Revert "middle: add implies_by to #[unstable]"

This reverts commit 224aec2.

Author: davidtwco

compiler/rustc_attr/src/builtin.rs

@@ -142,26 +142,6 @@ pub enum StabilityLevel {
         /// Relevant `rust-lang/rust` issue.
         issue: Option<NonZeroU32>,
         is_soft: bool,
-        /// If part of a feature is stabilized and a new feature is added for the remaining parts,
-        /// then the `implied_by` attribute is used to indicate which now-stable feature previously
-        /// contained a item.
-        ///
-        /// ```pseudo-Rust
-        /// #[unstable(feature = "foo", issue = "...")]
-        /// fn foo() {}
-        /// #[unstable(feature = "foo", issue = "...")]
-        /// fn foobar() {}
-        /// ```
-        ///
-        /// ...becomes...
-        ///
-        /// ```pseudo-Rust
-        /// #[stable(feature = "foo", since = "1.XX.X")]
-        /// fn foo() {}
-        /// #[unstable(feature = "foobar", issue = "...", implied_by = "foo")]
-        /// fn foobar() {}
-        /// ```
-        implied_by: Option<Symbol>,
     },
     /// `#[stable]`
     Stable {
@@ -302,7 +282,6 @@ where
                     let mut issue = None;
                     let mut issue_num = None;
                     let mut is_soft = false;
-                    let mut implied_by = None;
                     for meta in metas {
                         let Some(mi) = meta.meta_item() else {
                             handle_errors(
@@ -368,11 +347,6 @@ where
                                 }
                                 is_soft = true;
                             }
-                            sym::implied_by => {
-                                if !get(mi, &mut implied_by) {
-                                    continue 'outer;
-                                }
-                            }
                             _ => {
                                 handle_errors(
                                     &sess.parse_sess,
@@ -401,7 +375,6 @@ where
                                 reason: UnstableReason::from_opt_reason(reason),
                                 issue: issue_num,
                                 is_soft,
-                                implied_by,
                             };
                             if sym::unstable == meta_name {
                                 stab = Some((Stability { level, feature }, attr.span));

compiler/rustc_middle/src/middle/stability.rs

@@ -423,9 +423,7 @@ impl<'tcx> TyCtxt<'tcx> {
 
         match stability {
             Some(Stability {
-                level: attr::Unstable { reason, issue, is_soft, implied_by },
-                feature,
-                ..
+                level: attr::Unstable { reason, issue, is_soft }, feature, ..
             }) => {
                 if span.allows_unstable(feature) {
                     debug!("stability: skipping span={:?} since it is internal", span);
@@ -435,13 +433,6 @@ 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

compiler/rustc_passes/src/stability.rs

@@ -634,7 +634,6 @@ fn stability_index(tcx: TyCtxt<'_>, (): ()) -> Index {
                     reason: UnstableReason::Default,
                     issue: NonZeroU32::new(27812),
                     is_soft: false,
-                    implied_by: None,
                 },
                 feature: sym::rustc_private,
             };

compiler/rustc_resolve/src/macros.rs

@@ -796,16 +796,9 @@ impl<'a> Resolver<'a> {
     ) {
         let span = path.span;
         if let Some(stability) = &ext.stability {
-            if let StabilityLevel::Unstable { reason, issue, is_soft, implied_by } = stability.level
-            {
+            if let StabilityLevel::Unstable { reason, issue, is_soft } = stability.level {
                 let feature = stability.feature;
-
-                let is_allowed = |feature| {
-                    self.active_features.contains(&feature) || span.allows_unstable(feature)
-                };
-                let allowed_by_implication =
-                    implied_by.map(|feature| is_allowed(feature)).unwrap_or(false);
-                if !is_allowed(feature) && !allowed_by_implication {
+                if !self.active_features.contains(&feature) && !span.allows_unstable(feature) {
                     let lint_buffer = &mut self.lint_buffer;
                     let soft_handler =
                         |lint, span, msg: &_| lint_buffer.buffer_lint(lint, node_id, span, msg);

compiler/rustc_span/src/symbol.rs

@@ -800,7 +800,6 @@ symbols! {
         impl_lint_pass,
         impl_macros,
         impl_trait_in_bindings,
-        implied_by,
         import,
         import_shadowing,
         imported_main,