Rollup merge of #66895 - Centril:rustc_feature, r=oli-obk

Feature gating *declarations* => new crate `rustc_feature`

This PR moves the data-oriented parts of feature gating into its own crate, `rustc_feature`.
The parts consist of some data types as well as `accepted`, `active`, `removed`, and `builtin_attrs`.

Feature gate checking itself remains in `syntax::feature_gate::check`. The parts which define how to emit feature gate errors could probably be moved to `rustc_errors` or to the new `rustc_session` crate introduced in #66878. The visitor itself could probably be moved as a pass in `rustc_passes` depending on how the dependency edges work out.

The PR also contains some drive-by cleanup of feature gate checking. As such, the PR probably best read commit-by-commit.

r? @oli-obk
cc @petrochenkov
cc @Mark-Simulacrum

10 files changed, 49 insertions, 1758 deletions

diff --git a/src/libsyntax/Cargo.toml b/src/libsyntax/Cargo.toml
index dff23076c82..085c1760c80 100644
--- a/src/libsyntax/Cargo.toml
+++ b/src/libsyntax/Cargo.toml
@@ -18,6 +18,7 @@ lazy_static = "1.0.0"
 syntax_pos = { path = "../libsyntax_pos" }
 errors = { path = "../librustc_errors", package = "rustc_errors" }
 rustc_data_structures = { path = "../librustc_data_structures" }
+rustc_feature = { path = "../librustc_feature" }
 rustc_index = { path = "../librustc_index" }
 rustc_lexer = { path = "../librustc_lexer" }
 rustc_macros = { path = "../librustc_macros" }
diff --git a/src/libsyntax/attr/builtin.rs b/src/libsyntax/attr/builtin.rs
index c10541c8c7e..3c10f27b60a 100644
--- a/src/libsyntax/attr/builtin.rs
+++ b/src/libsyntax/attr/builtin.rs
@@ -1,7 +1,8 @@
 //! Parsing and validation of builtin attributes
 
+use super::{mark_used, MetaItemKind};
 use crate::ast::{self, Attribute, MetaItem, NestedMetaItem};
-use crate::feature_gate::{Features, GatedCfg};
+use crate::feature_gate::feature_err;
 use crate::print::pprust;
 use crate::sess::ParseSess;
 
@@ -9,12 +10,15 @@ use errors::{Applicability, Handler};
 use std::num::NonZeroU32;
 use syntax_pos::hygiene::Transparency;
 use syntax_pos::{symbol::Symbol, symbol::sym, Span};
+use rustc_feature::{Features, find_gated_cfg, GatedCfg, is_builtin_attr_name};
 use rustc_macros::HashStable_Generic;
 
-use super::{mark_used, MetaItemKind};
-
 use rustc_error_codes::*;
 
+pub fn is_builtin_attr(attr: &Attribute) -> bool {
+    attr.ident().filter(|ident| is_builtin_attr_name(ident.name)).is_some()
+}
+
 enum AttrError {
     MultipleItem(String),
     UnknownMetaItem(String, &'static [&'static str]),
@@ -24,31 +28,6 @@ enum AttrError {
     UnsupportedLiteral(&'static str, /* is_bytestr */ bool),
 }
 
-/// A template that the attribute input must match.
-/// Only top-level shape (`#[attr]` vs `#[attr(...)]` vs `#[attr = ...]`) is considered now.
-#[derive(Clone, Copy)]
-pub struct AttributeTemplate {
-    pub word: bool,
-    pub list: Option<&'static str>,
-    pub name_value_str: Option<&'static str>,
-}
-
-impl AttributeTemplate {
-    pub fn only_word() -> Self {
-        Self { word: true, list: None, name_value_str: None }
-    }
-
-    /// Checks that the given meta-item is compatible with this template.
-    pub fn compatible(&self, meta_item_kind: &ast::MetaItemKind) -> bool {
-        match meta_item_kind {
-            ast::MetaItemKind::Word => self.word,
-            ast::MetaItemKind::List(..) => self.list.is_some(),
-            ast::MetaItemKind::NameValue(lit) if lit.kind.is_str() => self.name_value_str.is_some(),
-            ast::MetaItemKind::NameValue(..) => false,
-        }
-    }
-}
-
 fn handle_errors(sess: &ParseSess, span: Span, error: AttrError) {
     let diag = &sess.span_diagnostic;
     match error {
@@ -555,8 +534,9 @@ pub fn find_crate_name(attrs: &[Attribute]) -> Option<Symbol> {
 /// Tests if a cfg-pattern matches the cfg set
 pub fn cfg_matches(cfg: &ast::MetaItem, sess: &ParseSess, features: Option<&Features>) -> bool {
     eval_condition(cfg, sess, &mut |cfg| {
-        if let (Some(feats), Some(gated_cfg)) = (features, GatedCfg::gate(cfg)) {
-            gated_cfg.check_and_emit(sess, feats);
+        let gate = find_gated_cfg(|sym| cfg.check_name(sym));
+        if let (Some(feats), Some(gated_cfg)) = (features, gate) {
+            gate_cfg(&gated_cfg, cfg.span, sess, feats);
         }
         let error = |span, msg| { sess.span_diagnostic.span_err(span, msg); true };
         if cfg.path.segments.len() != 1 {
@@ -585,12 +565,21 @@ pub fn cfg_matches(cfg: &ast::MetaItem, sess: &ParseSess, features: Option<&Feat
     })
 }
 
+fn gate_cfg(gated_cfg: &GatedCfg, cfg_span: Span, sess: &ParseSess, features: &Features) {
+    let (cfg, feature, has_feature) = gated_cfg;
+    if !has_feature(features) && !cfg_span.allows_unstable(*feature) {
+        let explain = format!("`cfg({})` is experimental and subject to change", cfg);
+        feature_err(sess, *feature, cfg_span, &explain).emit()
+    }
+}
+
 /// Evaluate a cfg-like condition (with `any` and `all`), using `eval` to
 /// evaluate individual items.
-pub fn eval_condition<F>(cfg: &ast::MetaItem, sess: &ParseSess, eval: &mut F)
-                         -> bool
-    where F: FnMut(&ast::MetaItem) -> bool
-{
+pub fn eval_condition(
+    cfg: &ast::MetaItem,
+    sess: &ParseSess,
+    eval: &mut impl FnMut(&ast::MetaItem) -> bool,
+) -> bool {
     match cfg.kind {
         ast::MetaItemKind::List(ref mis) => {
             for mi in mis.iter() {
diff --git a/src/libsyntax/feature_gate/accepted.rs b/src/libsyntax/feature_gate/accepted.rs
deleted file mode 100644
index dab83f48a03..00000000000
--- a/src/libsyntax/feature_gate/accepted.rs
+++ /dev/null
@@ -1,265 +0,0 @@
-//! List of the accepted feature gates.
-
-use crate::symbol::sym;
-use super::{State, Feature};
-
-macro_rules! declare_features {
-    ($(
-        $(#[doc = $doc:tt])* (accepted, $feature:ident, $ver:expr, $issue:expr, None),
-    )+) => {
-        /// 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: $issue,
-                    edition: None,
-                    description: concat!($($doc,)*),
-                }
-            ),+
-        ];
-    }
-}
-
-declare_features! (
-    // -------------------------------------------------------------------------
-    // feature-group-start: for testing purposes
-    // -------------------------------------------------------------------------
-
-    /// A temporary feature gate used to enable parser extensions needed
-    /// to bootstrap fix for #5723.
-    (accepted, issue_5723_bootstrap, "1.0.0", None, None),
-    /// These are used to test this portion of the compiler,
-    /// they don't actually mean anything.
-    (accepted, test_accepted_feature, "1.0.0", None, None),
-
-    // -------------------------------------------------------------------------
-    // feature-group-end: for testing purposes
-    // -------------------------------------------------------------------------
-
-    // -------------------------------------------------------------------------
-    // feature-group-start: accepted features
-    // -------------------------------------------------------------------------
-
-    /// Allows using associated `type`s in `trait`s.
-    (accepted, associated_types, "1.0.0", None, None),
-    /// Allows using assigning a default type to type parameters in algebraic data type definitions.
-    (accepted, default_type_params, "1.0.0", None, None),
-    // FIXME: explain `globs`.
-    (accepted, globs, "1.0.0", None, None),
-    /// Allows `macro_rules!` items.
-    (accepted, macro_rules, "1.0.0", None, None),
-    /// Allows use of `&foo[a..b]` as a slicing syntax.
-    (accepted, slicing_syntax, "1.0.0", None, None),
-    /// Allows struct variants `Foo { baz: u8, .. }` in enums (RFC 418).
-    (accepted, struct_variant, "1.0.0", None, None),
-    /// Allows indexing tuples.
-    (accepted, tuple_indexing, "1.0.0", None, None),
-    /// Allows the use of `if let` expressions.
-    (accepted, if_let, "1.0.0", None, None),
-    /// Allows the use of `while let` expressions.
-    (accepted, while_let, "1.0.0", None, None),
-    /// Allows using `#![no_std]`.
-    (accepted, no_std, "1.6.0", None, None),
-    /// Allows overloading augmented assignment operations like `a += b`.
-    (accepted, augmented_assignments, "1.8.0", Some(28235), None),
-    /// Allows empty structs and enum variants with braces.
-    (accepted, braced_empty_structs, "1.8.0", Some(29720), None),
-    /// Allows `#[deprecated]` attribute.
-    (accepted, deprecated, "1.9.0", Some(29935), None),
-    /// Allows macros to appear in the type position.
-    (accepted, type_macros, "1.13.0", Some(27245), None),
-    /// Allows use of the postfix `?` operator in expressions.
-    (accepted, question_mark, "1.13.0", Some(31436), None),
-    /// Allows `..` in tuple (struct) patterns.
-    (accepted, dotdot_in_tuple_patterns, "1.14.0", Some(33627), None),
-    /// Allows some increased flexibility in the name resolution rules,
-    /// especially around globs and shadowing (RFC 1560).
-    (accepted, item_like_imports, "1.15.0", Some(35120), None),
-    /// Allows using `Self` and associated types in struct expressions and patterns.
-    (accepted, more_struct_aliases, "1.16.0", Some(37544), None),
-    /// Allows elision of `'static` lifetimes in `static`s and `const`s.
-    (accepted, static_in_const, "1.17.0", Some(35897), None),
-    /// Allows field shorthands (`x` meaning `x: x`) in struct literal expressions.
-    (accepted, field_init_shorthand, "1.17.0", Some(37340), None),
-    /// Allows the definition recursive static items.
-    (accepted, static_recursion, "1.17.0", Some(29719), None),
-    /// Allows `pub(restricted)` visibilities (RFC 1422).
-    (accepted, pub_restricted, "1.18.0", Some(32409), None),
-    /// Allows `#![windows_subsystem]`.
-    (accepted, windows_subsystem, "1.18.0", Some(37499), None),
-    /// Allows `break {expr}` with a value inside `loop`s.
-    (accepted, loop_break_value, "1.19.0", Some(37339), None),
-    /// Allows numeric fields in struct expressions and patterns.
-    (accepted, relaxed_adts, "1.19.0", Some(35626), None),
-    /// Allows coercing non capturing closures to function pointers.
-    (accepted, closure_to_fn_coercion, "1.19.0", Some(39817), None),
-    /// Allows attributes on struct literal fields.
-    (accepted, struct_field_attributes, "1.20.0", Some(38814), None),
-    /// Allows the definition of associated constants in `trait` or `impl` blocks.
-    (accepted, associated_consts, "1.20.0", Some(29646), None),
-    /// Allows usage of the `compile_error!` macro.
-    (accepted, compile_error, "1.20.0", Some(40872), None),
-    /// Allows code like `let x: &'static u32 = &42` to work (RFC 1414).
-    (accepted, rvalue_static_promotion, "1.21.0", Some(38865), None),
-    /// Allows `Drop` types in constants (RFC 1440).
-    (accepted, drop_types_in_const, "1.22.0", Some(33156), None),
-    /// Allows the sysV64 ABI to be specified on all platforms
-    /// instead of just the platforms on which it is the C ABI.
-    (accepted, abi_sysv64, "1.24.0", Some(36167), None),
-    /// Allows `repr(align(16))` struct attribute (RFC 1358).
-    (accepted, repr_align, "1.25.0", Some(33626), None),
-    /// Allows '|' at beginning of match arms (RFC 1925).
-    (accepted, match_beginning_vert, "1.25.0", Some(44101), None),
-    /// Allows nested groups in `use` items (RFC 2128).
-    (accepted, use_nested_groups, "1.25.0", Some(44494), None),
-    /// Allows indexing into constant arrays.
-    (accepted, const_indexing, "1.26.0", Some(29947), None),
-    /// Allows using `a..=b` and `..=b` as inclusive range syntaxes.
-    (accepted, inclusive_range_syntax, "1.26.0", Some(28237), None),
-    /// Allows `..=` in patterns (RFC 1192).
-    (accepted, dotdoteq_in_patterns, "1.26.0", Some(28237), None),
-    /// Allows `fn main()` with return types which implements `Termination` (RFC 1937).
-    (accepted, termination_trait, "1.26.0", Some(43301), None),
-    /// Allows implementing `Clone` for closures where possible (RFC 2132).
-    (accepted, clone_closures, "1.26.0", Some(44490), None),
-    /// Allows implementing `Copy` for closures where possible (RFC 2132).
-    (accepted, copy_closures, "1.26.0", Some(44490), None),
-    /// Allows `impl Trait` in function arguments.
-    (accepted, universal_impl_trait, "1.26.0", Some(34511), None),
-    /// Allows `impl Trait` in function return types.
-    (accepted, conservative_impl_trait, "1.26.0", Some(34511), None),
-    /// Allows using the `u128` and `i128` types.
-    (accepted, i128_type, "1.26.0", Some(35118), None),
-    /// Allows default match binding modes (RFC 2005).
-    (accepted, match_default_bindings, "1.26.0", Some(42640), None),
-    /// Allows `'_` placeholder lifetimes.
-    (accepted, underscore_lifetimes, "1.26.0", Some(44524), None),
-    /// Allows attributes on lifetime/type formal parameters in generics (RFC 1327).
-    (accepted, generic_param_attrs, "1.27.0", Some(48848), None),
-    /// Allows `cfg(target_feature = "...")`.
-    (accepted, cfg_target_feature, "1.27.0", Some(29717), None),
-    /// Allows `#[target_feature(...)]`.
-    (accepted, target_feature, "1.27.0", None, None),
-    /// Allows using `dyn Trait` as a syntax for trait objects.
-    (accepted, dyn_trait, "1.27.0", Some(44662), None),
-    /// Allows `#[must_use]` on functions, and introduces must-use operators (RFC 1940).
-    (accepted, fn_must_use, "1.27.0", Some(43302), None),
-    /// Allows use of the `:lifetime` macro fragment specifier.
-    (accepted, macro_lifetime_matcher, "1.27.0", Some(34303), None),
-    /// Allows `#[test]` functions where the return type implements `Termination` (RFC 1937).
-    (accepted, termination_trait_test, "1.27.0", Some(48854), None),
-    /// Allows the `#[global_allocator]` attribute.
-    (accepted, global_allocator, "1.28.0", Some(27389), None),
-    /// Allows `#[repr(transparent)]` attribute on newtype structs.
-    (accepted, repr_transparent, "1.28.0", Some(43036), None),
-    /// Allows procedural macros in `proc-macro` crates.
-    (accepted, proc_macro, "1.29.0", Some(38356), None),
-    /// Allows `foo.rs` as an alternative to `foo/mod.rs`.
-    (accepted, non_modrs_mods, "1.30.0", Some(44660), None),
-    /// Allows use of the `:vis` macro fragment specifier
-    (accepted, macro_vis_matcher, "1.30.0", Some(41022), None),
-    /// Allows importing and reexporting macros with `use`,
-    /// enables macro modularization in general.
-    (accepted, use_extern_macros, "1.30.0", Some(35896), None),
-    /// Allows keywords to be escaped for use as identifiers.
-    (accepted, raw_identifiers, "1.30.0", Some(48589), None),
-    /// Allows attributes scoped to tools.
-    (accepted, tool_attributes, "1.30.0", Some(44690), None),
-    /// Allows multi-segment paths in attributes and derives.
-    (accepted, proc_macro_path_invoc, "1.30.0", Some(38356), None),
-    /// Allows all literals in attribute lists and values of key-value pairs.
-    (accepted, attr_literals, "1.30.0", Some(34981), None),
-    /// Allows inferring outlives requirements (RFC 2093).
-    (accepted, infer_outlives_requirements, "1.30.0", Some(44493), None),
-    /// Allows annotating functions conforming to `fn(&PanicInfo) -> !` with `#[panic_handler]`.
-    /// This defines the behavior of panics.
-    (accepted, panic_handler, "1.30.0", Some(44489), None),
-    /// Allows `#[used]` to preserve symbols (see llvm.used).
-    (accepted, used, "1.30.0", Some(40289), None),
-    /// Allows `crate` in paths.
-    (accepted, crate_in_paths, "1.30.0", Some(45477), None),
-    /// Allows resolving absolute paths as paths from other crates.
-    (accepted, extern_absolute_paths, "1.30.0", Some(44660), None),
-    /// Allows access to crate names passed via `--extern` through prelude.
-    (accepted, extern_prelude, "1.30.0", Some(44660), None),
-    /// Allows parentheses in patterns.
-    (accepted, pattern_parentheses, "1.31.0", Some(51087), None),
-    /// Allows the definition of `const fn` functions.
-    (accepted, min_const_fn, "1.31.0", Some(53555), None),
-    /// Allows scoped lints.
-    (accepted, tool_lints, "1.31.0", Some(44690), None),
-    /// Allows lifetime elision in `impl` headers. For example:
-    /// + `impl<I:Iterator> Iterator for &mut Iterator`
-    /// + `impl Debug for Foo<'_>`
-    (accepted, impl_header_lifetime_elision, "1.31.0", Some(15872), None),
-    /// Allows `extern crate foo as bar;`. This puts `bar` into extern prelude.
-    (accepted, extern_crate_item_prelude, "1.31.0", Some(55599), None),
-    /// Allows use of the `:literal` macro fragment specifier (RFC 1576).
-    (accepted, macro_literal_matcher, "1.32.0", Some(35625), None),
-    /// Allows use of `?` as the Kleene "at most one" operator in macros.
-    (accepted, macro_at_most_once_rep, "1.32.0", Some(48075), None),
-    /// Allows `Self` struct constructor (RFC 2302).
-    (accepted, self_struct_ctor, "1.32.0", Some(51994), None),
-    /// Allows `Self` in type definitions (RFC 2300).
-    (accepted, self_in_typedefs, "1.32.0", Some(49303), None),
-    /// Allows `use x::y;` to search `x` in the current scope.
-    (accepted, uniform_paths, "1.32.0", Some(53130), None),
-    /// Allows integer match exhaustiveness checking (RFC 2591).
-    (accepted, exhaustive_integer_patterns, "1.33.0", Some(50907), None),
-    /// Allows `use path as _;` and `extern crate c as _;`.
-    (accepted, underscore_imports, "1.33.0", Some(48216), None),
-    /// Allows `#[repr(packed(N))]` attribute on structs.
-    (accepted, repr_packed, "1.33.0", Some(33158), None),
-    /// Allows irrefutable patterns in `if let` and `while let` statements (RFC 2086).
-    (accepted, irrefutable_let_patterns, "1.33.0", Some(44495), None),
-    /// Allows calling `const unsafe fn` inside `unsafe` blocks in `const fn` functions.
-    (accepted, min_const_unsafe_fn, "1.33.0", Some(55607), None),
-    /// Allows let bindings, assignments and destructuring in `const` functions and constants.
-    /// As long as control flow is not implemented in const eval, `&&` and `||` may not be used
-    /// at the same time as let bindings.
-    (accepted, const_let, "1.33.0", Some(48821), None),
-    /// Allows `#[cfg_attr(predicate, multiple, attributes, here)]`.
-    (accepted, cfg_attr_multi, "1.33.0", Some(54881), None),
-    /// Allows top level or-patterns (`p | q`) in `if let` and `while let`.
-    (accepted, if_while_or_patterns, "1.33.0", Some(48215), None),
-    /// Allows `cfg(target_vendor = "...")`.
-    (accepted, cfg_target_vendor, "1.33.0", Some(29718), None),
-    /// Allows `extern crate self as foo;`.
-    /// This puts local crate root into extern prelude under name `foo`.
-    (accepted, extern_crate_self, "1.34.0", Some(56409), None),
-    /// Allows arbitrary delimited token streams in non-macro attributes.
-    (accepted, unrestricted_attribute_tokens, "1.34.0", Some(55208), None),
-    /// Allows paths to enum variants on type aliases including `Self`.
-    (accepted, type_alias_enum_variants, "1.37.0", Some(49683), None),
-    /// Allows using `#[repr(align(X))]` on enums with equivalent semantics
-    /// to wrapping an enum in a wrapper struct with `#[repr(align(X))]`.
-    (accepted, repr_align_enum, "1.37.0", Some(57996), None),
-    /// Allows `const _: TYPE = VALUE`.
-    (accepted, underscore_const_names, "1.37.0", Some(54912), None),
-    /// Allows free and inherent `async fn`s, `async` blocks, and `<expr>.await` expressions.
-    (accepted, async_await, "1.39.0", Some(50547), None),
-    /// Allows mixing bind-by-move in patterns and references to those identifiers in guards.
-    (accepted, bind_by_move_pattern_guards, "1.39.0", Some(15287), None),
-    /// Allows attributes in formal function parameters.
-    (accepted, param_attrs, "1.39.0", Some(60406), None),
-    /// Allows macro invocations in `extern {}` blocks.
-    (accepted, macros_in_extern, "1.40.0", Some(49476), None),
-    /// Allows future-proofing enums/structs with the `#[non_exhaustive]` attribute (RFC 2008).
-    (accepted, non_exhaustive, "1.40.0", Some(44109), None),
-    /// Allows calling constructor functions in `const fn`.
-    (accepted, const_constructor, "1.40.0", Some(61456), None),
-    /// Allows the use of `#[cfg(doctest)]`, set when rustdoc is collecting doctests.
-    (accepted, cfg_doctest, "1.40.0", Some(62210), None),
-    /// Allows the `!` type. Does not imply 'exhaustive_patterns' any more.
-    (accepted, never_type, "1.41.0", Some(35121), None),
-    /// Allows relaxing the coherence rules such that
-    /// `impl<T> ForeignTrait<LocalType> for ForeignType<T>` is permitted.
-    (accepted, re_rebalance_coherence, "1.41.0", Some(55437), None),
-
-    // -------------------------------------------------------------------------
-    // feature-group-end: accepted features
-    // -------------------------------------------------------------------------
-);
diff --git a/src/libsyntax/feature_gate/active.rs b/src/libsyntax/feature_gate/active.rs
deleted file mode 100644
index b04b30aa6bc..00000000000
--- a/src/libsyntax/feature_gate/active.rs
+++ /dev/null
@@ -1,554 +0,0 @@
-//! List of the active feature gates.
-
-use super::{State, Feature};
-
-use crate::edition::Edition;
-use crate::symbol::{Symbol, sym};
-
-use syntax_pos::Span;
-
-macro_rules! set {
-    ($field: ident) => {{
-        fn f(features: &mut Features, _: Span) {
-            features.$field = true;
-        }
-        f as fn(&mut Features, Span)
-    }}
-}
-
-macro_rules! declare_features {
-    ($(
-        $(#[doc = $doc:tt])* (active, $feature:ident, $ver:expr, $issue:expr, $edition:expr),
-    )+) => {
-        /// Represents active features that are currently being implemented or
-        /// currently being considered for addition/removal.
-        pub const ACTIVE_FEATURES:
-            &[Feature] =
-            &[$(
-                // (sym::$feature, $ver, $issue, $edition, set!($feature))
-                Feature {
-                    state: State::Active { set: set!($feature) },
-                    name: sym::$feature,
-                    since: $ver,
-                    issue: $issue,
-                    edition: $edition,
-                    description: concat!($($doc,)*),
-                }
-            ),+];
-
-        /// A set of features to be used by later passes.
-        #[derive(Clone)]
-        pub struct Features {
-            /// `#![feature]` attrs for language features, for error reporting.
-            pub declared_lang_features: Vec<(Symbol, Span, Option<Symbol>)>,
-            /// `#![feature]` attrs for non-language (library) features.
-            pub declared_lib_features: Vec<(Symbol, Span)>,
-            $(
-                $(#[doc = $doc])*
-                pub $feature: bool
-            ),+
-        }
-
-        impl Features {
-            pub fn new() -> Features {
-                Features {
-                    declared_lang_features: Vec::new(),
-                    declared_lib_features: Vec::new(),
-                    $($feature: false),+
-                }
-            }
-
-            pub fn walk_feature_fields<F>(&self, mut f: F)
-                where F: FnMut(&str, bool)
-            {
-                $(f(stringify!($feature), self.$feature);)+
-            }
-        }
-    };
-}
-
-impl Feature {
-    /// Sets this feature in `Features`. Panics if called on a non-active feature.
-    pub fn set(&self, features: &mut Features, span: Span) {
-        match self.state {
-            State::Active { set } => set(features, span),
-            _ => panic!("called `set` on feature `{}` which is not `active`", self.name)
-        }
-    }
-}
-
-// If you change this, please modify `src/doc/unstable-book` as well.
-//
-// Don't ever remove anything from this list; move them to `removed.rs`.
-//
-// The version numbers here correspond to the version in which the current status
-// was set. This is most important for knowing when a particular feature became
-// stable (active).
-//
-// Note that the features are grouped into internal/user-facing and then
-// sorted by version inside those groups. This is enforced with tidy.
-//
-// N.B., `tools/tidy/src/features.rs` parses this information directly out of the
-// source, so take care when modifying it.
-
-declare_features! (
-    // -------------------------------------------------------------------------
-    // feature-group-start: internal feature gates
-    // -------------------------------------------------------------------------
-
-    // no-tracking-issue-start
-
-    /// Allows using compiler's own crates.
-    (active, rustc_private, "1.0.0", Some(27812), None),
-
-    /// Allows using the `rust-intrinsic`'s "ABI".
-    (active, intrinsics, "1.0.0", None, None),
-
-    /// Allows using `#[lang = ".."]` attribute for linking items to special compiler logic.
-    (active, lang_items, "1.0.0", None, None),
-
-    /// Allows using the `#[stable]` and `#[unstable]` attributes.
-    (active, staged_api, "1.0.0", None, None),
-
-    /// Allows using `#[allow_internal_unstable]`. This is an
-    /// attribute on `macro_rules!` and can't use the attribute handling
-    /// below (it has to be checked before expansion possibly makes
-    /// macros disappear).
-    (active, allow_internal_unstable, "1.0.0", None, None),
-
-    /// Allows using `#[allow_internal_unsafe]`. This is an
-    /// attribute on `macro_rules!` and can't use the attribute handling
-    /// below (it has to be checked before expansion possibly makes
-    /// macros disappear).
-    (active, allow_internal_unsafe, "1.0.0", None, None),
-
-    /// Allows using `#[rustc_const_unstable(feature = "foo", ..)]` which
-    /// lets a function to be `const` when opted into with `#![feature(foo)]`.
-    (active, rustc_const_unstable, "1.0.0", None, None),
-
-    /// no-tracking-issue-end
-
-    /// Allows using `#[link_name="llvm.*"]`.
-    (active, link_llvm_intrinsics, "1.0.0", Some(29602), None),
-
-    /// Allows using `rustc_*` attributes (RFC 572).
-    (active, rustc_attrs, "1.0.0", Some(29642), None),
-
-    /// Allows using the `box $expr` syntax.
-    (active, box_syntax, "1.0.0", Some(49733), None),
-
-    /// Allows using `#[main]` to replace the entrypoint `#[lang = "start"]` calls.
-    (active, main, "1.0.0", Some(29634), None),
-
-    /// Allows using `#[start]` on a function indicating that it is the program entrypoint.
-    (active, start, "1.0.0", Some(29633), None),
-
-    /// Allows using the `#[fundamental]` attribute.
-    (active, fundamental, "1.0.0", Some(29635), None),
-
-    /// Allows using the `rust-call` ABI.
-    (active, unboxed_closures, "1.0.0", Some(29625), None),
-
-    /// Allows using the `#[linkage = ".."]` attribute.
-    (active, linkage, "1.0.0", Some(29603), None),
-
-    /// Allows features specific to OIBIT (auto traits).
-    (active, optin_builtin_traits, "1.0.0", Some(13231), None),
-
-    /// Allows using `box` in patterns (RFC 469).
-    (active, box_patterns, "1.0.0", Some(29641), None),
-
-    // no-tracking-issue-start
-
-    /// Allows using `#[prelude_import]` on glob `use` items.
-    (active, prelude_import, "1.2.0", None, None),
-
-    // no-tracking-issue-end
-
-    // no-tracking-issue-start
-
-    /// Allows using `#[omit_gdb_pretty_printer_section]`.
-    (active, omit_gdb_pretty_printer_section, "1.5.0", None, None),
-
-    /// Allows using the `vectorcall` ABI.
-    (active, abi_vectorcall, "1.7.0", None, None),
-
-    // no-tracking-issue-end
-
-    /// Allows using `#[structural_match]` which indicates that a type is structurally matchable.
-    (active, structural_match, "1.8.0", Some(31434), None),
-
-    /// Allows using the `may_dangle` attribute (RFC 1327).
-    (active, dropck_eyepatch, "1.10.0", Some(34761), None),
-
-    /// Allows using the `#![panic_runtime]` attribute.
-    (active, panic_runtime, "1.10.0", Some(32837), None),
-
-    /// Allows declaring with `#![needs_panic_runtime]` that a panic runtime is needed.
-    (active, needs_panic_runtime, "1.10.0", Some(32837), None),
-
-    // no-tracking-issue-start
-
-    /// Allows identifying the `compiler_builtins` crate.
-    (active, compiler_builtins, "1.13.0", None, None),
-
-    /// Allows using the `unadjusted` ABI; perma-unstable.
-    (active, abi_unadjusted, "1.16.0", None, None),
-
-    /// Allows identifying crates that contain sanitizer runtimes.
-    (active, sanitizer_runtime, "1.17.0", None, None),
-
-    /// Used to identify crates that contain the profiler runtime.
-    (active, profiler_runtime, "1.18.0", None, None),
-
-    /// Allows using the `thiscall` ABI.
-    (active, abi_thiscall, "1.19.0", None, None),
-
-    /// Allows using `#![needs_allocator]`, an implementation detail of `#[global_allocator]`.
-    (active, allocator_internals, "1.20.0", None, None),
-
-    /// Added for testing E0705; perma-unstable.
-    (active, test_2018_feature, "1.31.0", None, Some(Edition::Edition2018)),
-
-    // no-tracking-issue-end
-
-    // -------------------------------------------------------------------------
-    // feature-group-end: internal feature gates
-    // -------------------------------------------------------------------------
-
-    // -------------------------------------------------------------------------
-    // feature-group-start: actual feature gates (target features)
-    // -------------------------------------------------------------------------
-
-    // FIXME: Document these and merge with the list below.
-
-    // Unstable `#[target_feature]` directives.
-    (active, arm_target_feature, "1.27.0", Some(44839), None),
-    (active, aarch64_target_feature, "1.27.0", Some(44839), None),
-    (active, hexagon_target_feature, "1.27.0", Some(44839), None),
-    (active, powerpc_target_feature, "1.27.0", Some(44839), None),
-    (active, mips_target_feature, "1.27.0", Some(44839), None),
-    (active, avx512_target_feature, "1.27.0", Some(44839), None),
-    (active, mmx_target_feature, "1.27.0", Some(44839), None),
-    (active, sse4a_target_feature, "1.27.0", Some(44839), None),
-    (active, tbm_target_feature, "1.27.0", Some(44839), None),
-    (active, wasm_target_feature, "1.30.0", Some(44839), None),
-    (active, adx_target_feature, "1.32.0", Some(44839), None),
-    (active, cmpxchg16b_target_feature, "1.32.0", Some(44839), None),
-    (active, movbe_target_feature, "1.34.0", Some(44839), None),
-    (active, rtm_target_feature, "1.35.0", Some(44839), None),
-    (active, f16c_target_feature, "1.36.0", Some(44839), None),
-
-    // -------------------------------------------------------------------------
-    // feature-group-end: actual feature gates (target features)
-    // -------------------------------------------------------------------------
-
-    // -------------------------------------------------------------------------
-    // feature-group-start: actual feature gates
-    // -------------------------------------------------------------------------
-
-    /// Allows using the `#[link_args]` attribute.
-    (active, link_args, "1.0.0", Some(29596), None),
-
-    /// Allows defining identifiers beyond ASCII.
-    (active, non_ascii_idents, "1.0.0", Some(55467), None),
-
-    /// Allows using `#[plugin_registrar]` on functions.
-    (active, plugin_registrar, "1.0.0", Some(29597), None),
-
-    /// Allows using `#![plugin(myplugin)]`.
-    (active, plugin, "1.0.0", Some(29597), None),
-
-    /// Allows using `#[thread_local]` on `static` items.
-    (active, thread_local, "1.0.0", Some(29594), None),
-
-    /// Allows the use of SIMD types in functions declared in `extern` blocks.
-    (active, simd_ffi, "1.0.0", Some(27731), None),
-
-    /// Allows using non lexical lifetimes (RFC 2094).
-    (active, nll, "1.0.0", Some(43234), None),
-
-    /// Allows using slice patterns.
-    (active, slice_patterns, "1.0.0", Some(62254), None),
-
-    /// Allows the definition of `const` functions with some advanced features.
-    (active, const_fn, "1.2.0", Some(57563), None),
-
-    /// Allows associated type defaults.
-    (active, associated_type_defaults, "1.2.0", Some(29661), None),
-
-    /// Allows `#![no_core]`.
-    (active, no_core, "1.3.0", Some(29639), None),
-
-    /// Allows default type parameters to influence type inference.
-    (active, default_type_parameter_fallback, "1.3.0", Some(27336), None),
-
-    /// Allows `repr(simd)` and importing the various simd intrinsics.
-    (active, repr_simd, "1.4.0", Some(27731), None),
-
-    /// Allows `extern "platform-intrinsic" { ... }`.
-    (active, platform_intrinsics, "1.4.0", Some(27731), None),
-
-    /// Allows `#[unwind(..)]`.
-    ///
-    /// Permits specifying whether a function should permit unwinding or abort on unwind.
-    (active, unwind_attributes, "1.4.0", Some(58760), None),
-
-    /// Allows `#[no_debug]`.
-    (active, no_debug, "1.5.0", Some(29721), None),
-
-    /// Allows attributes on expressions and non-item statements.
-    (active, stmt_expr_attributes, "1.6.0", Some(15701), None),
-
-    /// Allows the use of type ascription in expressions.
-    (active, type_ascription, "1.6.0", Some(23416), None),
-
-    /// Allows `cfg(target_thread_local)`.
-    (active, cfg_target_thread_local, "1.7.0", Some(29594), None),
-
-    /// Allows specialization of implementations (RFC 1210).
-    (active, specialization, "1.7.0", Some(31844), None),
-
-    /// Allows using `#[naked]` on functions.
-    (active, naked_functions, "1.9.0", Some(32408), None),
-
-    /// Allows `cfg(target_has_atomic = "...")`.
-    (active, cfg_target_has_atomic, "1.9.0", Some(32976), None),
-
-    /// Allows `X..Y` patterns.
-    (active, exclusive_range_pattern, "1.11.0", Some(37854), None),
-
-    /// Allows exhaustive pattern matching on types that contain uninhabited types.
-    (active, exhaustive_patterns, "1.13.0", Some(51085), None),
-
-    /// Allows `union`s to implement `Drop`. Moreover, `union`s may now include fields
-    /// that don't implement `Copy` as long as they don't have any drop glue.
-    /// This is checked recursively. On encountering type variable where no progress can be made,
-    /// `T: Copy` is used as a substitute for "no drop glue".
-    ///
-    /// NOTE: A limited form of `union U { ... }` was accepted in 1.19.0.
-    (active, untagged_unions, "1.13.0", Some(55149), None),
-
-    /// Allows `#[link(..., cfg(..))]`.
-    (active, link_cfg, "1.14.0", Some(37406), None),
-
-    /// Allows `extern "ptx-*" fn()`.
-    (active, abi_ptx, "1.15.0", Some(38788), None),
-
-    /// Allows the `#[repr(i128)]` attribute for enums.
-    (active, repr128, "1.16.0", Some(35118), None),
-
-    /// Allows `#[link(kind="static-nobundle"...)]`.
-    (active, static_nobundle, "1.16.0", Some(37403), None),
-
-    /// Allows `extern "msp430-interrupt" fn()`.
-    (active, abi_msp430_interrupt, "1.16.0", Some(38487), None),
-
-    /// Allows declarative macros 2.0 (`macro`).
-    (active, decl_macro, "1.17.0", Some(39412), None),
-
-    /// Allows `extern "x86-interrupt" fn()`.
-    (active, abi_x86_interrupt, "1.17.0", Some(40180), None),
-
-    /// Allows overlapping impls of marker traits.
-    (active, overlapping_marker_traits, "1.18.0", Some(29864), None),
-
-    /// Allows a test to fail without failing the whole suite.
-    (active, allow_fail, "1.19.0", Some(46488), None),
-
-    /// Allows unsized tuple coercion.
-    (active, unsized_tuple_coercion, "1.20.0", Some(42877), None),
-
-    /// Allows defining generators.
-    (active, generators, "1.21.0", Some(43122), None),
-
-    /// Allows `#[doc(cfg(...))]`.
-    (active, doc_cfg, "1.21.0", Some(43781), None),
-
-    /// Allows `#[doc(masked)]`.
-    (active, doc_masked, "1.21.0", Some(44027), None),
-
-    /// Allows `#[doc(spotlight)]`.
-    (active, doc_spotlight, "1.22.0", Some(45040), None),
-
-    /// Allows `#[doc(include = "some-file")]`.
-    (active, external_doc, "1.22.0", Some(44732), None),
-
-    /// Allows using `crate` as visibility modifier, synonymous with `pub(crate)`.
-    (active, crate_visibility_modifier, "1.23.0", Some(53120), None),
-
-    /// Allows defining `extern type`s.
-    (active, extern_types, "1.23.0", Some(43467), None),
-
-    /// Allows trait methods with arbitrary self types.
-    (active, arbitrary_self_types, "1.23.0", Some(44874), None),
-
-    /// Allows in-band quantification of lifetime bindings (e.g., `fn foo(x: &'a u8) -> &'a u8`).
-    (active, in_band_lifetimes, "1.23.0", Some(44524), None),
-
-    /// Allows associated types to be generic, e.g., `type Foo<T>;` (RFC 1598).
-    (active, generic_associated_types, "1.23.0", Some(44265), None),
-
-    /// Allows defining `trait X = A + B;` alias items.
-    (active, trait_alias, "1.24.0", Some(41517), None),
-
-    /// Allows infering `'static` outlives requirements (RFC 2093).
-    (active, infer_static_outlives_requirements, "1.26.0", Some(54185), None),
-
-    /// Allows accessing fields of unions inside `const` functions.
-    (active, const_fn_union, "1.27.0", Some(51909), None),
-
-    /// Allows casting raw pointers to `usize` during const eval.
-    (active, const_raw_ptr_to_usize_cast, "1.27.0", Some(51910), None),
-
-    /// Allows dereferencing raw pointers during const eval.
-    (active, const_raw_ptr_deref, "1.27.0", Some(51911), None),
-
-    /// Allows comparing raw pointers during const eval.
-    (active, const_compare_raw_pointers, "1.27.0", Some(53020), None),
-
-    /// Allows `#[doc(alias = "...")]`.
-    (active, doc_alias, "1.27.0", Some(50146), None),
-
-    /// Allows inconsistent bounds in where clauses.
-    (active, trivial_bounds, "1.28.0", Some(48214), None),
-
-    /// Allows `'a: { break 'a; }`.
-    (active, label_break_value, "1.28.0", Some(48594), None),
-
-    /// Allows using `#[doc(keyword = "...")]`.
-    (active, doc_keyword, "1.28.0", Some(51315), None),
-
-    /// Allows reinterpretation of the bits of a value of one type as another
-    /// type during const eval.
-    (active, const_transmute, "1.29.0", Some(53605), None),
-
-    /// Allows using `try {...}` expressions.
-    (active, try_blocks, "1.29.0", Some(31436), None),
-
-    /// Allows defining an `#[alloc_error_handler]`.
-    (active, alloc_error_handler, "1.29.0", Some(51540), None),
-
-    /// Allows using the `amdgpu-kernel` ABI.
-    (active, abi_amdgpu_kernel, "1.29.0", Some(51575), None),
-
-    /// Allows panicking during const eval (producing compile-time errors).
-    (active, const_panic, "1.30.0", Some(51999), None),
-
-    /// Allows `#[marker]` on certain traits allowing overlapping implementations.
-    (active, marker_trait_attr, "1.30.0", Some(29864), None),
-
-    /// Allows macro invocations on modules expressions and statements and
-    /// procedural macros to expand to non-items.
-    (active, proc_macro_hygiene, "1.30.0", Some(54727), None),
-
-    /// Allows unsized rvalues at arguments and parameters.
-    (active, unsized_locals, "1.30.0", Some(48055), None),
-
-    /// Allows custom test frameworks with `#![test_runner]` and `#[test_case]`.
-    (active, custom_test_frameworks, "1.30.0", Some(50297), None),
-
-    /// Allows non-builtin attributes in inner attribute position.
-    (active, custom_inner_attributes, "1.30.0", Some(54726), None),
-
-    /// Allows `impl Trait` in bindings (`let`, `const`, `static`).
-    (active, impl_trait_in_bindings, "1.30.0", Some(63065), None),
-
-    /// Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.
-    (active, lint_reasons, "1.31.0", Some(54503), None),
-
-    /// Allows exhaustive integer pattern matching on `usize` and `isize`.
-    (active, precise_pointer_size_matching, "1.32.0", Some(56354), None),
-
-    /// Allows using `#[ffi_returns_twice]` on foreign functions.
-    (active, ffi_returns_twice, "1.34.0", Some(58314), None),
-
-    /// Allows const generic types (e.g. `struct Foo<const N: usize>(...);`).
-    (active, const_generics, "1.34.0", Some(44580), None),
-
-    /// Allows using `#[optimize(X)]`.
-    (active, optimize_attribute, "1.34.0", Some(54882), None),
-
-    /// Allows using C-variadics.
-    (active, c_variadic, "1.34.0", Some(44930), None),
-
-    /// Allows the user of associated type bounds.
-    (active, associated_type_bounds, "1.34.0", Some(52662), None),
-
-    /// Allows `if/while p && let q = r && ...` chains.
-    (active, let_chains, "1.37.0", Some(53667), None),
-
-    /// Allows #[repr(transparent)] on enums (RFC 2645).
-    (active, transparent_enums, "1.37.0", Some(60405), None),
-
-    /// Allows #[repr(transparent)] on unions (RFC 2645).
-    (active, transparent_unions, "1.37.0", Some(60405), None),
-
-    /// Allows explicit discriminants on non-unit enum variants.
-    (active, arbitrary_enum_discriminant, "1.37.0", Some(60553), None),
-
-    /// Allows `impl Trait` with multiple unrelated lifetimes.
-    (active, member_constraints, "1.37.0", Some(61977), None),
-
-    /// Allows `async || body` closures.
-    (active, async_closure, "1.37.0", Some(62290), None),
-
-    /// Allows `[x; N]` where `x` is a constant (RFC 2203).
-    (active, const_in_array_repeat_expressions, "1.37.0", Some(49147), None),
-
-    /// Allows `impl Trait` to be used inside type aliases (RFC 2515).
-    (active, type_alias_impl_trait, "1.38.0", Some(63063), None),
-
-    /// Allows the use of or-patterns (e.g., `0 | 1`).
-    (active, or_patterns, "1.38.0", Some(54883), None),
-
-    /// Allows the definition of `const extern fn` and `const unsafe extern fn`.
-    (active, const_extern_fn, "1.40.0", Some(64926), None),
-
-    /// Allows the use of raw-dylibs (RFC 2627).
-    (active, raw_dylib, "1.40.0", Some(58713), None),
-
-    /// Allows `#[track_caller]` to be used which provides
-    /// accurate caller location reporting during panic (RFC 2091).
-    (active, track_caller, "1.40.0", Some(47809), None),
-
-    /// Allows making `dyn Trait` well-formed even if `Trait` is not object safe.
-    /// In that case, `dyn Trait: Trait` does not hold. Moreover, coercions and
-    /// casts in safe Rust to `dyn Trait` for such a `Trait` is also forbidden.
-    (active, object_safe_for_dispatch, "1.40.0", Some(43561), None),
-
-    /// Allows using the `efiapi` ABI.
-    (active, abi_efiapi, "1.40.0", Some(65815), None),
-
-    /// Allows `&raw const $place_expr` and `&raw mut $place_expr` expressions.
-    (active, raw_ref_op, "1.41.0", Some(64490), None),
-
-    /// Allows diverging expressions to fall back to `!` rather than `()`.
-    (active, never_type_fallback, "1.41.0", Some(65992), None),
-
-    /// Allows using the `#[register_attr]` attribute.
-    (active, register_attr, "1.41.0", Some(66080), None),
-
-    /// Allows using the `#[register_tool]` attribute.
-    (active, register_tool, "1.41.0", Some(66079), None),
-
-    /// Allows the use of `if` and `match` in constants.
-    (active, const_if_match, "1.41.0", Some(49146), None),
-
-    // -------------------------------------------------------------------------
-    // feature-group-end: actual feature gates
-    // -------------------------------------------------------------------------
-);
-
-/// Some features are known to be incomplete and using them is likely to have
-/// unanticipated results, such as compiler crashes. We warn the user about these
-/// to alert them.
-pub const INCOMPLETE_FEATURES: &[Symbol] = &[
-    sym::impl_trait_in_bindings,
-    sym::generic_associated_types,
-    sym::const_generics,
-    sym::or_patterns,
-    sym::let_chains,
-    sym::raw_dylib,
-    sym::track_caller,
-];
diff --git a/src/libsyntax/feature_gate/builtin_attrs.rs b/src/libsyntax/feature_gate/builtin_attrs.rs
deleted file mode 100644
index a9f41633f30..00000000000
--- a/src/libsyntax/feature_gate/builtin_attrs.rs
+++ /dev/null
@@ -1,608 +0,0 @@
-//! Built-in attributes and `cfg` flag gating.
-
-use AttributeType::*;
-use AttributeGate::*;
-
-use super::check::{emit_feature_err, GateIssue};
-use super::check::{Stability, EXPLAIN_ALLOW_INTERNAL_UNSAFE, EXPLAIN_ALLOW_INTERNAL_UNSTABLE};
-use super::active::Features;
-
-use crate::ast;
-use crate::attr::AttributeTemplate;
-use crate::sess::ParseSess;
-use crate::symbol::{Symbol, sym};
-
-use syntax_pos::Span;
-use rustc_data_structures::fx::FxHashMap;
-use lazy_static::lazy_static;
-
-type GateFn = fn(&Features) -> bool;
-
-macro_rules! cfg_fn {
-    ($field: ident) => {
-        (|features| { features.$field }) as GateFn
-    }
-}
-
-/// `cfg(...)`'s that are feature gated.
-const GATED_CFGS: &[(Symbol, Symbol, GateFn)] = &[
-    // (name in cfg, feature, function to check if the feature is enabled)
-    (sym::target_thread_local, sym::cfg_target_thread_local, cfg_fn!(cfg_target_thread_local)),
-    (sym::target_has_atomic, sym::cfg_target_has_atomic, cfg_fn!(cfg_target_has_atomic)),
-    (sym::target_has_atomic_load_store, sym::cfg_target_has_atomic, cfg_fn!(cfg_target_has_atomic)),
-];
-
-#[derive(Debug)]
-pub struct GatedCfg {
-    span: Span,
-    index: usize,
-}
-
-impl GatedCfg {
-    pub fn gate(cfg: &ast::MetaItem) -> Option<GatedCfg> {
-        GATED_CFGS.iter()
-                  .position(|info| cfg.check_name(info.0))
-                  .map(|idx| {
-                      GatedCfg {
-                          span: cfg.span,
-                          index: idx
-                      }
-                  })
-    }
-
-    pub fn check_and_emit(&self, sess: &ParseSess, features: &Features) {
-        let (cfg, feature, has_feature) = GATED_CFGS[self.index];
-        if !has_feature(features) && !self.span.allows_unstable(feature) {
-            let explain = format!("`cfg({})` is experimental and subject to change", cfg);
-            emit_feature_err(sess, feature, self.span, GateIssue::Language, &explain);
-        }
-    }
-}
-
-// If you change this, please modify `src/doc/unstable-book` as well. You must
-// move that documentation into the relevant place in the other docs, and
-// remove the chapter on the flag.
-
-#[derive(Copy, Clone, PartialEq, Debug)]
-pub enum AttributeType {
-    /// Normal, builtin attribute that is consumed
-    /// by the compiler before the unused_attribute check
-    Normal,
-
-    /// Builtin attribute that may not be consumed by the compiler
-    /// before the unused_attribute check. These attributes
-    /// will be ignored by the unused_attribute lint
-    Whitelisted,
-
-    /// Builtin attribute that is only allowed at the crate level
-    CrateLevel,
-}
-
-#[derive(Clone, Copy)]
-pub enum AttributeGate {
-    /// Is gated by a given feature gate, reason
-    /// and function to check if enabled
-    Gated(Stability, Symbol, &'static str, fn(&Features) -> bool),
-
-    /// Ungated attribute, can be used on all release channels
-    Ungated,
-}
-
-// fn() is not Debug
-impl std::fmt::Debug for AttributeGate {
-    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match *self {
-            Self::Gated(ref stab, name, expl, _) =>
-                write!(fmt, "Gated({:?}, {}, {})", stab, name, expl),
-            Self::Ungated => write!(fmt, "Ungated")
-        }
-    }
-}
-
-impl AttributeGate {
-    fn is_deprecated(&self) -> bool {
-        match *self {
-            Self::Gated(Stability::Deprecated(_, _), ..) => true,
-            _ => false,
-        }
-    }
-}
-
-/// A convenience macro for constructing attribute templates.
-/// E.g., `template!(Word, List: "description")` means that the attribute
-/// supports forms `#[attr]` and `#[attr(description)]`.
-macro_rules! template {
-    (Word) => { template!(@ true, None, None) };
-    (List: $descr: expr) => { template!(@ false, Some($descr), None) };
-    (NameValueStr: $descr: expr) => { template!(@ false, None, Some($descr)) };
-    (Word, List: $descr: expr) => { template!(@ true, Some($descr), None) };
-    (Word, NameValueStr: $descr: expr) => { template!(@ true, None, Some($descr)) };
-    (List: $descr1: expr, NameValueStr: $descr2: expr) => {
-        template!(@ false, Some($descr1), Some($descr2))
-    };
-    (Word, List: $descr1: expr, NameValueStr: $descr2: expr) => {
-        template!(@ true, Some($descr1), Some($descr2))
-    };
-    (@ $word: expr, $list: expr, $name_value_str: expr) => { AttributeTemplate {
-        word: $word, list: $list, name_value_str: $name_value_str
-    } };
-}
-
-macro_rules! ungated {
-    ($attr:ident, $typ:expr, $tpl:expr $(,)?) => {
-        (sym::$attr, $typ, $tpl, Ungated)
-    };
-}
-
-macro_rules! gated {
-    ($attr:ident, $typ:expr, $tpl:expr, $gate:ident, $msg:expr $(,)?) => {
-        (sym::$attr, $typ, $tpl, Gated(Stability::Unstable, sym::$gate, $msg, cfg_fn!($gate)))
-    };
-    ($attr:ident, $typ:expr, $tpl:expr, $msg:expr $(,)?) => {
-        (sym::$attr, $typ, $tpl, Gated(Stability::Unstable, sym::$attr, $msg, cfg_fn!($attr)))
-    };
-}
-
-macro_rules! rustc_attr {
-    (TEST, $attr:ident, $typ:expr, $tpl:expr $(,)?) => {
-        rustc_attr!(
-            $attr, $typ, $tpl,
-            concat!("the `#[", stringify!($attr), "]` attribute is just used for rustc unit tests \
-                and will never be stable",
-            ),
-        )
-    };
-    ($attr:ident, $typ:expr, $tpl:expr, $msg:expr $(,)?) => {
-        (sym::$attr, $typ, $tpl,
-         Gated(Stability::Unstable, sym::rustc_attrs, $msg, cfg_fn!(rustc_attrs)))
-    };
-}
-
-macro_rules! experimental {
-    ($attr:ident) => {
-        concat!("the `#[", stringify!($attr), "]` attribute is an experimental feature")
-    }
-}
-
-const IMPL_DETAIL: &str = "internal implementation detail";
-const INTERNAL_UNSTABLE: &str = "this is an internal attribute that will never be stable";
-
-pub type BuiltinAttribute = (Symbol, AttributeType, AttributeTemplate, AttributeGate);
-
-/// Attributes that have a special meaning to rustc or rustdoc.
-pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
-    // ==========================================================================
-    // Stable attributes:
-    // ==========================================================================
-
-    // Condtional compilation:
-    ungated!(cfg, Normal, template!(List: "predicate")),
-    ungated!(cfg_attr, Normal, template!(List: "predicate, attr1, attr2, ...")),
-
-    // Testing:
-    ungated!(ignore, Normal, template!(Word, NameValueStr: "reason")),
-    ungated!(
-        should_panic, Normal,
-        template!(Word, List: r#"expected = "reason"#, NameValueStr: "reason"),
-    ),
-    // FIXME(Centril): This can be used on stable but shouldn't.
-    ungated!(reexport_test_harness_main, Normal, template!(NameValueStr: "name")),
-
-    // Macros:
-    ungated!(derive, Normal, template!(List: "Trait1, Trait2, ...")),
-    ungated!(automatically_derived, Normal, template!(Word)),
-    // FIXME(#14407)
-    ungated!(macro_use, Normal, template!(Word, List: "name1, name2, ...")),
-    ungated!(macro_escape, Normal, template!(Word)), // Deprecated synonym for `macro_use`.
-    ungated!(macro_export, Normal, template!(Word, List: "local_inner_macros")),
-    ungated!(proc_macro, Normal, template!(Word)),
-    ungated!(
-        proc_macro_derive, Normal,
-        template!(List: "TraitName, /*opt*/ attributes(name1, name2, ...)"),
-    ),
-    ungated!(proc_macro_attribute, Normal, template!(Word)),
-
-    // Lints:
-    ungated!(warn, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#)),
-    ungated!(allow, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#)),
-    ungated!(forbid, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#)),
-    ungated!(deny, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#)),
-    ungated!(must_use, Whitelisted, template!(Word, NameValueStr: "reason")),
-    // FIXME(#14407)
-    ungated!(
-        deprecated, Normal,
-        template!(
-            Word,
-            List: r#"/*opt*/ since = "version", /*opt*/ note = "reason""#,
-            NameValueStr: "reason"
-        ),
-    ),
-
-    // Crate properties:
-    ungated!(crate_name, CrateLevel, template!(NameValueStr: "name")),
-    ungated!(crate_type, CrateLevel, template!(NameValueStr: "bin|lib|...")),
-    ungated!(crate_id, CrateLevel, template!(NameValueStr: "ignored")),
-
-    // ABI, linking, symbols, and FFI
-    ungated!(
-        link, Whitelisted,
-        template!(List: r#"name = "...", /*opt*/ kind = "dylib|static|...", /*opt*/ cfg = "...""#),
-    ),
-    ungated!(link_name, Whitelisted, template!(NameValueStr: "name")),
-    ungated!(no_link, Normal, template!(Word)),
-    ungated!(repr, Normal, template!(List: "C")),
-    ungated!(export_name, Whitelisted, template!(NameValueStr: "name")),
-    ungated!(link_section, Whitelisted, template!(NameValueStr: "name")),
-    ungated!(no_mangle, Whitelisted, template!(Word)),
-    ungated!(used, Whitelisted, template!(Word)),
-
-    // Limits:
-    ungated!(recursion_limit, CrateLevel, template!(NameValueStr: "N")),
-    ungated!(type_length_limit, CrateLevel, template!(NameValueStr: "N")),
-
-    // Entry point:
-    ungated!(main, Normal, template!(Word)),
-    ungated!(start, Normal, template!(Word)),
-    ungated!(no_start, CrateLevel, template!(Word)),
-    ungated!(no_main, CrateLevel, template!(Word)),
-
-    // Modules, prelude, and resolution:
-    ungated!(path, Normal, template!(NameValueStr: "file")),
-    ungated!(no_std, CrateLevel, template!(Word)),
-    ungated!(no_implicit_prelude, Normal, template!(Word)),
-    ungated!(non_exhaustive, Whitelisted, template!(Word)),
-
-    // Runtime
-    ungated!(windows_subsystem, Whitelisted, template!(NameValueStr: "windows|console")),
-    ungated!(panic_handler, Normal, template!(Word)), // RFC 2070
-
-    // Code generation:
-    ungated!(inline, Whitelisted, template!(Word, List: "always|never")),
-    ungated!(cold, Whitelisted, template!(Word)),
-    ungated!(no_builtins, Whitelisted, template!(Word)),
-    ungated!(target_feature, Whitelisted, template!(List: r#"enable = "name""#)),
-
-    // FIXME: #14408 whitelist docs since rustdoc looks at them
-    ungated!(doc, Whitelisted, template!(List: "hidden|inline|...", NameValueStr: "string")),
-
-    // ==========================================================================
-    // Unstable attributes:
-    // ==========================================================================
-
-    // Linking:
-    gated!(naked, Whitelisted, template!(Word), naked_functions, experimental!(naked)),
-    gated!(
-        link_args, Normal, template!(NameValueStr: "args"),
-        "the `link_args` attribute is experimental and not portable across platforms, \
-        it is recommended to use `#[link(name = \"foo\")] instead",
-    ),
-    gated!(
-        link_ordinal, Whitelisted, template!(List: "ordinal"), raw_dylib,
-        experimental!(link_ordinal)
-    ),
-
-    // Plugins:
-    (
-        sym::plugin_registrar, Normal, template!(Word),
-        Gated(
-            Stability::Deprecated(
-                "https://github.com/rust-lang/rust/pull/64675",
-                Some("may be removed in a future compiler version"),
-            ),
-            sym::plugin_registrar,
-            "compiler plugins are deprecated",
-            cfg_fn!(plugin_registrar)
-        )
-    ),
-    (
-        sym::plugin, CrateLevel, template!(List: "name|name(args)"),
-        Gated(
-            Stability::Deprecated(
-                "https://github.com/rust-lang/rust/pull/64675",
-                Some("may be removed in a future compiler version"),
-            ),
-            sym::plugin,
-            "compiler plugins are deprecated",
-            cfg_fn!(plugin)
-        )
-    ),
-
-    // Testing:
-    gated!(allow_fail, Normal, template!(Word), experimental!(allow_fail)),
-    gated!(
-        test_runner, CrateLevel, template!(List: "path"), custom_test_frameworks,
-        "custom test frameworks are an unstable feature",
-    ),
-    // RFC #1268
-    gated!(marker, Normal, template!(Word), marker_trait_attr, experimental!(marker)),
-    gated!(
-        thread_local, Whitelisted, template!(Word),
-        "`#[thread_local]` is an experimental feature, and does not currently handle destructors",
-    ),
-    gated!(no_core, CrateLevel, template!(Word), experimental!(no_core)),
-    // RFC 2412
-    gated!(
-        optimize, Whitelisted, template!(List: "size|speed"), optimize_attribute,
-        experimental!(optimize),
-    ),
-
-    gated!(ffi_returns_twice, Whitelisted, template!(Word), experimental!(ffi_returns_twice)),
-    gated!(track_caller, Whitelisted, template!(Word), experimental!(track_caller)),
-    gated!(
-        register_attr, CrateLevel, template!(List: "attr1, attr2, ..."),
-        experimental!(register_attr),
-    ),
-    gated!(
-        register_tool, CrateLevel, template!(List: "tool1, tool2, ..."),
-        experimental!(register_tool),
-    ),
-
-    // ==========================================================================
-    // Internal attributes: Stability, deprecation, and unsafe:
-    // ==========================================================================
-
-    ungated!(feature, CrateLevel, template!(List: "name1, name1, ...")),
-    // FIXME(#14407) -- only looked at on-demand so we can't
-    // guarantee they'll have already been checked.
-    ungated!(
-        rustc_deprecated, Whitelisted,
-        template!(List: r#"since = "version", reason = "...""#)
-    ),
-    // FIXME(#14407)
-    ungated!(stable, Whitelisted, template!(List: r#"feature = "name", since = "version""#)),
-    // FIXME(#14407)
-    ungated!(
-        unstable, Whitelisted,
-        template!(List: r#"feature = "name", reason = "...", issue = "N""#),
-    ),
-    gated!(
-        rustc_const_unstable, Normal, template!(List: r#"feature = "name""#),
-        "the `#[rustc_const_unstable]` attribute is an internal feature",
-    ),
-    gated!(
-        allow_internal_unstable, Normal, template!(Word, List: "feat1, feat2, ..."),
-        EXPLAIN_ALLOW_INTERNAL_UNSTABLE,
-    ),
-    gated!(allow_internal_unsafe, Normal, template!(Word), EXPLAIN_ALLOW_INTERNAL_UNSAFE),
-
-    // ==========================================================================
-    // Internal attributes: Type system related:
-    // ==========================================================================
-
-    gated!(fundamental, Whitelisted, template!(Word), experimental!(fundamental)),
-    gated!(
-        // RFC #1445.
-        structural_match, Whitelisted, template!(Word),
-        "the semantics of constant patterns is not yet settled",
-    ),
-    gated!(
-        may_dangle, Normal, template!(Word), dropck_eyepatch,
-        "`may_dangle` has unstable semantics and may be removed in the future",
-    ),
-
-    // ==========================================================================
-    // Internal attributes: Runtime related:
-    // ==========================================================================
-
-    rustc_attr!(rustc_allocator, Whitelisted, template!(Word), IMPL_DETAIL),
-    rustc_attr!(rustc_allocator_nounwind, Whitelisted, template!(Word), IMPL_DETAIL),
-    gated!(alloc_error_handler, Normal, template!(Word), experimental!(alloc_error_handler)),
-    gated!(
-        default_lib_allocator, Whitelisted, template!(Word), allocator_internals,
-        experimental!(default_lib_allocator),
-    ),
-    gated!(
-        needs_allocator, Normal, template!(Word), allocator_internals,
-        experimental!(needs_allocator),
-    ),
-    gated!(panic_runtime, Whitelisted, template!(Word), experimental!(panic_runtime)),
-    gated!(needs_panic_runtime, Whitelisted, template!(Word), experimental!(needs_panic_runtime)),
-    gated!(
-        unwind, Whitelisted, template!(List: "allowed|aborts"), unwind_attributes,
-        experimental!(unwind),
-    ),
-    gated!(
-        compiler_builtins, Whitelisted, template!(Word),
-        "the `#[compiler_builtins]` attribute is used to identify the `compiler_builtins` crate \
-        which contains compiler-rt intrinsics and will never be stable",
-    ),
-    gated!(
-        sanitizer_runtime, Whitelisted, template!(Word),
-        "the `#[sanitizer_runtime]` attribute is used to identify crates that contain the runtime \
-        of a sanitizer and will never be stable",
-    ),
-    gated!(
-        profiler_runtime, Whitelisted, template!(Word),
-        "the `#[profiler_runtime]` attribute is used to identify the `profiler_builtins` crate \
-        which contains the profiler runtime and will never be stable",
-    ),
-
-    // ==========================================================================
-    // Internal attributes, Linkage:
-    // ==========================================================================
-
-    gated!(
-        linkage, Whitelisted, template!(NameValueStr: "external|internal|..."),
-        "the `linkage` attribute is experimental and not portable across platforms",
-    ),
-    rustc_attr!(rustc_std_internal_symbol, Whitelisted, template!(Word), INTERNAL_UNSTABLE),
-
-    // ==========================================================================
-    // Internal attributes, Macro related:
-    // ==========================================================================
-
-    rustc_attr!(rustc_builtin_macro, Whitelisted, template!(Word), IMPL_DETAIL),
-    rustc_attr!(rustc_proc_macro_decls, Normal, template!(Word), INTERNAL_UNSTABLE),
-    rustc_attr!(
-        rustc_macro_transparency, Whitelisted,
-        template!(NameValueStr: "transparent|semitransparent|opaque"),
-        "used internally for testing macro hygiene",
-    ),
-
-    // ==========================================================================
-    // Internal attributes, Diagnostics related:
-    // ==========================================================================
-
-    rustc_attr!(
-        rustc_on_unimplemented, Whitelisted,
-        template!(
-            List: r#"/*opt*/ message = "...", /*opt*/ label = "...", /*opt*/ note = "...""#,
-            NameValueStr: "message"
-        ),
-        INTERNAL_UNSTABLE
-    ),
-    // Whitelists "identity-like" conversion methods to suggest on type mismatch.
-    rustc_attr!(rustc_conversion_suggestion, Whitelisted, template!(Word), INTERNAL_UNSTABLE),
-
-    // ==========================================================================
-    // Internal attributes, Const related:
-    // ==========================================================================
-
-    rustc_attr!(rustc_promotable, Whitelisted, template!(Word), IMPL_DETAIL),
-    rustc_attr!(rustc_allow_const_fn_ptr, Whitelisted, template!(Word), IMPL_DETAIL),
-    rustc_attr!(rustc_args_required_const, Whitelisted, template!(List: "N"), INTERNAL_UNSTABLE),
-
-    // ==========================================================================
-    // Internal attributes, Layout related:
-    // ==========================================================================
-
-    rustc_attr!(
-        rustc_layout_scalar_valid_range_start, Whitelisted, template!(List: "value"),
-        "the `#[rustc_layout_scalar_valid_range_start]` attribute is just used to enable \
-        niche optimizations in libcore and will never be stable",
-    ),
-    rustc_attr!(
-        rustc_layout_scalar_valid_range_end, Whitelisted, template!(List: "value"),
-        "the `#[rustc_layout_scalar_valid_range_end]` attribute is just used to enable \
-        niche optimizations in libcore and will never be stable",
-    ),
-    rustc_attr!(
-        rustc_nonnull_optimization_guaranteed, Whitelisted, template!(Word),
-        "the `#[rustc_nonnull_optimization_guaranteed]` attribute is just used to enable \
-        niche optimizations in libcore and will never be stable",
-    ),
-
-    // ==========================================================================
-    // Internal attributes, Misc:
-    // ==========================================================================
-    gated!(
-        lang, Normal, template!(NameValueStr: "name"), lang_items,
-        "language items are subject to change",
-    ),
-    (
-        sym::rustc_diagnostic_item,
-        Normal,
-        template!(NameValueStr: "name"),
-        Gated(
-            Stability::Unstable,
-            sym::rustc_attrs,
-            "diagnostic items compiler internal support for linting",
-            cfg_fn!(rustc_attrs),
-        ),
-    ),
-    (
-        sym::no_debug, Whitelisted, template!(Word),
-        Gated(
-            Stability::Deprecated("https://github.com/rust-lang/rust/issues/29721", None),
-            sym::no_debug,
-            "the `#[no_debug]` attribute was an experimental feature that has been \
-            deprecated due to lack of demand",
-            cfg_fn!(no_debug)
-        )
-    ),
-    gated!(
-        // Used in resolve:
-        prelude_import, Whitelisted, template!(Word),
-        "`#[prelude_import]` is for use by rustc only",
-    ),
-    gated!(
-        rustc_paren_sugar, Normal, template!(Word), unboxed_closures,
-        "unboxed_closures are still evolving",
-    ),
-    rustc_attr!(
-        rustc_inherit_overflow_checks, Whitelisted, template!(Word),
-        "the `#[rustc_inherit_overflow_checks]` attribute is just used to control \
-        overflow checking behavior of several libcore functions that are inlined \
-        across crates and will never be stable",
-    ),
-    rustc_attr!(rustc_reservation_impl, Normal, template!(NameValueStr: "reservation message"),
-                "the `#[rustc_reservation_impl]` attribute is internally used \
-                 for reserving for `for<T> From<!> for T` impl"
-    ),
-    rustc_attr!(
-        rustc_test_marker, Normal, template!(Word),
-        "the `#[rustc_test_marker]` attribute is used internally to track tests",
-    ),
-
-    // ==========================================================================
-    // Internal attributes, Testing:
-    // ==========================================================================
-
-    rustc_attr!(TEST, rustc_outlives, Normal, template!(Word)),
-    rustc_attr!(TEST, rustc_variance, Normal, template!(Word)),
-    rustc_attr!(TEST, rustc_layout, Normal, template!(List: "field1, field2, ...")),
-    rustc_attr!(TEST, rustc_regions, Normal, template!(Word)),
-    rustc_attr!(
-        TEST, rustc_error, Whitelisted,
-        template!(Word, List: "delay_span_bug_from_inside_query")
-    ),
-    rustc_attr!(TEST, rustc_dump_user_substs, Whitelisted, template!(Word)),
-    rustc_attr!(TEST, rustc_if_this_changed, Whitelisted, template!(Word, List: "DepNode")),
-    rustc_attr!(TEST, rustc_then_this_would_need, Whitelisted, template!(List: "DepNode")),
-    rustc_attr!(
-        TEST, rustc_dirty, Whitelisted,
-        template!(List: r#"cfg = "...", /*opt*/ label = "...", /*opt*/ except = "...""#),
-    ),
-    rustc_attr!(
-        TEST, rustc_clean, Whitelisted,
-        template!(List: r#"cfg = "...", /*opt*/ label = "...", /*opt*/ except = "...""#),
-    ),
-    rustc_attr!(
-        TEST, rustc_partition_reused, Whitelisted,
-        template!(List: r#"cfg = "...", module = "...""#),
-    ),
-    rustc_attr!(
-        TEST, rustc_partition_codegened, Whitelisted,
-        template!(List: r#"cfg = "...", module = "...""#),
-    ),
-    rustc_attr!(
-        TEST, rustc_expected_cgu_reuse, Whitelisted,
-        template!(List: r#"cfg = "...", module = "...", kind = "...""#),
-    ),
-    rustc_attr!(TEST, rustc_synthetic, Whitelisted, template!(Word)),
-    rustc_attr!(TEST, rustc_symbol_name, Whitelisted, template!(Word)),
-    rustc_attr!(TEST, rustc_def_path, Whitelisted, template!(Word)),
-    rustc_attr!(TEST, rustc_mir, Whitelisted, template!(List: "arg1, arg2, ...")),
-    rustc_attr!(TEST, rustc_dump_program_clauses, Whitelisted, template!(Word)),
-    rustc_attr!(TEST, rustc_dump_env_program_clauses, Whitelisted, template!(Word)),
-    rustc_attr!(TEST, rustc_object_lifetime_default, Whitelisted, template!(Word)),
-    rustc_attr!(TEST, rustc_dummy, Normal, template!(Word /* doesn't matter*/)),
-    gated!(
-        omit_gdb_pretty_printer_section, Whitelisted, template!(Word),
-        "the `#[omit_gdb_pretty_printer_section]` attribute is just used for the Rust test suite",
-    ),
-];
-
-pub fn deprecated_attributes() -> Vec<&'static BuiltinAttribute> {
-    BUILTIN_ATTRIBUTES.iter().filter(|(.., gate)| gate.is_deprecated()).collect()
-}
-
-pub fn is_builtin_attr_name(name: ast::Name) -> bool {
-    BUILTIN_ATTRIBUTE_MAP.get(&name).is_some()
-}
-
-pub fn is_builtin_attr(attr: &ast::Attribute) -> bool {
-    attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name)).is_some()
-}
-
-lazy_static! {
-    pub static ref BUILTIN_ATTRIBUTE_MAP: FxHashMap<Symbol, &'static BuiltinAttribute> = {
-        let mut map = FxHashMap::default();
-        for attr in BUILTIN_ATTRIBUTES.iter() {
-            if map.insert(attr.0, attr).is_some() {
-                panic!("duplicate builtin attribute `{}`", attr.0);
-            }
-        }
-        map
-    };
-}
diff --git a/src/libsyntax/feature_gate/check.rs b/src/libsyntax/feature_gate/check.rs
index ec0eaa56812..3d2c3b1d4f9 100644
--- a/src/libsyntax/feature_gate/check.rs
+++ b/src/libsyntax/feature_gate/check.rs
@@ -1,7 +1,7 @@
-use super::{active::{ACTIVE_FEATURES, Features}, Feature, State as FeatureState};
-use super::accepted::ACCEPTED_FEATURES;
-use super::removed::{REMOVED_FEATURES, STABLE_REMOVED_FEATURES};
-use super::builtin_attrs::{AttributeGate, BUILTIN_ATTRIBUTE_MAP};
+use rustc_feature::{ACCEPTED_FEATURES, ACTIVE_FEATURES, REMOVED_FEATURES, STABLE_REMOVED_FEATURES};
+use rustc_feature::{AttributeGate, BUILTIN_ATTRIBUTE_MAP};
+use rustc_feature::{Features, Feature, State as FeatureState, UnstableFeatures};
+use rustc_feature::{find_feature_issue, GateIssue};
 
 use crate::ast::{self, AssocTyConstraint, AssocTyConstraintKind, NodeId};
 use crate::ast::{GenericParam, GenericParamKind, PatKind, RangeEnd, VariantData};
@@ -19,18 +19,6 @@ use log::debug;
 
 use rustc_error_codes::*;
 
-
-use std::env;
-use std::num::NonZeroU32;
-
-#[derive(Copy, Clone, Debug)]
-pub enum Stability {
-    Unstable,
-    // First argument is tracking issue link; second argument is an optional
-    // help message, which defaults to "remove this attribute"
-    Deprecated(&'static str, Option<&'static str>),
-}
-
 macro_rules! gate_feature_fn {
     ($cx: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr, $level: expr) => {{
         let (cx, has_feature, span,
@@ -59,30 +47,6 @@ pub fn check_attribute(attr: &ast::Attribute, parse_sess: &ParseSess, features:
     PostExpansionVisitor { parse_sess, features }.visit_attribute(attr)
 }
 
-fn find_lang_feature_issue(feature: Symbol) -> Option<NonZeroU32> {
-    if let Some(info) = ACTIVE_FEATURES.iter().find(|t| t.name == feature) {
-        // FIXME (#28244): enforce that active features have issue numbers
-        // assert!(info.issue().is_some())
-        info.issue()
-    } else {
-        // search in Accepted, Removed, or Stable Removed features
-        let found = ACCEPTED_FEATURES
-            .iter()
-            .chain(REMOVED_FEATURES)
-            .chain(STABLE_REMOVED_FEATURES)
-            .find(|t| t.name == feature);
-        match found {
-            Some(found) => found.issue(),
-            None => panic!("feature `{}` is not declared anywhere", feature),
-        }
-    }
-}
-
-pub enum GateIssue {
-    Language,
-    Library(Option<NonZeroU32>)
-}
-
 #[derive(Debug, Copy, Clone, PartialEq)]
 pub enum GateStrength {
     /// A hard error. (Most feature gates should use this.)
@@ -91,41 +55,35 @@ pub enum GateStrength {
     Soft,
 }
 
-pub fn emit_feature_err(
-    sess: &ParseSess,
+pub fn feature_err<'a>(
+    sess: &'a ParseSess,
     feature: Symbol,
-    span: Span,
-    issue: GateIssue,
+    span: impl Into<MultiSpan>,
     explain: &str,
-) {
-    feature_err(sess, feature, span, issue, explain).emit();
+) -> DiagnosticBuilder<'a> {
+    feature_err_issue(sess, feature, span, GateIssue::Language, explain)
 }
 
-pub fn feature_err<'a, S: Into<MultiSpan>>(
+pub fn feature_err_issue<'a>(
     sess: &'a ParseSess,
     feature: Symbol,
-    span: S,
+    span: impl Into<MultiSpan>,
     issue: GateIssue,
     explain: &str,
 ) -> DiagnosticBuilder<'a> {
     leveled_feature_err(sess, feature, span, issue, explain, GateStrength::Hard)
 }
 
-fn leveled_feature_err<'a, S: Into<MultiSpan>>(
+fn leveled_feature_err<'a>(
     sess: &'a ParseSess,
     feature: Symbol,
-    span: S,
+    span: impl Into<MultiSpan>,
     issue: GateIssue,
     explain: &str,
     level: GateStrength,
 ) -> DiagnosticBuilder<'a> {
     let diag = &sess.span_diagnostic;
 
-    let issue = match issue {
-        GateIssue::Language => find_lang_feature_issue(feature),
-        GateIssue::Library(lib) => lib,
-    };
-
     let mut err = match level {
         GateStrength::Hard => {
             diag.struct_span_err_with_code(span, explain, stringify_error_code!(E0658))
@@ -133,7 +91,7 @@ fn leveled_feature_err<'a, S: Into<MultiSpan>>(
         GateStrength::Soft => diag.struct_span_warn(span, explain),
     };
 
-    if let Some(n) = issue {
+    if let Some(n) = find_feature_issue(feature, issue) {
         err.note(&format!(
             "for more information, see https://github.com/rust-lang/rust/issues/{}",
             n,
@@ -156,20 +114,6 @@ fn leveled_feature_err<'a, S: Into<MultiSpan>>(
 
 }
 
-const EXPLAIN_BOX_SYNTAX: &str =
-    "box expression syntax is experimental; you can call `Box::new` instead";
-
-pub const EXPLAIN_STMT_ATTR_SYNTAX: &str =
-    "attributes on expressions are experimental";
-
-pub const EXPLAIN_ALLOW_INTERNAL_UNSTABLE: &str =
-    "allow_internal_unstable side-steps feature gating and stability checks";
-pub const EXPLAIN_ALLOW_INTERNAL_UNSAFE: &str =
-    "allow_internal_unsafe side-steps the unsafe_code lint";
-
-pub const EXPLAIN_UNSIZED_TUPLE_COERCION: &str =
-    "unsized tuple coercion is not stable enough for use and is subject to change";
-
 struct PostExpansionVisitor<'a> {
     parse_sess: &'a ParseSess,
     features: &'a Features,
@@ -282,7 +226,6 @@ impl<'a> PostExpansionVisitor<'a> {
                 self.parse_sess,
                 sym::arbitrary_enum_discriminant,
                 discriminant_spans.clone(),
-                crate::feature_gate::GateIssue::Language,
                 "custom discriminant values are not allowed in enums with tuple or struct variants",
             );
             for sp in discriminant_spans {
@@ -529,7 +472,10 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
     fn visit_expr(&mut self, e: &'a ast::Expr) {
         match e.kind {
             ast::ExprKind::Box(_) => {
-                gate_feature_post!(&self, box_syntax, e.span, EXPLAIN_BOX_SYNTAX);
+                gate_feature_post!(
+                    &self, box_syntax, e.span,
+                    "box expression syntax is experimental; you can call `Box::new` instead"
+                );
             }
             ast::ExprKind::Type(..) => {
                 // To avoid noise about type ascription in common syntax errors, only emit if it
@@ -695,7 +641,7 @@ pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
         err.emit();
     }
 
-    let mut features = Features::new();
+    let mut features = Features::default();
     let mut edition_enabled_features = FxHashMap::default();
 
     for &edition in ALL_EDITIONS {
@@ -900,40 +846,6 @@ pub fn check_crate(krate: &ast::Crate,
     visit::walk_crate(&mut visitor, krate);
 }
 
-#[derive(Clone, Copy, Hash)]
-pub enum UnstableFeatures {
-    /// Hard errors for unstable features are active, as on beta/stable channels.
-    Disallow,
-    /// Allow features to be activated, as on nightly.
-    Allow,
-    /// Errors are bypassed for bootstrapping. This is required any time
-    /// during the build that feature-related lints are set to warn or above
-    /// because the build turns on warnings-as-errors and uses lots of unstable
-    /// features. As a result, this is always required for building Rust itself.
-    Cheat
-}
-
-impl UnstableFeatures {
-    pub fn from_environment() -> UnstableFeatures {
-        // `true` if this is a feature-staged build, i.e., on the beta or stable channel.
-        let disable_unstable_features = option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some();
-        // `true` if we should enable unstable features for bootstrapping.
-        let bootstrap = env::var("RUSTC_BOOTSTRAP").is_ok();
-        match (disable_unstable_features, bootstrap) {
-            (_, true) => UnstableFeatures::Cheat,
-            (true, _) => UnstableFeatures::Disallow,
-            (false, _) => UnstableFeatures::Allow
-        }
-    }
-
-    pub fn is_nightly_build(&self) -> bool {
-        match *self {
-            UnstableFeatures::Allow | UnstableFeatures::Cheat => true,
-            UnstableFeatures::Disallow => false,
-        }
-    }
-}
-
 fn maybe_stage_features(span_handler: &Handler, krate: &ast::Crate, unstable: UnstableFeatures) {
     if !unstable.is_nightly_build() {
         for attr in krate.attrs.iter().filter(|attr| attr.check_name(sym::feature)) {
diff --git a/src/libsyntax/feature_gate/mod.rs b/src/libsyntax/feature_gate/mod.rs
deleted file mode 100644
index c4418c0f0f6..00000000000
--- a/src/libsyntax/feature_gate/mod.rs
+++ /dev/null
@@ -1,71 +0,0 @@
-//! # Feature gating
-//!
-//! This module implements the gating necessary for preventing certain compiler
-//! features from being used by default. This module will crawl a pre-expanded
-//! AST to ensure that there are no features which are used that are not
-//! enabled.
-//!
-//! Features are enabled in programs via the crate-level attributes of
-//! `#![feature(...)]` with a comma-separated list of features.
-//!
-//! For the purpose of future feature-tracking, once code for detection of feature
-//! gate usage is added, *do not remove it again* even once the feature
-//! becomes stable.
-
-mod accepted;
-mod removed;
-mod active;
-mod builtin_attrs;
-mod check;
-
-use crate::{edition::Edition, symbol::Symbol};
-use std::fmt;
-use std::num::NonZeroU32;
-use syntax_pos::Span;
-
-#[derive(Clone, Copy)]
-pub enum State {
-    Accepted,
-    Active { set: fn(&mut Features, Span) },
-    Removed { reason: Option<&'static str> },
-    Stabilized { 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"),
-            State::Stabilized { .. } => write!(f, "stabilized"),
-        }
-    }
-}
-
-#[derive(Debug, Clone)]
-pub struct Feature {
-    state: State,
-    name: Symbol,
-    since: &'static str,
-    issue: Option<u32>,  // FIXME: once #58732 is done make this an Option<NonZeroU32>
-    edition: Option<Edition>,
-    description: &'static str,
-}
-
-impl Feature {
-    fn issue(&self) -> Option<NonZeroU32> {
-        self.issue.and_then(|i| NonZeroU32::new(i))
-    }
-}
-
-pub use active::{Features, INCOMPLETE_FEATURES};
-pub use builtin_attrs::{
-    AttributeGate, AttributeType, GatedCfg,
-    BuiltinAttribute, BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
-    deprecated_attributes, is_builtin_attr,  is_builtin_attr_name,
-};
-pub use check::{
-    check_crate, check_attribute, get_features, feature_err, emit_feature_err,
-    Stability, GateIssue, UnstableFeatures,
-    EXPLAIN_STMT_ATTR_SYNTAX, EXPLAIN_UNSIZED_TUPLE_COERCION,
-};
diff --git a/src/libsyntax/feature_gate/removed.rs b/src/libsyntax/feature_gate/removed.rs
deleted file mode 100644
index f0aa74c65df..00000000000
--- a/src/libsyntax/feature_gate/removed.rs
+++ /dev/null
@@ -1,116 +0,0 @@
-//! List of the removed feature gates.
-
-use crate::symbol::sym;
-use super::{State, Feature};
-
-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: $reason },
-                    name: sym::$feature,
-                    since: $ver,
-                    issue: $issue,
-                    edition: None,
-                    description: concat!($($doc,)*),
-                }
-            ),+
-        ];
-    };
-
-    ($(
-        $(#[doc = $doc:tt])* (stable_removed, $feature:ident, $ver:expr, $issue:expr, None),
-    )+) => {
-        /// Represents stable features which have since been removed (it was once Accepted)
-        pub const STABLE_REMOVED_FEATURES: &[Feature] = &[
-            $(
-                Feature {
-                    state: State::Stabilized { reason: None },
-                    name: sym::$feature,
-                    since: $ver,
-                    issue: $issue,
-                    edition: None,
-                    description: concat!($($doc,)*),
-                }
-            ),+
-        ];
-    };
-}
-
-declare_features! (
-    // -------------------------------------------------------------------------
-    // feature-group-start: removed features
-    // -------------------------------------------------------------------------
-
-    (removed, import_shadowing, "1.0.0", None, None, None),
-    (removed, managed_boxes, "1.0.0", None, None, None),
-    /// Allows use of unary negate on unsigned integers, e.g., -e for e: u8
-    (removed, negate_unsigned, "1.0.0", Some(29645), None, None),
-    (removed, reflect, "1.0.0", Some(27749), None, None),
-    /// A way to temporarily opt out of opt in copy. This will *never* be accepted.
-    (removed, opt_out_copy, "1.0.0", None, None, None),
-    (removed, quad_precision_float, "1.0.0", None, None, None),
-    (removed, struct_inherit, "1.0.0", None, None, None),
-    (removed, test_removed_feature, "1.0.0", None, None, None),
-    (removed, visible_private_types, "1.0.0", None, None, None),
-    (removed, unsafe_no_drop_flag, "1.0.0", None, None, None),
-    /// Allows using items which are missing stability attributes
-    (removed, unmarked_api, "1.0.0", None, None, None),
-    (removed, allocator, "1.0.0", None, None, None),
-    (removed, simd, "1.0.0", Some(27731), None,
-     Some("removed in favor of `#[repr(simd)]`")),
-    (removed, advanced_slice_patterns, "1.0.0", Some(62254), None,
-     Some("merged into `#![feature(slice_patterns)]`")),
-    (removed, macro_reexport, "1.0.0", Some(29638), None,
-     Some("subsumed by `pub use`")),
-    /// Allows using custom attributes (RFC 572).
-    (removed, custom_attribute, "1.0.0", Some(29642), None,
-     Some("removed in favor of `#![register_tool]` and `#![register_attr]`")),
-    (removed, pushpop_unsafe, "1.2.0", None, None, None),
-    (removed, needs_allocator, "1.4.0", Some(27389), None,
-     Some("subsumed by `#![feature(allocator_internals)]`")),
-    (removed, proc_macro_mod, "1.27.0", Some(54727), None,
-     Some("subsumed by `#![feature(proc_macro_hygiene)]`")),
-    (removed, proc_macro_expr, "1.27.0", Some(54727), None,
-     Some("subsumed by `#![feature(proc_macro_hygiene)]`")),
-    (removed, proc_macro_non_items, "1.27.0", Some(54727), None,
-     Some("subsumed by `#![feature(proc_macro_hygiene)]`")),
-    (removed, proc_macro_gen, "1.27.0", Some(54727), None,
-     Some("subsumed by `#![feature(proc_macro_hygiene)]`")),
-    (removed, panic_implementation, "1.28.0", Some(44489), None,
-     Some("subsumed by `#[panic_handler]`")),
-    /// Allows the use of `#[derive(Anything)]` as sugar for `#[derive_Anything]`.
-    (removed, custom_derive, "1.32.0", Some(29644), None,
-     Some("subsumed by `#[proc_macro_derive]`")),
-    /// Paths of the form: `extern::foo::bar`
-    (removed, extern_in_paths, "1.33.0", Some(55600), None,
-     Some("subsumed by `::foo::bar` paths")),
-    (removed, quote, "1.33.0", Some(29601), None, None),
-    /// Allows using `#[unsafe_destructor_blind_to_params]` (RFC 1238).
-    (removed, dropck_parametricity, "1.38.0", Some(28498), None, None),
-    (removed, await_macro, "1.38.0", Some(50547), None,
-     Some("subsumed by `.await` syntax")),
-    /// Allows defining `existential type`s.
-    (removed, existential_type, "1.38.0", Some(63063), None,
-     Some("removed in favor of `#![feature(type_alias_impl_trait)]`")),
-    /// Allows using the macros:
-    /// + `__diagnostic_used`
-    /// + `__register_diagnostic`
-    /// +`__build_diagnostic_array`
-    (removed, rustc_diagnostic_macros, "1.38.0", None, None, None),
-    /// Allows using `#[on_unimplemented(..)]` on traits.
-    /// (Moved to `rustc_attrs`.)
-    (removed, on_unimplemented, "1.40.0", None, None, None),
-
-    // -------------------------------------------------------------------------
-    // feature-group-end: removed features
-    // -------------------------------------------------------------------------
-);
-
-declare_features! (
-    (stable_removed, no_stack_check, "1.0.0", None, None),
-);
diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index 22b49862f49..3d4a5d624c1 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -92,7 +92,10 @@ pub mod attr;
 pub mod expand;
 pub use syntax_pos::source_map;
 pub mod entry;
-pub mod feature_gate;
+pub mod feature_gate {
+    mod check;
+    pub use check::{check_crate, check_attribute, get_features, feature_err, feature_err_issue};
+}
 pub mod mut_visit;
 pub mod ptr;
 pub mod show_span;
diff --git a/src/libsyntax/sess.rs b/src/libsyntax/sess.rs
index 740e9dfe459..aa9217c1b69 100644
--- a/src/libsyntax/sess.rs
+++ b/src/libsyntax/sess.rs
@@ -3,15 +3,15 @@
 
 use crate::ast::{CrateConfig, NodeId};
 use crate::early_buffered_lints::{BufferedEarlyLint, BufferedEarlyLintId};
-use crate::source_map::{SourceMap, FilePathMapping};
-use crate::feature_gate::UnstableFeatures;
 
 use errors::{Applicability, emitter::SilentEmitter, Handler, ColorConfig, DiagnosticBuilder};
 use rustc_data_structures::fx::{FxHashSet, FxHashMap};
 use rustc_data_structures::sync::{Lrc, Lock, Once};
+use rustc_feature::UnstableFeatures;
 use syntax_pos::{Symbol, Span, MultiSpan};
 use syntax_pos::edition::Edition;
 use syntax_pos::hygiene::ExpnId;
+use syntax_pos::source_map::{SourceMap, FilePathMapping};
 
 use std::path::PathBuf;
 use std::str;