3 files changed, 36 insertions, 10 deletions
diff --git a/compiler/rustc_middle/src/middle/mod.rs b/compiler/rustc_middle/src/middle/mod.rs
index fc35cafcc77..8dc68b1f5a8 100644
--- a/compiler/rustc_middle/src/middle/mod.rs
+++ b/compiler/rustc_middle/src/middle/mod.rs
@@ -3,14 +3,14 @@ pub mod dependency_format;
 pub mod exported_symbols;
 pub mod lang_items;
 pub mod lib_features {
-    use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-    use rustc_span::symbol::Symbol;
+    use rustc_data_structures::fx::FxHashMap;
+    use rustc_span::{symbol::Symbol, Span};
 
     #[derive(HashStable, Debug)]
     pub struct LibFeatures {
-        // A map from feature to stabilisation version.
-        pub stable: FxHashMap<Symbol, Symbol>,
-        pub unstable: FxHashSet<Symbol>,
+        /// A map from feature to stabilisation version.
+        pub stable: FxHashMap<Symbol, (Symbol, Span)>,
+        pub unstable: FxHashMap<Symbol, Span>,
     }
 
     impl LibFeatures {
@@ -18,8 +18,8 @@ pub mod lib_features {
             let mut all_features: Vec<_> = self
                 .stable
                 .iter()
-                .map(|(f, s)| (*f, Some(*s)))
-                .chain(self.unstable.iter().map(|f| (*f, None)))
+                .map(|(f, (s, _))| (*f, Some(*s)))
+                .chain(self.unstable.iter().map(|(f, _)| (*f, None)))
                 .collect();
             all_features.sort_unstable_by(|a, b| a.0.as_str().partial_cmp(b.0.as_str()).unwrap());
             all_features
diff --git a/compiler/rustc_middle/src/middle/stability.rs b/compiler/rustc_middle/src/middle/stability.rs
index 96e068a3601..0fbad3f0f0f 100644
--- a/compiler/rustc_middle/src/middle/stability.rs
+++ b/compiler/rustc_middle/src/middle/stability.rs
@@ -62,6 +62,19 @@ pub struct Index {
     pub stab_map: FxHashMap<LocalDefId, Stability>,
     pub const_stab_map: FxHashMap<LocalDefId, ConstStability>,
     pub depr_map: FxHashMap<LocalDefId, DeprecationEntry>,
+    /// Mapping from feature name to feature name based on the `implied_by` field of `#[unstable]`
+    /// attributes. If a `#[unstable(feature = "implier", implied_by = "impliee")]` attribute
+    /// exists, then this map will have a `impliee -> implier` entry.
+    ///
+    /// This mapping is necessary unless both the `#[stable]` and `#[unstable]` attributes should
+    /// specify their implications (both `implies` and `implied_by`). If only one of the two
+    /// attributes do (as in the current implementation, `implied_by` in `#[unstable]`), then this
+    /// mapping is necessary for diagnostics. When a "unnecessary feature attribute" error is
+    /// reported, only the `#[stable]` attribute information is available, so the map is necessary
+    /// to know that the feature implies another feature. If it were reversed, and the `#[stable]`
+    /// attribute had an `implies` meta item, then a map would be necessary when avoiding a "use of
+    /// unstable feature" error for a feature that was implied.
+    pub implications: FxHashMap<Symbol, Symbol>,
 }
 
 impl Index {
@@ -423,7 +436,9 @@ impl<'tcx> TyCtxt<'tcx> {
 
         match stability {
             Some(Stability {
-                level: attr::Unstable { reason, issue, is_soft }, feature, ..
+                level: attr::Unstable { reason, issue, is_soft, implied_by },
+                feature,
+                ..
             }) => {
                 if span.allows_unstable(feature) {
                     debug!("stability: skipping span={:?} since it is internal", span);
@@ -433,6 +448,13 @@ impl<'tcx> TyCtxt<'tcx> {
                     return EvalResult::Allow;
                 }
 
+                // If this item was previously part of a now-stabilized feature which is still
+                // active (i.e. the user hasn't removed the attribute for the stabilized feature
+                // yet) then allow use of this item.
+                if let Some(implied_by) = implied_by && self.features().active(implied_by) {
+                    return EvalResult::Allow;
+                }
+
                 // When we're compiling the compiler itself we may pull in
                 // crates from crates.io, but those crates may depend on other
                 // crates also pulled in from crates.io. We want to ideally be
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 0581ef41f66..466a0fc25f7 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -1634,11 +1634,15 @@ rustc_queries! {
         storage(ArenaCacheSelector<'tcx>)
         desc { "calculating the lib features map" }
     }
-    query defined_lib_features(_: CrateNum)
-        -> &'tcx [(Symbol, Option<Symbol>)] {
+    query defined_lib_features(_: CrateNum) -> &'tcx [(Symbol, Option<Symbol>)] {
         desc { "calculating the lib features defined in a crate" }
         separate_provide_extern
     }
+    query stability_implications(_: CrateNum) -> FxHashMap<Symbol, Symbol> {
+        storage(ArenaCacheSelector<'tcx>)
+        desc { "calculating the implications between `#[unstable]` features defined in a crate" }
+        separate_provide_extern
+    }
     /// Whether the function is an intrinsic
     query is_intrinsic(def_id: DefId) -> bool {
         desc { |tcx| "is_intrinsic({})", tcx.def_path_str(def_id) }