feature {
                     name: sym::$feature,
                     since: $ver,
                     issue: to_nonzero($issue),
                     edition: $edition,
-                }
-            ),+];
+                },
+                // Sets this feature's corresponding bool within `features`.
+                set_enabled: |features| features.$feature = true,
+            }),+
+        ];
 
         /// A set of features to be used by later passes.
         #[derive(Clone, Default, Debug)]
@@ -134,16 +136,6 @@ macro_rules! declare_features {
     };
 }
 
-impl Feature {
-    /// Sets this feature in `Features`. Panics if called on a non-active feature.
-    pub fn set(&self, features: &mut Features) {
-        match self.state {
-            State::Active { set } => set(features),
-            _ => panic!("called `set` on feature `{}` which is not `active`", self.name),
-        }
-    }
-}
-
 // See https://rustc-dev-guide.rust-lang.org/feature-gates.html#feature-gates for more
 // documentation about handling feature gates.
 //

compiler/rustc_feature/src/lib.rs

@@ -24,29 +24,10 @@ mod removed;
 mod tests;
 
 use rustc_span::{edition::Edition, symbol::Symbol};
-use std::fmt;
 use std::num::NonZeroU32;
 
-#[derive(Clone, Copy)]
-pub enum State {
-    Accepted,
-    Active { set: fn(&mut Features) },
-    Removed { reason: Option<&'static str> },
-}
-
-impl fmt::Debug for State {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
-            State::Accepted { .. } => write!(f, "accepted"),
-            State::Active { .. } => write!(f, "active"),
-            State::Removed { .. } => write!(f, "removed"),
-        }
-    }
-}
-
 #[derive(Debug, Clone)]
 pub struct Feature {
-    pub state: State,
     pub name: Symbol,
     pub since: &'static str,
     issue: Option<NonZeroU32>,
@@ -106,17 +87,16 @@ impl UnstableFeatures {
 
 fn find_lang_feature_issue(feature: Symbol) -> Option<NonZeroU32> {
     // Search in all the feature lists.
-    let found = []
-        .iter()
-        .chain(ACTIVE_FEATURES)
-        .chain(ACCEPTED_FEATURES)
-        .chain(REMOVED_FEATURES)
-        .find(|t| t.name == feature);
-
-    match found {
-        Some(found) => found.issue,
-        None => panic!("feature `{feature}` is not declared anywhere"),
+    if let Some(f) = ACTIVE_FEATURES.iter().find(|f| f.feature.name == feature) {
+        return f.feature.issue;
     }
+    if let Some(f) = ACCEPTED_FEATURES.iter().find(|f| f.name == feature) {
+        return f.issue;
+    }
+    if let Some(f) = REMOVED_FEATURES.iter().find(|f| f.feature.name == feature) {
+        return f.feature.issue;
+    }
+    panic!("feature `{feature}` is not declared anywhere");
 }
 
 const fn to_nonzero(n: Option<u32>) -> Option<NonZeroU32> {

compiler/rustc_feature/src/removed.rs

@@ -1,23 +1,28 @@
 //! List of the removed feature gates.
 
-use super::{to_nonzero, Feature, State};
+use super::{to_nonzero, Feature};
 use rustc_span::symbol::sym;
 
+pub struct RemovedFeature {
+    pub feature: Feature,
+    pub reason: Option<&'static str>,
+}
+
 macro_rules! declare_features {
     ($(
         $(#[doc = $doc:tt])* (removed, $feature:ident, $ver:expr, $issue:expr, None, $reason:expr),
     )+) => {
         /// Represents unstable features which have since been removed (it was once Active)
-        pub const REMOVED_FEATURES: &[Feature] = &[
-            $(
-                Feature {
-                    state: State::Removed { reason: $reason },
+        pub const REMOVED_FEATURES: &[RemovedFeature] = &[
+            $(RemovedFeature {
+                feature: Feature {
                     name: sym::$feature,
                     since: $ver,
                     issue: to_nonzero($issue),
                     edition: None,
-                }
-            ),+
+                },
+                reason: $reason
+            }),+
         ];
     };
 }