Emit error when marking as unstable specifying an stable feature

compiler/rustc_passes/messages.ftl

@@ -745,6 +745,12 @@ passes_unrecognized_repr_hint =
     unrecognized representation hint
     .help = valid reprs are `Rust` (default), `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
+passes_unstable_attr_for_already_stable_feature =
+    can't mark as unstable using an already stable feature
+    .label = this feature is already stable
+    .item = the stability attribute annotates this item
+    .help = Consider removing the attribute
+
 passes_unused =
     unused attribute
     .suggestion = remove this attribute

compiler/rustc_passes/src/errors.rs

@@ -1496,6 +1496,17 @@ pub(crate) struct CannotStabilizeDeprecated {
     pub item_sp: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_unstable_attr_for_already_stable_feature)]
+pub(crate) struct UnstableAttrForAlreadyStableFeature {
+    #[primary_span]
+    #[label]
+    #[help]
+    pub span: Span,
+    #[label(passes_item)]
+    pub item_sp: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_missing_stability_attr)]
 pub(crate) struct MissingStabilityAttr<'a> {

compiler/rustc_passes/src/stability.rs

@@ -10,6 +10,7 @@ use rustc_attr::{
 };
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::unord::{ExtendUnord, UnordMap, UnordSet};
+use rustc_feature::ACCEPTED_FEATURES;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{CRATE_DEF_ID, LOCAL_CRATE, LocalDefId, LocalModDefId};
@@ -246,12 +247,27 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
                 }
             }
 
+            if let Stability { level: Unstable { .. }, feature } = stab {
+                if ACCEPTED_FEATURES.iter().find(|f| f.name == feature).is_some() {
+                    self.tcx
+                        .dcx()
+                        .emit_err(errors::UnstableAttrForAlreadyStableFeature { span, item_sp });
+                }
+            }
             if let Stability { level: Unstable { implied_by: Some(implied_by), .. }, feature } =
                 stab
             {
                 self.index.implications.insert(implied_by, feature);
             }
 
+            if let Some(ConstStability { level: Unstable { .. }, feature, .. }) = const_stab {
+                if ACCEPTED_FEATURES.iter().find(|f| f.name == feature).is_some() {
+                    self.tcx.dcx().emit_err(errors::UnstableAttrForAlreadyStableFeature {
+                        span: const_span.unwrap(), // If const_stab contains Some(..), same is true for const_span
+                        item_sp,
+                    });
+                }
+            }
             if let Some(ConstStability {
                 level: Unstable { implied_by: Some(implied_by), .. },
                 feature,

tests/ui/feature-gates/unstable-attribute-rejects-already-stable-features.rs

@@ -0,0 +1,11 @@
+//! Ensure #[unstable] doesn't accept already stable features
+//@ check-fail
+
+#![feature(staged_api)]
+#![stable(feature = "rust_test", since = "1.0.0")]
+
+#[unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
+#[rustc_const_unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
+const fn my_fun() {}
+
+fn main() {}