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