compiler/rustc_ast_passes/src/feature_gate.rs

   1 use rustc_ast as ast;
   2 use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
   3 use rustc_ast::{AssocConstraint, AssocConstraintKind, NodeId};
   4 use rustc_ast::{PatKind, RangeEnd};
   5 use rustc_errors::{struct_span_err, Applicability, StashKey};
   6 use rustc_feature::{AttributeGate, BuiltinAttribute, Features, GateIssue, BUILTIN_ATTRIBUTE_MAP};
   7 use rustc_session::parse::{feature_err, feature_err_issue, feature_warn};
   8 use rustc_session::Session;
   9 use rustc_span::source_map::Spanned;
  10 use rustc_span::symbol::sym;
  11 use rustc_span::Span;
  12 use rustc_target::spec::abi;
  13
  14 macro_rules! gate_feature_fn {
  15     ($visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr, $help: expr) => {{
  16         let (visitor, has_feature, span, name, explain, help) =
  17             (&*$visitor, $has_feature, $span, $name, $explain, $help);
  18         let has_feature: bool = has_feature(visitor.features);
  19         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
  20         if !has_feature && !span.allows_unstable($name) {
  21             feature_err(&visitor.sess.parse_sess, name, span, explain).help(help).emit();
  22         }
  23     }};
  24     ($visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr) => {{
  25         let (visitor, has_feature, span, name, explain) =
  26             (&*$visitor, $has_feature, $span, $name, $explain);
  27         let has_feature: bool = has_feature(visitor.features);
  28         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
  29         if !has_feature && !span.allows_unstable($name) {
  30             feature_err(&visitor.sess.parse_sess, name, span, explain).emit();
  31         }
  32     }};
  33     (future_incompatible; $visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr) => {{
  34         let (visitor, has_feature, span, name, explain) =
  35             (&*$visitor, $has_feature, $span, $name, $explain);
  36         let has_feature: bool = has_feature(visitor.features);
  37         debug!(
  38             "gate_feature(feature = {:?}, span = {:?}); has? {} (future_incompatible)",
  39             name, span, has_feature
  40         );
  41         if !has_feature && !span.allows_unstable($name) {
  42             feature_warn(&visitor.sess.parse_sess, name, span, explain);
  43         }
  44     }};
  45 }
  46
  47 macro_rules! gate_feature_post {
  48     ($visitor: expr, $feature: ident, $span: expr, $explain: expr, $help: expr) => {
  49         gate_feature_fn!($visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain, $help)
  50     };
  51     ($visitor: expr, $feature: ident, $span: expr, $explain: expr) => {
  52         gate_feature_fn!($visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain)
  53     };
  54     (future_incompatible; $visitor: expr, $feature: ident, $span: expr, $explain: expr) => {
  55         gate_feature_fn!(future_incompatible; $visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain)
  56     };
  57 }
  58
  59 pub fn check_attribute(attr: &ast::Attribute, sess: &Session, features: &Features) {
  60     PostExpansionVisitor { sess, features }.visit_attribute(attr)
  61 }
  62
  63 struct PostExpansionVisitor<'a> {
  64     sess: &'a Session,
  65
  66     // `sess` contains a `Features`, but this might not be that one.
  67     features: &'a Features,
  68 }
  69
  70 impl<'a> PostExpansionVisitor<'a> {
  71     fn check_abi(&self, abi: ast::StrLit, constness: ast::Const) {
  72         let ast::StrLit { symbol_unescaped, span, .. } = abi;
  73
  74         if let ast::Const::Yes(_) = constness {
  75             match symbol_unescaped {
  76                 // Stable
  77                 sym::Rust | sym::C => {}
  78                 abi => gate_feature_post!(
  79                     &self,
  80                     const_extern_fn,
  81                     span,
  82                     &format!("`{}` as a `const fn` ABI is unstable", abi)
  83                 ),
  84             }
  85         }
  86
  87         match abi::is_enabled(&self.features, span, symbol_unescaped.as_str()) {
  88             Ok(()) => (),
  89             Err(abi::AbiDisabled::Unstable { feature, explain }) => {
  90                 feature_err_issue(
  91                     &self.sess.parse_sess,
  92                     feature,
  93                     span,
  94                     GateIssue::Language,
  95                     explain,
  96                 )
  97                 .emit();
  98             }
  99             Err(abi::AbiDisabled::Unrecognized) => {
 100                 if self.sess.opts.pretty.map_or(true, |ppm| ppm.needs_hir()) {
 101                     self.sess.parse_sess.span_diagnostic.delay_span_bug(
 102                         span,
 103                         &format!(
 104                             "unrecognized ABI not caught in lowering: {}",
 105                             symbol_unescaped.as_str()
 106                         ),
 107                     );
 108                 }
 109             }
 110         }
 111     }
 112
 113     fn check_extern(&self, ext: ast::Extern, constness: ast::Const) {
 114         if let ast::Extern::Explicit(abi, _) = ext {
 115             self.check_abi(abi, constness);
 116         }
 117     }
 118
 119     /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
 120     fn check_impl_trait(&self, ty: &ast::Ty) {
 121         struct ImplTraitVisitor<'a> {
 122             vis: &'a PostExpansionVisitor<'a>,
 123         }
 124         impl Visitor<'_> for ImplTraitVisitor<'_> {
 125             fn visit_ty(&mut self, ty: &ast::Ty) {
 126                 if let ast::TyKind::ImplTrait(..) = ty.kind {
 127                     gate_feature_post!(
 128                         &self.vis,
 129                         type_alias_impl_trait,
 130                         ty.span,
 131                         "`impl Trait` in type aliases is unstable"
 132                     );
 133                 }
 134                 visit::walk_ty(self, ty);
 135             }
 136         }
 137         ImplTraitVisitor { vis: self }.visit_ty(ty);
 138     }
 139 }
 140
 141 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 142     fn visit_attribute(&mut self, attr: &ast::Attribute) {
 143         let attr_info = attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name));
 144         // Check feature gates for built-in attributes.
 145         if let Some(BuiltinAttribute {
 146             gate: AttributeGate::Gated(_, name, descr, has_feature),
 147             ..
 148         }) = attr_info
 149         {
 150             gate_feature_fn!(self, has_feature, attr.span, *name, descr);
 151         }
 152         // Check unstable flavors of the `#[doc]` attribute.
 153         if attr.has_name(sym::doc) {
 154             for nested_meta in attr.meta_item_list().unwrap_or_default() {
 155                 macro_rules! gate_doc { ($($name:ident => $feature:ident)*) => {
 156                     $(if nested_meta.has_name(sym::$name) {
 157                         let msg = concat!("`#[doc(", stringify!($name), ")]` is experimental");
 158                         gate_feature_post!(self, $feature, attr.span, msg);
 159                     })*
 160                 }}
 161
 162                 gate_doc!(
 163                     cfg => doc_cfg
 164                     cfg_hide => doc_cfg_hide
 165                     masked => doc_masked
 166                     notable_trait => doc_notable_trait
 167                 );
 168
 169                 if nested_meta.has_name(sym::keyword) {
 170                     let msg = "`#[doc(keyword)]` is meant for internal use only";
 171                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
 172                 }
 173
 174                 if nested_meta.has_name(sym::fake_variadic) {
 175                     let msg = "`#[doc(fake_variadic)]` is meant for internal use only";
 176                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
 177                 }
 178             }
 179         }
 180
 181         // Emit errors for non-staged-api crates.
 182         if !self.features.staged_api {
 183             if attr.has_name(sym::unstable)
 184                 || attr.has_name(sym::stable)
 185                 || attr.has_name(sym::rustc_const_unstable)
 186                 || attr.has_name(sym::rustc_const_stable)
 187                 || attr.has_name(sym::rustc_default_body_unstable)
 188             {
 189                 struct_span_err!(
 190                     self.sess,
 191                     attr.span,
 192                     E0734,
 193                     "stability attributes may not be used outside of the standard library",
 194                 )
 195                 .emit();
 196             }
 197         }
 198     }
 199
 200     fn visit_item(&mut self, i: &'a ast::Item) {
 201         match &i.kind {
 202             ast::ItemKind::ForeignMod(foreign_module) => {
 203                 if let Some(abi) = foreign_module.abi {
 204                     self.check_abi(abi, ast::Const::No);
 205                 }
 206             }
 207
 208             ast::ItemKind::Fn(..) => {
 209                 if self.sess.contains_name(&i.attrs, sym::start) {
 210                     gate_feature_post!(
 211                         &self,
 212                         start,
 213                         i.span,
 214                         "`#[start]` functions are experimental \
 215                          and their signature may change \
 216                          over time"
 217                     );
 218                 }
 219             }
 220
 221             ast::ItemKind::Struct(..) => {
 222                 for attr in self.sess.filter_by_name(&i.attrs, sym::repr) {
 223                     for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
 224                         if item.has_name(sym::simd) {
 225                             gate_feature_post!(
 226                                 &self,
 227                                 repr_simd,
 228                                 attr.span,
 229                                 "SIMD types are experimental and possibly buggy"
 230                             );
 231                         }
 232                     }
 233                 }
 234             }
 235
 236             ast::ItemKind::Impl(box ast::Impl { polarity, defaultness, of_trait, .. }) => {
 237                 if let &ast::ImplPolarity::Negative(span) = polarity {
 238                     gate_feature_post!(
 239                         &self,
 240                         negative_impls,
 241                         span.to(of_trait.as_ref().map_or(span, |t| t.path.span)),
 242                         "negative trait bounds are not yet fully implemented; \
 243                          use marker types for now"
 244                     );
 245                 }
 246
 247                 if let ast::Defaultness::Default(_) = defaultness {
 248                     gate_feature_post!(&self, specialization, i.span, "specialization is unstable");
 249                 }
 250             }
 251
 252             ast::ItemKind::Trait(box ast::Trait { is_auto: ast::IsAuto::Yes, .. }) => {
 253                 gate_feature_post!(
 254                     &self,
 255                     auto_traits,
 256                     i.span,
 257                     "auto traits are experimental and possibly buggy"
 258                 );
 259             }
 260
 261             ast::ItemKind::TraitAlias(..) => {
 262                 gate_feature_post!(&self, trait_alias, i.span, "trait aliases are experimental");
 263             }
 264
 265             ast::ItemKind::MacroDef(ast::MacroDef { macro_rules: false, .. }) => {
 266                 let msg = "`macro` is experimental";
 267                 gate_feature_post!(&self, decl_macro, i.span, msg);
 268             }
 269
 270             ast::ItemKind::TyAlias(box ast::TyAlias { ty: Some(ty), .. }) => {
 271                 self.check_impl_trait(&ty)
 272             }
 273
 274             _ => {}
 275         }
 276
 277         visit::walk_item(self, i);
 278     }
 279
 280     fn visit_foreign_item(&mut self, i: &'a ast::ForeignItem) {
 281         match i.kind {
 282             ast::ForeignItemKind::Fn(..) | ast::ForeignItemKind::Static(..) => {
 283                 let link_name = self.sess.first_attr_value_str_by_name(&i.attrs, sym::link_name);
 284                 let links_to_llvm =
 285                     link_name.map_or(false, |val| val.as_str().starts_with("llvm."));
 286                 if links_to_llvm {
 287                     gate_feature_post!(
 288                         &self,
 289                         link_llvm_intrinsics,
 290                         i.span,
 291                         "linking to LLVM intrinsics is experimental"
 292                     );
 293                 }
 294             }
 295             ast::ForeignItemKind::TyAlias(..) => {
 296                 gate_feature_post!(&self, extern_types, i.span, "extern types are experimental");
 297             }
 298             ast::ForeignItemKind::MacCall(..) => {}
 299         }
 300
 301         visit::walk_foreign_item(self, i)
 302     }
 303
 304     fn visit_ty(&mut self, ty: &'a ast::Ty) {
 305         match &ty.kind {
 306             ast::TyKind::BareFn(bare_fn_ty) => {
 307                 // Function pointers cannot be `const`
 308                 self.check_extern(bare_fn_ty.ext, ast::Const::No);
 309             }
 310             ast::TyKind::Never => {
 311                 gate_feature_post!(&self, never_type, ty.span, "the `!` type is experimental");
 312             }
 313             ast::TyKind::TraitObject(_, ast::TraitObjectSyntax::DynStar, ..) => {
 314                 gate_feature_post!(&self, dyn_star, ty.span, "dyn* trait objects are unstable");
 315             }
 316             _ => {}
 317         }
 318         visit::walk_ty(self, ty)
 319     }
 320
 321     fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FnRetTy) {
 322         if let ast::FnRetTy::Ty(output_ty) = ret_ty {
 323             if let ast::TyKind::Never = output_ty.kind {
 324                 // Do nothing.
 325             } else {
 326                 self.visit_ty(output_ty)
 327             }
 328         }
 329     }
 330
 331     fn visit_stmt(&mut self, stmt: &'a ast::Stmt) {
 332         if let ast::StmtKind::Semi(expr) = &stmt.kind
 333             && let ast::ExprKind::Assign(lhs, _, _) = &expr.kind
 334             && let ast::ExprKind::Type(..) = lhs.kind
 335             && self.sess.parse_sess.span_diagnostic.err_count() == 0
 336             && !self.features.type_ascription
 337             && !lhs.span.allows_unstable(sym::type_ascription)
 338         {
 339             // When we encounter a statement of the form `foo: Ty = val;`, this will emit a type
 340             // ascription error, but the likely intention was to write a `let` statement. (#78907).
 341             feature_err(
 342                 &self.sess.parse_sess,
 343                 sym::type_ascription,
 344                 lhs.span,
 345                 "type ascription is experimental",
 346             ).span_suggestion_verbose(
 347                 lhs.span.shrink_to_lo(),
 348                 "you might have meant to introduce a new binding",
 349                 "let ".to_string(),
 350                 Applicability::MachineApplicable,
 351             ).emit();
 352         }
 353         visit::walk_stmt(self, stmt);
 354     }
 355
 356     fn visit_expr(&mut self, e: &'a ast::Expr) {
 357         match e.kind {
 358             ast::ExprKind::Box(_) => {
 359                 gate_feature_post!(
 360                     &self,
 361                     box_syntax,
 362                     e.span,
 363                     "box expression syntax is experimental; you can call `Box::new` instead"
 364                 );
 365             }
 366             ast::ExprKind::Type(..) => {
 367                 if self.sess.parse_sess.span_diagnostic.err_count() == 0 {
 368                     // To avoid noise about type ascription in common syntax errors,
 369                     // only emit if it is the *only* error.
 370                     gate_feature_post!(
 371                         &self,
 372                         type_ascription,
 373                         e.span,
 374                         "type ascription is experimental"
 375                     );
 376                 } else {
 377                     // And if it isn't, cancel the early-pass warning.
 378                     self.sess
 379                         .parse_sess
 380                         .span_diagnostic
 381                         .steal_diagnostic(e.span, StashKey::EarlySyntaxWarning)
 382                         .map(|err| err.cancel());
 383                 }
 384             }
 385             ast::ExprKind::TryBlock(_) => {
 386                 gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
 387             }
 388             _ => {}
 389         }
 390         visit::walk_expr(self, e)
 391     }
 392
 393     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
 394         match &pattern.kind {
 395             PatKind::Slice(pats) => {
 396                 for pat in pats {
 397                     let inner_pat = match &pat.kind {
 398                         PatKind::Ident(.., Some(pat)) => pat,
 399                         _ => pat,
 400                     };
 401                     if let PatKind::Range(Some(_), None, Spanned { .. }) = inner_pat.kind {
 402                         gate_feature_post!(
 403                             &self,
 404                             half_open_range_patterns_in_slices,
 405                             pat.span,
 406                             "`X..` patterns in slices are experimental"
 407                         );
 408                     }
 409                 }
 410             }
 411             PatKind::Box(..) => {
 412                 gate_feature_post!(
 413                     &self,
 414                     box_patterns,
 415                     pattern.span,
 416                     "box pattern syntax is experimental"
 417                 );
 418             }
 419             PatKind::Range(_, Some(_), Spanned { node: RangeEnd::Excluded, .. }) => {
 420                 gate_feature_post!(
 421                     &self,
 422                     exclusive_range_pattern,
 423                     pattern.span,
 424                     "exclusive range pattern syntax is experimental"
 425                 );
 426             }
 427             _ => {}
 428         }
 429         visit::walk_pat(self, pattern)
 430     }
 431
 432     fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
 433         if let Some(header) = fn_kind.header() {
 434             // Stability of const fn methods are covered in `visit_assoc_item` below.
 435             self.check_extern(header.ext, header.constness);
 436         }
 437
 438         if fn_kind.ctxt() != Some(FnCtxt::Foreign) && fn_kind.decl().c_variadic() {
 439             gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
 440         }
 441
 442         visit::walk_fn(self, fn_kind)
 443     }
 444
 445     fn visit_assoc_constraint(&mut self, constraint: &'a AssocConstraint) {
 446         if let AssocConstraintKind::Bound { .. } = constraint.kind {
 447             gate_feature_post!(
 448                 &self,
 449                 associated_type_bounds,
 450                 constraint.span,
 451                 "associated type bounds are unstable"
 452             )
 453         }
 454         visit::walk_assoc_constraint(self, constraint)
 455     }
 456
 457     fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
 458         let is_fn = match &i.kind {
 459             ast::AssocItemKind::Fn(_) => true,
 460             ast::AssocItemKind::Type(box ast::TyAlias { ty, .. }) => {
 461                 if let (Some(_), AssocCtxt::Trait) = (ty, ctxt) {
 462                     gate_feature_post!(
 463                         &self,
 464                         associated_type_defaults,
 465                         i.span,
 466                         "associated type defaults are unstable"
 467                     );
 468                 }
 469                 if let Some(ty) = ty {
 470                     self.check_impl_trait(ty);
 471                 }
 472                 false
 473             }
 474             _ => false,
 475         };
 476         if let ast::Defaultness::Default(_) = i.kind.defaultness() {
 477             // Limit `min_specialization` to only specializing functions.
 478             gate_feature_fn!(
 479                 &self,
 480                 |x: &Features| x.specialization || (is_fn && x.min_specialization),
 481                 i.span,
 482                 sym::specialization,
 483                 "specialization is unstable"
 484             );
 485         }
 486         visit::walk_assoc_item(self, i, ctxt)
 487     }
 488 }
 489
 490 pub fn check_crate(krate: &ast::Crate, sess: &Session) {
 491     maybe_stage_features(sess, krate);
 492     check_incompatible_features(sess);
 493     let mut visitor = PostExpansionVisitor { sess, features: &sess.features_untracked() };
 494
 495     let spans = sess.parse_sess.gated_spans.spans.borrow();
 496     macro_rules! gate_all {
 497         ($gate:ident, $msg:literal, $help:literal) => {
 498             if let Some(spans) = spans.get(&sym::$gate) {
 499                 for span in spans {
 500                     gate_feature_post!(&visitor, $gate, *span, $msg, $help);
 501                 }
 502             }
 503         };
 504         ($gate:ident, $msg:literal) => {
 505             if let Some(spans) = spans.get(&sym::$gate) {
 506                 for span in spans {
 507                     gate_feature_post!(&visitor, $gate, *span, $msg);
 508                 }
 509             }
 510         };
 511     }
 512     gate_all!(
 513         if_let_guard,
 514         "`if let` guards are experimental",
 515         "you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`"
 516     );
 517     gate_all!(let_chains, "`let` expressions in this position are unstable");
 518     gate_all!(
 519         async_closure,
 520         "async closures are unstable",
 521         "to use an async block, remove the `||`: `async {`"
 522     );
 523     gate_all!(
 524         closure_lifetime_binder,
 525         "`for<...>` binders for closures are experimental",
 526         "consider removing `for<...>`"
 527     );
 528     gate_all!(more_qualified_paths, "usage of qualified paths in this context is experimental");
 529     gate_all!(generators, "yield syntax is experimental");
 530     gate_all!(raw_ref_op, "raw address of syntax is experimental");
 531     gate_all!(const_trait_impl, "const trait impls are experimental");
 532     gate_all!(
 533         half_open_range_patterns_in_slices,
 534         "half-open range patterns in slices are unstable"
 535     );
 536     gate_all!(inline_const, "inline-const is experimental");
 537     gate_all!(inline_const_pat, "inline-const in pattern position is experimental");
 538     gate_all!(associated_const_equality, "associated const equality is incomplete");
 539     gate_all!(yeet_expr, "`do yeet` expression is experimental");
 540
 541     // All uses of `gate_all!` below this point were added in #65742,
 542     // and subsequently disabled (with the non-early gating readded).
 543     // We emit an early future-incompatible warning for these.
 544     // New syntax gates should go above here to get a hard error gate.
 545     macro_rules! gate_all {
 546         ($gate:ident, $msg:literal) => {
 547             for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
 548                 gate_feature_post!(future_incompatible; &visitor, $gate, *span, $msg);
 549             }
 550         };
 551     }
 552
 553     gate_all!(trait_alias, "trait aliases are experimental");
 554     gate_all!(associated_type_bounds, "associated type bounds are unstable");
 555     gate_all!(decl_macro, "`macro` is experimental");
 556     gate_all!(box_patterns, "box pattern syntax is experimental");
 557     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
 558     gate_all!(try_blocks, "`try` blocks are unstable");
 559     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
 560     gate_all!(type_ascription, "type ascription is experimental");
 561
 562     visit::walk_crate(&mut visitor, krate);
 563 }
 564
 565 fn maybe_stage_features(sess: &Session, krate: &ast::Crate) {
 566     // checks if `#![feature]` has been used to enable any lang feature
 567     // does not check the same for lib features unless there's at least one
 568     // declared lang feature
 569     if !sess.opts.unstable_features.is_nightly_build() {
 570         let lang_features = &sess.features_untracked().declared_lang_features;
 571         if lang_features.len() == 0 {
 572             return;
 573         }
 574         for attr in krate.attrs.iter().filter(|attr| attr.has_name(sym::feature)) {
 575             let mut err = struct_span_err!(
 576                 sess.parse_sess.span_diagnostic,
 577                 attr.span,
 578                 E0554,
 579                 "`#![feature]` may not be used on the {} release channel",
 580                 option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)")
 581             );
 582             let mut all_stable = true;
 583             for ident in
 584                 attr.meta_item_list().into_iter().flatten().flat_map(|nested| nested.ident())
 585             {
 586                 let name = ident.name;
 587                 let stable_since = lang_features
 588                     .iter()
 589                     .flat_map(|&(feature, _, since)| if feature == name { since } else { None })
 590                     .next();
 591                 if let Some(since) = stable_since {
 592                     err.help(&format!(
 593                         "the feature `{}` has been stable since {} and no longer requires \
 594                                   an attribute to enable",
 595                         name, since
 596                     ));
 597                 } else {
 598                     all_stable = false;
 599                 }
 600             }
 601             if all_stable {
 602                 err.span_suggestion(
 603                     attr.span,
 604                     "remove the attribute",
 605                     "",
 606                     Applicability::MachineApplicable,
 607                 );
 608             }
 609             err.emit();
 610         }
 611     }
 612 }
 613
 614 fn check_incompatible_features(sess: &Session) {
 615     let features = sess.features_untracked();
 616
 617     let declared_features = features
 618         .declared_lang_features
 619         .iter()
 620         .copied()
 621         .map(|(name, span, _)| (name, span))
 622         .chain(features.declared_lib_features.iter().copied());
 623
 624     for (f1, f2) in rustc_feature::INCOMPATIBLE_FEATURES
 625         .iter()
 626         .filter(|&&(f1, f2)| features.enabled(f1) && features.enabled(f2))
 627     {
 628         if let Some((f1_name, f1_span)) = declared_features.clone().find(|(name, _)| name == f1) {
 629             if let Some((f2_name, f2_span)) = declared_features.clone().find(|(name, _)| name == f2)
 630             {
 631                 let spans = vec![f1_span, f2_span];
 632                 sess.struct_span_err(
 633                     spans.clone(),
 634                     &format!(
 635                         "features `{}` and `{}` are incompatible, using them at the same time \
 636                         is not allowed",
 637                         f1_name, f2_name
 638                     ),
 639                 )
 640                 .help("remove one of these features")
 641                 .emit();
 642             }
 643         }
 644     }
 645 }