Overhaul config::features.

The new way of doing things:
- Avoids some code duplication.
- Distinguishes the `crate_edition` (which comes from `--edition`) and
  the `features_edition` (which combines `--edition` along with any
  `rustc_20XX_preview` features), which is useful.
- Has a simpler initial loop, one that just looks for
  `rustc_20XX_preview` features in order to compute `features_edition`.
- Creates a fallible alternative to `Features::enabled`, which is
  useful.

It's not easy to see how exactly the old and new code are equivalent,
but it's reassuring to know that the test coverage is quite good for
this stuff.

Author: nnethercote

compiler/rustc_expand/src/config.rs

@@ -13,7 +13,7 @@ use rustc_ast::NodeId;
 use rustc_ast::{self as ast, AttrStyle, Attribute, HasAttrs, HasTokens, MetaItem};
 use rustc_attr as attr;
 use rustc_data_structures::flat_map_in_place::FlatMapInPlace;
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::FxHashSet;
 use rustc_feature::{Feature, Features, State as FeatureState};
 use rustc_feature::{ACCEPTED_FEATURES, ACTIVE_FEATURES, REMOVED_FEATURES};
 use rustc_parse::validate_attr;
@@ -55,55 +55,37 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
     }
 
     let mut features = Features::default();
-    let mut edition_enabled_features = FxHashMap::default();
-    let crate_edition = sess.edition();
-
-    // Enable edition umbrella feature-gates based on the crate edition.
-    // - enable `rust_2015_preview` always
-    // - enable `rust_2018_preview` if the crate edition is 2018 or higher
-    // - enable `rust_2021_preview` if the crate edition is 2021 or higher
-    // - etc.
-    for &edition in ALL_EDITIONS {
-        if edition <= crate_edition {
-            edition_enabled_features.insert(edition.feature_name(), edition);
-        }
-    }
 
-    // Enable edition-dependent features based on the crate edition.
-    // - E.g. `test_2018_feature` if the crate edition is 2018 or higher
-    for feature in active_features_up_to(crate_edition) {
-        feature.set(&mut features);
-        edition_enabled_features.insert(feature.name, crate_edition);
-    }
+    // The edition from `--edition`.
+    let crate_edition = sess.edition();
 
-    // Enable edition umbrella feature-gates that are declared in the code. If
-    // present, enable edition-specific features based on that.
-    // - E.g. enable `test_2018_feature` if the crate edition is 2015 but
-    //   `rust_2018_preview` is present
+    // The maximum of (a) the edition from `--edition` and (b) any edition
+    // umbrella feature-gates declared in the code.
+    // - E.g. if `crate_edition` is 2015 but `rust_2018_preview` is present,
+    //   `feature_edition` is 2018
+    let mut features_edition = crate_edition;
     for attr in krate_attrs {
         for mi in feature_list(attr) {
-            if !mi.is_word() {
-                continue;
-            }
-
-            let name = mi.name_or_empty();
-
-            let edition = ALL_EDITIONS.iter().find(|e| name == e.feature_name()).copied();
-            if let Some(edition) = edition {
-                if edition <= crate_edition {
-                    continue;
-                }
-
-                for feature in active_features_up_to(edition) {
-                    // FIXME(Manishearth) there is currently no way to set
-                    // lib features by edition
-                    feature.set(&mut features);
-                    edition_enabled_features.insert(feature.name, edition);
+            if mi.is_word() {
+                let name = mi.name_or_empty();
+                let edition = ALL_EDITIONS.iter().find(|e| name == e.feature_name()).copied();
+                if let Some(edition) = edition && edition > features_edition {
+                    features_edition = edition;
                 }
             }
         }
     }
 
+    // Enable edition-dependent features based on `features_edition`.
+    // - E.g. enable `test_2018_feature` if `features_edition` is 2018 or higher
+    let mut edition_enabled_features = FxHashSet::default();
+    for feature in active_features_up_to(features_edition) {
+        // FIXME(Manishearth) there is currently no way to set lib features by
+        // edition.
+        edition_enabled_features.insert(feature.name);
+        feature.set(&mut features);
+    }
+
     // Process all features declared in the code.
     for attr in krate_attrs {
         for mi in feature_list(attr) {
@@ -128,30 +110,37 @@ pub fn features(sess: &Session, krate_attrs: &[Attribute]) -> Features {
                 }
             };
 
-            // If the declared feature is edition-specific and already enabled
-            // due to the crate edition or a declared edition umbrella
-            // feature-gate, give a warning.
-            // - E.g. warn if `test_2018_feature` is declared when the crate
-            //   edition is 2018 or higher
+            // If the declared feature is an edition umbrella feature-gate,
+            // warn if it was redundant w.r.t. `crate_edition`.
+            // - E.g. warn if `rust_2018_preview` is declared when
+            //   `crate_edition` is 2018
+            // - E.g. don't warn if `rust_2018_preview` is declared when
+            //   `crate_edition` is 2015.
+            if let Some(&edition) = ALL_EDITIONS.iter().find(|e| name == e.feature_name()) {
+                if edition <= crate_edition {
+                    sess.emit_warning(FeatureIncludedInEdition {
+                        span: mi.span(),
+                        feature: name,
+                        edition,
+                    });
+                }
+                continue;
+            }
+
+            // If the declared feature is edition-dependent and was already
+            // enabled due to `feature_edition`, give a warning.
             // - E.g. warn if `test_2018_feature` is declared when
-            //   `rust_2018_preview` or higher is declared.
-            if let Some(&edition) = edition_enabled_features.get(&name) {
+            //   `feature_edition` is 2018 or higher.
+            if edition_enabled_features.contains(&name) {
                 sess.emit_warning(FeatureIncludedInEdition {
                     span: mi.span(),
                     feature: name,
-                    edition,
+                    edition: features_edition,
                 });
                 continue;
             }
 
-            // If the declared feature is an edition umbrella feature-gate,
-            // ignore it, because it was already handled above.
-            // - E.g. `rust_20XX_preview`
-            if ALL_EDITIONS.iter().any(|e| name == e.feature_name()) {
-                continue;
-            }
-
-            // If the declared feature is removed, issue an error.
+            // If the declared feature has been removed, issue an error.
             if let Some(Feature { state, .. }) = REMOVED_FEATURES.iter().find(|f| name == f.name) {
                 if let FeatureState::Removed { reason } = state {
                     sess.emit_err(FeatureRemoved {