src/librustc_passes/stability.rs

   1 //! A pass that annotates every item and method with its stability level,
   2 //! propagating default levels lexically from parent to children ast nodes.
   3
   4 use rustc::hir::map::Map;
   5 use rustc::lint;
   6 use rustc::middle::privacy::AccessLevels;
   7 use rustc::middle::stability::{DeprecationEntry, Index};
   8 use rustc::session::parse::feature_err;
   9 use rustc::session::Session;
  10 use rustc::traits::misc::can_type_implement_copy;
  11 use rustc::ty::query::Providers;
  12 use rustc::ty::TyCtxt;
  13 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  14 use rustc_errors::struct_span_err;
  15 use rustc_hir as hir;
  16 use rustc_hir::def::{DefKind, Res};
  17 use rustc_hir::def_id::{DefId, CRATE_DEF_INDEX, LOCAL_CRATE};
  18 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
  19 use rustc_hir::{Generics, HirId, Item, StructField, Variant};
  20 use rustc_span::symbol::{sym, Symbol};
  21 use rustc_span::Span;
  22 use syntax::ast::Attribute;
  23 use syntax::attr::{self, Stability};
  24
  25 use std::cmp::Ordering;
  26 use std::mem::replace;
  27 use std::num::NonZeroU32;
  28
  29 #[derive(PartialEq)]
  30 enum AnnotationKind {
  31     // Annotation is required if not inherited from unstable parents
  32     Required,
  33     // Annotation is useless, reject it
  34     Prohibited,
  35     // Annotation itself is useless, but it can be propagated to children
  36     Container,
  37 }
  38
  39 // A private tree-walker for producing an Index.
  40 struct Annotator<'a, 'tcx> {
  41     tcx: TyCtxt<'tcx>,
  42     index: &'a mut Index<'tcx>,
  43     parent_stab: Option<&'tcx Stability>,
  44     parent_depr: Option<DeprecationEntry>,
  45     in_trait_impl: bool,
  46 }
  47
  48 impl<'a, 'tcx> Annotator<'a, 'tcx> {
  49     // Determine the stability for a node based on its attributes and inherited
  50     // stability. The stability is recorded in the index and used as the parent.
  51     fn annotate<F>(
  52         &mut self,
  53         hir_id: HirId,
  54         attrs: &[Attribute],
  55         item_sp: Span,
  56         kind: AnnotationKind,
  57         visit_children: F,
  58     ) where
  59         F: FnOnce(&mut Self),
  60     {
  61         if self.tcx.features().staged_api {
  62             // This crate explicitly wants staged API.
  63             debug!("annotate(id = {:?}, attrs = {:?})", hir_id, attrs);
  64             if let Some(..) = attr::find_deprecation(&self.tcx.sess.parse_sess, attrs, item_sp) {
  65                 self.tcx.sess.span_err(
  66                     item_sp,
  67                     "`#[deprecated]` cannot be used in staged API; \
  68                                                  use `#[rustc_deprecated]` instead",
  69                 );
  70             }
  71             let (stab, const_stab) =
  72                 attr::find_stability(&self.tcx.sess.parse_sess, attrs, item_sp);
  73             if let Some(const_stab) = const_stab {
  74                 let const_stab = self.tcx.intern_const_stability(const_stab);
  75                 self.index.const_stab_map.insert(hir_id, const_stab);
  76             }
  77             if let Some(mut stab) = stab {
  78                 // Error if prohibited, or can't inherit anything from a container.
  79                 if kind == AnnotationKind::Prohibited
  80                     || (kind == AnnotationKind::Container
  81                         && stab.level.is_stable()
  82                         && stab.rustc_depr.is_none())
  83                 {
  84                     self.tcx.sess.span_err(item_sp, "This stability annotation is useless");
  85                 }
  86
  87                 debug!("annotate: found {:?}", stab);
  88                 // If parent is deprecated and we're not, inherit this by merging
  89                 // deprecated_since and its reason.
  90                 if let Some(parent_stab) = self.parent_stab {
  91                     if parent_stab.rustc_depr.is_some() && stab.rustc_depr.is_none() {
  92                         stab.rustc_depr = parent_stab.rustc_depr
  93                     }
  94                 }
  95
  96                 let stab = self.tcx.intern_stability(stab);
  97
  98                 // Check if deprecated_since < stable_since. If it is,
  99                 // this is *almost surely* an accident.
 100                 if let (
 101                     &Some(attr::RustcDeprecation { since: dep_since, .. }),
 102                     &attr::Stable { since: stab_since },
 103                 ) = (&stab.rustc_depr, &stab.level)
 104                 {
 105                     // Explicit version of iter::order::lt to handle parse errors properly
 106                     for (dep_v, stab_v) in
 107                         dep_since.as_str().split('.').zip(stab_since.as_str().split('.'))
 108                     {
 109                         if let (Ok(dep_v), Ok(stab_v)) = (dep_v.parse::<u64>(), stab_v.parse()) {
 110                             match dep_v.cmp(&stab_v) {
 111                                 Ordering::Less => {
 112                                     self.tcx.sess.span_err(
 113                                         item_sp,
 114                                         "An API can't be stabilized \
 115                                                                      after it is deprecated",
 116                                     );
 117                                     break;
 118                                 }
 119                                 Ordering::Equal => continue,
 120                                 Ordering::Greater => break,
 121                             }
 122                         } else {
 123                             // Act like it isn't less because the question is now nonsensical,
 124                             // and this makes us not do anything else interesting.
 125                             self.tcx.sess.span_err(
 126                                 item_sp,
 127                                 "Invalid stability or deprecation \
 128                                                              version found",
 129                             );
 130                             break;
 131                         }
 132                     }
 133                 }
 134
 135                 self.index.stab_map.insert(hir_id, stab);
 136
 137                 let orig_parent_stab = replace(&mut self.parent_stab, Some(stab));
 138                 visit_children(self);
 139                 self.parent_stab = orig_parent_stab;
 140             } else {
 141                 debug!("annotate: not found, parent = {:?}", self.parent_stab);
 142                 if let Some(stab) = self.parent_stab {
 143                     if stab.level.is_unstable() {
 144                         self.index.stab_map.insert(hir_id, stab);
 145                     }
 146                 }
 147                 visit_children(self);
 148             }
 149         } else {
 150             // Emit errors for non-staged-api crates.
 151             let unstable_attrs = [
 152                 sym::unstable,
 153                 sym::stable,
 154                 sym::rustc_deprecated,
 155                 sym::rustc_const_unstable,
 156                 sym::rustc_const_stable,
 157             ];
 158             for attr in attrs {
 159                 let name = attr.name_or_empty();
 160                 if unstable_attrs.contains(&name) {
 161                     attr::mark_used(attr);
 162                     struct_span_err!(
 163                         self.tcx.sess,
 164                         attr.span,
 165                         E0734,
 166                         "stability attributes may not be used outside of the standard library",
 167                     )
 168                     .emit();
 169                 }
 170             }
 171
 172             // Propagate unstability.  This can happen even for non-staged-api crates in case
 173             // -Zforce-unstable-if-unmarked is set.
 174             if let Some(stab) = self.parent_stab {
 175                 if stab.level.is_unstable() {
 176                     self.index.stab_map.insert(hir_id, stab);
 177                 }
 178             }
 179
 180             if let Some(depr) = attr::find_deprecation(&self.tcx.sess.parse_sess, attrs, item_sp) {
 181                 if kind == AnnotationKind::Prohibited {
 182                     self.tcx.sess.span_err(item_sp, "This deprecation annotation is useless");
 183                 }
 184
 185                 // `Deprecation` is just two pointers, no need to intern it
 186                 let depr_entry = DeprecationEntry::local(depr, hir_id);
 187                 self.index.depr_map.insert(hir_id, depr_entry.clone());
 188
 189                 let orig_parent_depr = replace(&mut self.parent_depr, Some(depr_entry));
 190                 visit_children(self);
 191                 self.parent_depr = orig_parent_depr;
 192             } else if let Some(parent_depr) = self.parent_depr.clone() {
 193                 self.index.depr_map.insert(hir_id, parent_depr);
 194                 visit_children(self);
 195             } else {
 196                 visit_children(self);
 197             }
 198         }
 199     }
 200 }
 201
 202 impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
 203     /// Because stability levels are scoped lexically, we want to walk
 204     /// nested items in the context of the outer item, so enable
 205     /// deep-walking.
 206     type Map = Map<'tcx>;
 207
 208     fn nested_visit_map(&mut self) -> NestedVisitorMap<'_, Self::Map> {
 209         NestedVisitorMap::All(&self.tcx.hir())
 210     }
 211
 212     fn visit_item(&mut self, i: &'tcx Item<'tcx>) {
 213         let orig_in_trait_impl = self.in_trait_impl;
 214         let mut kind = AnnotationKind::Required;
 215         match i.kind {
 216             // Inherent impls and foreign modules serve only as containers for other items,
 217             // they don't have their own stability. They still can be annotated as unstable
 218             // and propagate this unstability to children, but this annotation is completely
 219             // optional. They inherit stability from their parents when unannotated.
 220             hir::ItemKind::Impl { of_trait: None, .. } | hir::ItemKind::ForeignMod(..) => {
 221                 self.in_trait_impl = false;
 222                 kind = AnnotationKind::Container;
 223             }
 224             hir::ItemKind::Impl { of_trait: Some(_), .. } => {
 225                 self.in_trait_impl = true;
 226             }
 227             hir::ItemKind::Struct(ref sd, _) => {
 228                 if let Some(ctor_hir_id) = sd.ctor_hir_id() {
 229                     self.annotate(ctor_hir_id, &i.attrs, i.span, AnnotationKind::Required, |_| {})
 230                 }
 231             }
 232             _ => {}
 233         }
 234
 235         self.annotate(i.hir_id, &i.attrs, i.span, kind, |v| intravisit::walk_item(v, i));
 236         self.in_trait_impl = orig_in_trait_impl;
 237     }
 238
 239     fn visit_trait_item(&mut self, ti: &'tcx hir::TraitItem<'tcx>) {
 240         self.annotate(ti.hir_id, &ti.attrs, ti.span, AnnotationKind::Required, |v| {
 241             intravisit::walk_trait_item(v, ti);
 242         });
 243     }
 244
 245     fn visit_impl_item(&mut self, ii: &'tcx hir::ImplItem<'tcx>) {
 246         let kind =
 247             if self.in_trait_impl { AnnotationKind::Prohibited } else { AnnotationKind::Required };
 248         self.annotate(ii.hir_id, &ii.attrs, ii.span, kind, |v| {
 249             intravisit::walk_impl_item(v, ii);
 250         });
 251     }
 252
 253     fn visit_variant(&mut self, var: &'tcx Variant<'tcx>, g: &'tcx Generics<'tcx>, item_id: HirId) {
 254         self.annotate(var.id, &var.attrs, var.span, AnnotationKind::Required, |v| {
 255             if let Some(ctor_hir_id) = var.data.ctor_hir_id() {
 256                 v.annotate(ctor_hir_id, &var.attrs, var.span, AnnotationKind::Required, |_| {});
 257             }
 258
 259             intravisit::walk_variant(v, var, g, item_id)
 260         })
 261     }
 262
 263     fn visit_struct_field(&mut self, s: &'tcx StructField<'tcx>) {
 264         self.annotate(s.hir_id, &s.attrs, s.span, AnnotationKind::Required, |v| {
 265             intravisit::walk_struct_field(v, s);
 266         });
 267     }
 268
 269     fn visit_foreign_item(&mut self, i: &'tcx hir::ForeignItem<'tcx>) {
 270         self.annotate(i.hir_id, &i.attrs, i.span, AnnotationKind::Required, |v| {
 271             intravisit::walk_foreign_item(v, i);
 272         });
 273     }
 274
 275     fn visit_macro_def(&mut self, md: &'tcx hir::MacroDef<'tcx>) {
 276         self.annotate(md.hir_id, &md.attrs, md.span, AnnotationKind::Required, |_| {});
 277     }
 278 }
 279
 280 struct MissingStabilityAnnotations<'a, 'tcx> {
 281     tcx: TyCtxt<'tcx>,
 282     access_levels: &'a AccessLevels,
 283 }
 284
 285 impl<'a, 'tcx> MissingStabilityAnnotations<'a, 'tcx> {
 286     fn check_missing_stability(&self, hir_id: HirId, span: Span, name: &str) {
 287         let stab = self.tcx.stability().local_stability(hir_id);
 288         let is_error =
 289             !self.tcx.sess.opts.test && stab.is_none() && self.access_levels.is_reachable(hir_id);
 290         if is_error {
 291             self.tcx.sess.span_err(span, &format!("{} has missing stability attribute", name));
 292         }
 293     }
 294 }
 295
 296 impl<'a, 'tcx> Visitor<'tcx> for MissingStabilityAnnotations<'a, 'tcx> {
 297     type Map = Map<'tcx>;
 298
 299     fn nested_visit_map(&mut self) -> NestedVisitorMap<'_, Self::Map> {
 300         NestedVisitorMap::OnlyBodies(&self.tcx.hir())
 301     }
 302
 303     fn visit_item(&mut self, i: &'tcx Item<'tcx>) {
 304         match i.kind {
 305             // Inherent impls and foreign modules serve only as containers for other items,
 306             // they don't have their own stability. They still can be annotated as unstable
 307             // and propagate this unstability to children, but this annotation is completely
 308             // optional. They inherit stability from their parents when unannotated.
 309             hir::ItemKind::Impl { of_trait: None, .. } | hir::ItemKind::ForeignMod(..) => {}
 310
 311             _ => self.check_missing_stability(i.hir_id, i.span, i.kind.descriptive_variant()),
 312         }
 313
 314         intravisit::walk_item(self, i)
 315     }
 316
 317     fn visit_trait_item(&mut self, ti: &'tcx hir::TraitItem<'tcx>) {
 318         self.check_missing_stability(ti.hir_id, ti.span, "item");
 319         intravisit::walk_trait_item(self, ti);
 320     }
 321
 322     fn visit_impl_item(&mut self, ii: &'tcx hir::ImplItem<'tcx>) {
 323         let impl_def_id = self.tcx.hir().local_def_id(self.tcx.hir().get_parent_item(ii.hir_id));
 324         if self.tcx.impl_trait_ref(impl_def_id).is_none() {
 325             self.check_missing_stability(ii.hir_id, ii.span, "item");
 326         }
 327         intravisit::walk_impl_item(self, ii);
 328     }
 329
 330     fn visit_variant(&mut self, var: &'tcx Variant<'tcx>, g: &'tcx Generics<'tcx>, item_id: HirId) {
 331         self.check_missing_stability(var.id, var.span, "variant");
 332         intravisit::walk_variant(self, var, g, item_id);
 333     }
 334
 335     fn visit_struct_field(&mut self, s: &'tcx StructField<'tcx>) {
 336         self.check_missing_stability(s.hir_id, s.span, "field");
 337         intravisit::walk_struct_field(self, s);
 338     }
 339
 340     fn visit_foreign_item(&mut self, i: &'tcx hir::ForeignItem<'tcx>) {
 341         self.check_missing_stability(i.hir_id, i.span, i.kind.descriptive_variant());
 342         intravisit::walk_foreign_item(self, i);
 343     }
 344
 345     fn visit_macro_def(&mut self, md: &'tcx hir::MacroDef<'tcx>) {
 346         self.check_missing_stability(md.hir_id, md.span, "macro");
 347     }
 348 }
 349
 350 fn new_index(tcx: TyCtxt<'tcx>) -> Index<'tcx> {
 351     let is_staged_api =
 352         tcx.sess.opts.debugging_opts.force_unstable_if_unmarked || tcx.features().staged_api;
 353     let mut staged_api = FxHashMap::default();
 354     staged_api.insert(LOCAL_CRATE, is_staged_api);
 355     let mut index = Index {
 356         staged_api,
 357         stab_map: Default::default(),
 358         const_stab_map: Default::default(),
 359         depr_map: Default::default(),
 360         active_features: Default::default(),
 361     };
 362
 363     let active_lib_features = &tcx.features().declared_lib_features;
 364     let active_lang_features = &tcx.features().declared_lang_features;
 365
 366     // Put the active features into a map for quick lookup.
 367     index.active_features = active_lib_features
 368         .iter()
 369         .map(|&(s, ..)| s)
 370         .chain(active_lang_features.iter().map(|&(s, ..)| s))
 371         .collect();
 372
 373     {
 374         let krate = tcx.hir().krate();
 375         let mut annotator = Annotator {
 376             tcx,
 377             index: &mut index,
 378             parent_stab: None,
 379             parent_depr: None,
 380             in_trait_impl: false,
 381         };
 382
 383         // If the `-Z force-unstable-if-unmarked` flag is passed then we provide
 384         // a parent stability annotation which indicates that this is private
 385         // with the `rustc_private` feature. This is intended for use when
 386         // compiling librustc crates themselves so we can leverage crates.io
 387         // while maintaining the invariant that all sysroot crates are unstable
 388         // by default and are unable to be used.
 389         if tcx.sess.opts.debugging_opts.force_unstable_if_unmarked {
 390             let reason = "this crate is being loaded from the sysroot, an \
 391                           unstable location; did you mean to load this crate \
 392                           from crates.io via `Cargo.toml` instead?";
 393             let stability = tcx.intern_stability(Stability {
 394                 level: attr::StabilityLevel::Unstable {
 395                     reason: Some(Symbol::intern(reason)),
 396                     issue: NonZeroU32::new(27812),
 397                     is_soft: false,
 398                 },
 399                 feature: sym::rustc_private,
 400                 rustc_depr: None,
 401             });
 402             annotator.parent_stab = Some(stability);
 403         }
 404
 405         annotator.annotate(
 406             hir::CRATE_HIR_ID,
 407             &krate.attrs,
 408             krate.span,
 409             AnnotationKind::Required,
 410             |v| intravisit::walk_crate(v, krate),
 411         );
 412     }
 413     return index;
 414 }
 415
 416 /// Cross-references the feature names of unstable APIs with enabled
 417 /// features and possibly prints errors.
 418 fn check_mod_unstable_api_usage(tcx: TyCtxt<'_>, module_def_id: DefId) {
 419     tcx.hir().visit_item_likes_in_module(module_def_id, &mut Checker { tcx }.as_deep_visitor());
 420 }
 421
 422 pub(crate) fn provide(providers: &mut Providers<'_>) {
 423     *providers = Providers { check_mod_unstable_api_usage, ..*providers };
 424     providers.stability_index = |tcx, cnum| {
 425         assert_eq!(cnum, LOCAL_CRATE);
 426         tcx.arena.alloc(new_index(tcx))
 427     };
 428 }
 429
 430 struct Checker<'tcx> {
 431     tcx: TyCtxt<'tcx>,
 432 }
 433
 434 impl Visitor<'tcx> for Checker<'tcx> {
 435     type Map = Map<'tcx>;
 436
 437     /// Because stability levels are scoped lexically, we want to walk
 438     /// nested items in the context of the outer item, so enable
 439     /// deep-walking.
 440     fn nested_visit_map(&mut self) -> NestedVisitorMap<'_, Self::Map> {
 441         NestedVisitorMap::OnlyBodies(&self.tcx.hir())
 442     }
 443
 444     fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
 445         match item.kind {
 446             hir::ItemKind::ExternCrate(_) => {
 447                 // compiler-generated `extern crate` items have a dummy span.
 448                 if item.span.is_dummy() {
 449                     return;
 450                 }
 451
 452                 let def_id = self.tcx.hir().local_def_id(item.hir_id);
 453                 let cnum = match self.tcx.extern_mod_stmt_cnum(def_id) {
 454                     Some(cnum) => cnum,
 455                     None => return,
 456                 };
 457                 let def_id = DefId { krate: cnum, index: CRATE_DEF_INDEX };
 458                 self.tcx.check_stability(def_id, Some(item.hir_id), item.span);
 459             }
 460
 461             // For implementations of traits, check the stability of each item
 462             // individually as it's possible to have a stable trait with unstable
 463             // items.
 464             hir::ItemKind::Impl { of_trait: Some(ref t), items, .. } => {
 465                 if let Res::Def(DefKind::Trait, trait_did) = t.path.res {
 466                     for impl_item_ref in items {
 467                         let impl_item = self.tcx.hir().impl_item(impl_item_ref.id);
 468                         let trait_item_def_id = self
 469                             .tcx
 470                             .associated_items(trait_did)
 471                             .find(|item| item.ident.name == impl_item.ident.name)
 472                             .map(|item| item.def_id);
 473                         if let Some(def_id) = trait_item_def_id {
 474                             // Pass `None` to skip deprecation warnings.
 475                             self.tcx.check_stability(def_id, None, impl_item.span);
 476                         }
 477                     }
 478                 }
 479             }
 480
 481             // There's no good place to insert stability check for non-Copy unions,
 482             // so semi-randomly perform it here in stability.rs
 483             hir::ItemKind::Union(..) if !self.tcx.features().untagged_unions => {
 484                 let def_id = self.tcx.hir().local_def_id(item.hir_id);
 485                 let adt_def = self.tcx.adt_def(def_id);
 486                 let ty = self.tcx.type_of(def_id);
 487
 488                 if adt_def.has_dtor(self.tcx) {
 489                     feature_err(
 490                         &self.tcx.sess.parse_sess,
 491                         sym::untagged_unions,
 492                         item.span,
 493                         "unions with `Drop` implementations are unstable",
 494                     )
 495                     .emit();
 496                 } else {
 497                     let param_env = self.tcx.param_env(def_id);
 498                     if !can_type_implement_copy(self.tcx, param_env, ty).is_ok() {
 499                         feature_err(
 500                             &self.tcx.sess.parse_sess,
 501                             sym::untagged_unions,
 502                             item.span,
 503                             "unions with non-`Copy` fields are unstable",
 504                         )
 505                         .emit();
 506                     }
 507                 }
 508             }
 509
 510             _ => (/* pass */),
 511         }
 512         intravisit::walk_item(self, item);
 513     }
 514
 515     fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, id: hir::HirId) {
 516         if let Some(def_id) = path.res.opt_def_id() {
 517             self.tcx.check_stability(def_id, Some(id), path.span)
 518         }
 519         intravisit::walk_path(self, path)
 520     }
 521 }
 522
 523 /// Given the list of enabled features that were not language features (i.e., that
 524 /// were expected to be library features), and the list of features used from
 525 /// libraries, identify activated features that don't exist and error about them.
 526 pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
 527     let access_levels = &tcx.privacy_access_levels(LOCAL_CRATE);
 528
 529     if tcx.stability().staged_api[&LOCAL_CRATE] {
 530         let krate = tcx.hir().krate();
 531         let mut missing = MissingStabilityAnnotations { tcx, access_levels };
 532         missing.check_missing_stability(hir::CRATE_HIR_ID, krate.span, "crate");
 533         intravisit::walk_crate(&mut missing, krate);
 534         krate.visit_all_item_likes(&mut missing.as_deep_visitor());
 535     }
 536
 537     let declared_lang_features = &tcx.features().declared_lang_features;
 538     let mut lang_features = FxHashSet::default();
 539     for &(feature, span, since) in declared_lang_features {
 540         if let Some(since) = since {
 541             // Warn if the user has enabled an already-stable lang feature.
 542             unnecessary_stable_feature_lint(tcx, span, feature, since);
 543         }
 544         if !lang_features.insert(feature) {
 545             // Warn if the user enables a lang feature multiple times.
 546             duplicate_feature_err(tcx.sess, span, feature);
 547         }
 548     }
 549
 550     let declared_lib_features = &tcx.features().declared_lib_features;
 551     let mut remaining_lib_features = FxHashMap::default();
 552     for (feature, span) in declared_lib_features {
 553         if remaining_lib_features.contains_key(&feature) {
 554             // Warn if the user enables a lib feature multiple times.
 555             duplicate_feature_err(tcx.sess, *span, *feature);
 556         }
 557         remaining_lib_features.insert(feature, span.clone());
 558     }
 559     // `stdbuild` has special handling for `libc`, so we need to
 560     // recognise the feature when building std.
 561     // Likewise, libtest is handled specially, so `test` isn't
 562     // available as we'd like it to be.
 563     // FIXME: only remove `libc` when `stdbuild` is active.
 564     // FIXME: remove special casing for `test`.
 565     remaining_lib_features.remove(&Symbol::intern("libc"));
 566     remaining_lib_features.remove(&sym::test);
 567
 568     let check_features = |remaining_lib_features: &mut FxHashMap<_, _>, defined_features: &[_]| {
 569         for &(feature, since) in defined_features {
 570             if let Some(since) = since {
 571                 if let Some(span) = remaining_lib_features.get(&feature) {
 572                     // Warn if the user has enabled an already-stable lib feature.
 573                     unnecessary_stable_feature_lint(tcx, *span, feature, since);
 574                 }
 575             }
 576             remaining_lib_features.remove(&feature);
 577             if remaining_lib_features.is_empty() {
 578                 break;
 579             }
 580         }
 581     };
 582
 583     // We always collect the lib features declared in the current crate, even if there are
 584     // no unknown features, because the collection also does feature attribute validation.
 585     let local_defined_features = tcx.lib_features().to_vec();
 586     if !remaining_lib_features.is_empty() {
 587         check_features(&mut remaining_lib_features, &local_defined_features);
 588
 589         for &cnum in &*tcx.crates() {
 590             if remaining_lib_features.is_empty() {
 591                 break;
 592             }
 593             check_features(&mut remaining_lib_features, tcx.defined_lib_features(cnum));
 594         }
 595     }
 596
 597     for (feature, span) in remaining_lib_features {
 598         struct_span_err!(tcx.sess, span, E0635, "unknown feature `{}`", feature).emit();
 599     }
 600
 601     // FIXME(#44232): the `used_features` table no longer exists, so we
 602     // don't lint about unused features. We should reenable this one day!
 603 }
 604
 605 fn unnecessary_stable_feature_lint(tcx: TyCtxt<'_>, span: Span, feature: Symbol, since: Symbol) {
 606     tcx.lint_hir(
 607         lint::builtin::STABLE_FEATURES,
 608         hir::CRATE_HIR_ID,
 609         span,
 610         &format!(
 611             "the feature `{}` has been stable since {} and no longer requires \
 612                   an attribute to enable",
 613             feature, since
 614         ),
 615     );
 616 }
 617
 618 fn duplicate_feature_err(sess: &Session, span: Span, feature: Symbol) {
 619     struct_span_err!(sess, span, E0636, "the feature `{}` has already been declared", feature)
 620         .emit();
 621 }