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::{AssocTyConstraint, AssocTyConstraintKind, NodeId};
   4 use rustc_ast::{PatKind, RangeEnd, VariantData};
   5 use rustc_errors::struct_span_err;
   6 use rustc_feature::{AttributeGate, BUILTIN_ATTRIBUTE_MAP};
   7 use rustc_feature::{Features, GateIssue};
   8 use rustc_session::parse::{feature_err, feature_err_issue};
   9 use rustc_session::Session;
  10 use rustc_span::source_map::Spanned;
  11 use rustc_span::symbol::{sym, Symbol};
  12 use rustc_span::Span;
  13
  14 use tracing::debug;
  15
  16 macro_rules! gate_feature_fn {
  17     ($visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr) => {{
  18         let (visitor, has_feature, span, name, explain) =
  19             (&*$visitor, $has_feature, $span, $name, $explain);
  20         let has_feature: bool = has_feature(visitor.features);
  21         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
  22         if !has_feature && !span.allows_unstable($name) {
  23             feature_err_issue(&visitor.sess.parse_sess, name, span, GateIssue::Language, explain)
  24                 .emit();
  25         }
  26     }};
  27 }
  28
  29 macro_rules! gate_feature_post {
  30     ($visitor: expr, $feature: ident, $span: expr, $explain: expr) => {
  31         gate_feature_fn!($visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain)
  32     };
  33 }
  34
  35 pub fn check_attribute(attr: &ast::Attribute, sess: &Session, features: &Features) {
  36     PostExpansionVisitor { sess, features }.visit_attribute(attr)
  37 }
  38
  39 struct PostExpansionVisitor<'a> {
  40     sess: &'a Session,
  41
  42     // `sess` contains a `Features`, but this might not be that one.
  43     features: &'a Features,
  44 }
  45
  46 impl<'a> PostExpansionVisitor<'a> {
  47     fn check_abi(&self, abi: ast::StrLit) {
  48         let ast::StrLit { symbol_unescaped, span, .. } = abi;
  49
  50         match &*symbol_unescaped.as_str() {
  51             // Stable
  52             "Rust" | "C" | "cdecl" | "stdcall" | "fastcall" | "aapcs" | "win64" | "sysv64"
  53             | "system" => {}
  54             "rust-intrinsic" => {
  55                 gate_feature_post!(&self, intrinsics, span, "intrinsics are subject to change");
  56             }
  57             "platform-intrinsic" => {
  58                 gate_feature_post!(
  59                     &self,
  60                     platform_intrinsics,
  61                     span,
  62                     "platform intrinsics are experimental and possibly buggy"
  63                 );
  64             }
  65             "vectorcall" => {
  66                 gate_feature_post!(
  67                     &self,
  68                     abi_vectorcall,
  69                     span,
  70                     "vectorcall is experimental and subject to change"
  71                 );
  72             }
  73             "thiscall" => {
  74                 gate_feature_post!(
  75                     &self,
  76                     abi_thiscall,
  77                     span,
  78                     "thiscall is experimental and subject to change"
  79                 );
  80             }
  81             "rust-call" => {
  82                 gate_feature_post!(
  83                     &self,
  84                     unboxed_closures,
  85                     span,
  86                     "rust-call ABI is subject to change"
  87                 );
  88             }
  89             "ptx-kernel" => {
  90                 gate_feature_post!(
  91                     &self,
  92                     abi_ptx,
  93                     span,
  94                     "PTX ABIs are experimental and subject to change"
  95                 );
  96             }
  97             "unadjusted" => {
  98                 gate_feature_post!(
  99                     &self,
 100                     abi_unadjusted,
 101                     span,
 102                     "unadjusted ABI is an implementation detail and perma-unstable"
 103                 );
 104             }
 105             "msp430-interrupt" => {
 106                 gate_feature_post!(
 107                     &self,
 108                     abi_msp430_interrupt,
 109                     span,
 110                     "msp430-interrupt ABI is experimental and subject to change"
 111                 );
 112             }
 113             "x86-interrupt" => {
 114                 gate_feature_post!(
 115                     &self,
 116                     abi_x86_interrupt,
 117                     span,
 118                     "x86-interrupt ABI is experimental and subject to change"
 119                 );
 120             }
 121             "amdgpu-kernel" => {
 122                 gate_feature_post!(
 123                     &self,
 124                     abi_amdgpu_kernel,
 125                     span,
 126                     "amdgpu-kernel ABI is experimental and subject to change"
 127                 );
 128             }
 129             "avr-interrupt" | "avr-non-blocking-interrupt" => {
 130                 gate_feature_post!(
 131                     &self,
 132                     abi_avr_interrupt,
 133                     span,
 134                     "avr-interrupt and avr-non-blocking-interrupt ABIs are experimental and subject to change"
 135                 );
 136             }
 137             "efiapi" => {
 138                 gate_feature_post!(
 139                     &self,
 140                     abi_efiapi,
 141                     span,
 142                     "efiapi ABI is experimental and subject to change"
 143                 );
 144             }
 145             abi => self
 146                 .sess
 147                 .parse_sess
 148                 .span_diagnostic
 149                 .delay_span_bug(span, &format!("unrecognized ABI not caught in lowering: {}", abi)),
 150         }
 151     }
 152
 153     fn check_extern(&self, ext: ast::Extern) {
 154         if let ast::Extern::Explicit(abi) = ext {
 155             self.check_abi(abi);
 156         }
 157     }
 158
 159     fn maybe_report_invalid_custom_discriminants(&self, variants: &[ast::Variant]) {
 160         let has_fields = variants.iter().any(|variant| match variant.data {
 161             VariantData::Tuple(..) | VariantData::Struct(..) => true,
 162             VariantData::Unit(..) => false,
 163         });
 164
 165         let discriminant_spans = variants
 166             .iter()
 167             .filter(|variant| match variant.data {
 168                 VariantData::Tuple(..) | VariantData::Struct(..) => false,
 169                 VariantData::Unit(..) => true,
 170             })
 171             .filter_map(|variant| variant.disr_expr.as_ref().map(|c| c.value.span))
 172             .collect::<Vec<_>>();
 173
 174         if !discriminant_spans.is_empty() && has_fields {
 175             let mut err = feature_err(
 176                 &self.sess.parse_sess,
 177                 sym::arbitrary_enum_discriminant,
 178                 discriminant_spans.clone(),
 179                 "custom discriminant values are not allowed in enums with tuple or struct variants",
 180             );
 181             for sp in discriminant_spans {
 182                 err.span_label(sp, "disallowed custom discriminant");
 183             }
 184             for variant in variants.iter() {
 185                 match &variant.data {
 186                     VariantData::Struct(..) => {
 187                         err.span_label(variant.span, "struct variant defined here");
 188                     }
 189                     VariantData::Tuple(..) => {
 190                         err.span_label(variant.span, "tuple variant defined here");
 191                     }
 192                     VariantData::Unit(..) => {}
 193                 }
 194             }
 195             err.emit();
 196         }
 197     }
 198
 199     fn check_gat(&self, generics: &ast::Generics, span: Span) {
 200         if !generics.params.is_empty() {
 201             gate_feature_post!(
 202                 &self,
 203                 generic_associated_types,
 204                 span,
 205                 "generic associated types are unstable"
 206             );
 207         }
 208         if !generics.where_clause.predicates.is_empty() {
 209             gate_feature_post!(
 210                 &self,
 211                 generic_associated_types,
 212                 span,
 213                 "where clauses on associated types are unstable"
 214             );
 215         }
 216     }
 217
 218     /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
 219     fn check_impl_trait(&self, ty: &ast::Ty) {
 220         struct ImplTraitVisitor<'a> {
 221             vis: &'a PostExpansionVisitor<'a>,
 222         }
 223         impl Visitor<'_> for ImplTraitVisitor<'_> {
 224             fn visit_ty(&mut self, ty: &ast::Ty) {
 225                 if let ast::TyKind::ImplTrait(..) = ty.kind {
 226                     gate_feature_post!(
 227                         &self.vis,
 228                         type_alias_impl_trait,
 229                         ty.span,
 230                         "`impl Trait` in type aliases is unstable"
 231                     );
 232                 }
 233                 visit::walk_ty(self, ty);
 234             }
 235         }
 236         ImplTraitVisitor { vis: self }.visit_ty(ty);
 237     }
 238 }
 239
 240 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 241     fn visit_attribute(&mut self, attr: &ast::Attribute) {
 242         let attr_info =
 243             attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name)).map(|a| **a);
 244         // Check feature gates for built-in attributes.
 245         if let Some((.., AttributeGate::Gated(_, name, descr, has_feature))) = attr_info {
 246             gate_feature_fn!(self, has_feature, attr.span, name, descr);
 247         }
 248         // Check unstable flavors of the `#[doc]` attribute.
 249         if self.sess.check_name(attr, sym::doc) {
 250             for nested_meta in attr.meta_item_list().unwrap_or_default() {
 251                 macro_rules! gate_doc { ($($name:ident => $feature:ident)*) => {
 252                     $(if nested_meta.has_name(sym::$name) {
 253                         let msg = concat!("`#[doc(", stringify!($name), ")]` is experimental");
 254                         gate_feature_post!(self, $feature, attr.span, msg);
 255                     })*
 256                 }}
 257
 258                 gate_doc!(
 259                     include => external_doc
 260                     cfg => doc_cfg
 261                     masked => doc_masked
 262                     spotlight => doc_spotlight
 263                     keyword => doc_keyword
 264                 );
 265             }
 266         }
 267     }
 268
 269     fn visit_name(&mut self, sp: Span, name: Symbol) {
 270         if !name.as_str().is_ascii() {
 271             gate_feature_post!(
 272                 &self,
 273                 non_ascii_idents,
 274                 self.sess.parse_sess.source_map().guess_head_span(sp),
 275                 "non-ascii idents are not fully supported"
 276             );
 277         }
 278     }
 279
 280     fn visit_item(&mut self, i: &'a ast::Item) {
 281         match i.kind {
 282             ast::ItemKind::ForeignMod(ref foreign_module) => {
 283                 if let Some(abi) = foreign_module.abi {
 284                     self.check_abi(abi);
 285                 }
 286             }
 287
 288             ast::ItemKind::Fn(..) => {
 289                 if self.sess.contains_name(&i.attrs[..], sym::plugin_registrar) {
 290                     gate_feature_post!(
 291                         &self,
 292                         plugin_registrar,
 293                         i.span,
 294                         "compiler plugins are experimental and possibly buggy"
 295                     );
 296                 }
 297                 if self.sess.contains_name(&i.attrs[..], sym::start) {
 298                     gate_feature_post!(
 299                         &self,
 300                         start,
 301                         i.span,
 302                         "`#[start]` functions are experimental \
 303                          and their signature may change \
 304                          over time"
 305                     );
 306                 }
 307                 if self.sess.contains_name(&i.attrs[..], sym::main) {
 308                     gate_feature_post!(
 309                         &self,
 310                         main,
 311                         i.span,
 312                         "declaration of a non-standard `#[main]` \
 313                          function may change over time, for now \
 314                          a top-level `fn main()` is required"
 315                     );
 316                 }
 317             }
 318
 319             ast::ItemKind::Struct(..) => {
 320                 for attr in self.sess.filter_by_name(&i.attrs[..], sym::repr) {
 321                     for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
 322                         if item.has_name(sym::simd) {
 323                             gate_feature_post!(
 324                                 &self,
 325                                 repr_simd,
 326                                 attr.span,
 327                                 "SIMD types are experimental and possibly buggy"
 328                             );
 329                         }
 330                     }
 331                 }
 332             }
 333
 334             ast::ItemKind::Enum(ast::EnumDef { ref variants, .. }, ..) => {
 335                 for variant in variants {
 336                     match (&variant.data, &variant.disr_expr) {
 337                         (ast::VariantData::Unit(..), _) => {}
 338                         (_, Some(disr_expr)) => gate_feature_post!(
 339                             &self,
 340                             arbitrary_enum_discriminant,
 341                             disr_expr.value.span,
 342                             "discriminants on non-unit variants are experimental"
 343                         ),
 344                         _ => {}
 345                     }
 346                 }
 347
 348                 let has_feature = self.features.arbitrary_enum_discriminant;
 349                 if !has_feature && !i.span.allows_unstable(sym::arbitrary_enum_discriminant) {
 350                     self.maybe_report_invalid_custom_discriminants(&variants);
 351                 }
 352             }
 353
 354             ast::ItemKind::Impl { polarity, defaultness, ref of_trait, .. } => {
 355                 if let ast::ImplPolarity::Negative(span) = polarity {
 356                     gate_feature_post!(
 357                         &self,
 358                         negative_impls,
 359                         span.to(of_trait.as_ref().map(|t| t.path.span).unwrap_or(span)),
 360                         "negative trait bounds are not yet fully implemented; \
 361                          use marker types for now"
 362                     );
 363                 }
 364
 365                 if let ast::Defaultness::Default(_) = defaultness {
 366                     gate_feature_post!(&self, specialization, i.span, "specialization is unstable");
 367                 }
 368             }
 369
 370             ast::ItemKind::Trait(ast::IsAuto::Yes, ..) => {
 371                 gate_feature_post!(
 372                     &self,
 373                     auto_traits,
 374                     i.span,
 375                     "auto traits are experimental and possibly buggy"
 376                 );
 377             }
 378
 379             ast::ItemKind::TraitAlias(..) => {
 380                 gate_feature_post!(&self, trait_alias, i.span, "trait aliases are experimental");
 381             }
 382
 383             ast::ItemKind::MacroDef(ast::MacroDef { macro_rules: false, .. }) => {
 384                 let msg = "`macro` is experimental";
 385                 gate_feature_post!(&self, decl_macro, i.span, msg);
 386             }
 387
 388             ast::ItemKind::TyAlias(_, _, _, Some(ref ty)) => self.check_impl_trait(&ty),
 389
 390             _ => {}
 391         }
 392
 393         visit::walk_item(self, i);
 394     }
 395
 396     fn visit_foreign_item(&mut self, i: &'a ast::ForeignItem) {
 397         match i.kind {
 398             ast::ForeignItemKind::Fn(..) | ast::ForeignItemKind::Static(..) => {
 399                 let link_name = self.sess.first_attr_value_str_by_name(&i.attrs, sym::link_name);
 400                 let links_to_llvm =
 401                     link_name.map_or(false, |val| val.as_str().starts_with("llvm."));
 402                 if links_to_llvm {
 403                     gate_feature_post!(
 404                         &self,
 405                         link_llvm_intrinsics,
 406                         i.span,
 407                         "linking to LLVM intrinsics is experimental"
 408                     );
 409                 }
 410             }
 411             ast::ForeignItemKind::TyAlias(..) => {
 412                 gate_feature_post!(&self, extern_types, i.span, "extern types are experimental");
 413             }
 414             ast::ForeignItemKind::MacCall(..) => {}
 415         }
 416
 417         visit::walk_foreign_item(self, i)
 418     }
 419
 420     fn visit_ty(&mut self, ty: &'a ast::Ty) {
 421         match ty.kind {
 422             ast::TyKind::BareFn(ref bare_fn_ty) => {
 423                 self.check_extern(bare_fn_ty.ext);
 424             }
 425             ast::TyKind::Never => {
 426                 gate_feature_post!(&self, never_type, ty.span, "the `!` type is experimental");
 427             }
 428             _ => {}
 429         }
 430         visit::walk_ty(self, ty)
 431     }
 432
 433     fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FnRetTy) {
 434         if let ast::FnRetTy::Ty(ref output_ty) = *ret_ty {
 435             if let ast::TyKind::Never = output_ty.kind {
 436                 // Do nothing.
 437             } else {
 438                 self.visit_ty(output_ty)
 439             }
 440         }
 441     }
 442
 443     fn visit_expr(&mut self, e: &'a ast::Expr) {
 444         match e.kind {
 445             ast::ExprKind::Box(_) => {
 446                 gate_feature_post!(
 447                     &self,
 448                     box_syntax,
 449                     e.span,
 450                     "box expression syntax is experimental; you can call `Box::new` instead"
 451                 );
 452             }
 453             ast::ExprKind::Type(..) => {
 454                 // To avoid noise about type ascription in common syntax errors, only emit if it
 455                 // is the *only* error.
 456                 if self.sess.parse_sess.span_diagnostic.err_count() == 0 {
 457                     gate_feature_post!(
 458                         &self,
 459                         type_ascription,
 460                         e.span,
 461                         "type ascription is experimental"
 462                     );
 463                 }
 464             }
 465             ast::ExprKind::TryBlock(_) => {
 466                 gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
 467             }
 468             ast::ExprKind::Block(_, opt_label) => {
 469                 if let Some(label) = opt_label {
 470                     gate_feature_post!(
 471                         &self,
 472                         label_break_value,
 473                         label.ident.span,
 474                         "labels on blocks are unstable"
 475                     );
 476                 }
 477             }
 478             _ => {}
 479         }
 480         visit::walk_expr(self, e)
 481     }
 482
 483     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
 484         match &pattern.kind {
 485             PatKind::Box(..) => {
 486                 gate_feature_post!(
 487                     &self,
 488                     box_patterns,
 489                     pattern.span,
 490                     "box pattern syntax is experimental"
 491                 );
 492             }
 493             PatKind::Range(_, _, Spanned { node: RangeEnd::Excluded, .. }) => {
 494                 gate_feature_post!(
 495                     &self,
 496                     exclusive_range_pattern,
 497                     pattern.span,
 498                     "exclusive range pattern syntax is experimental"
 499                 );
 500             }
 501             _ => {}
 502         }
 503         visit::walk_pat(self, pattern)
 504     }
 505
 506     fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
 507         if let Some(header) = fn_kind.header() {
 508             // Stability of const fn methods are covered in `visit_assoc_item` below.
 509             self.check_extern(header.ext);
 510
 511             if let (ast::Const::Yes(_), ast::Extern::Implicit)
 512             | (ast::Const::Yes(_), ast::Extern::Explicit(_)) = (header.constness, header.ext)
 513             {
 514                 gate_feature_post!(
 515                     &self,
 516                     const_extern_fn,
 517                     span,
 518                     "`const extern fn` definitions are unstable"
 519                 );
 520             }
 521         }
 522
 523         if fn_kind.ctxt() != Some(FnCtxt::Foreign) && fn_kind.decl().c_variadic() {
 524             gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
 525         }
 526
 527         visit::walk_fn(self, fn_kind, span)
 528     }
 529
 530     fn visit_assoc_ty_constraint(&mut self, constraint: &'a AssocTyConstraint) {
 531         if let AssocTyConstraintKind::Bound { .. } = constraint.kind {
 532             gate_feature_post!(
 533                 &self,
 534                 associated_type_bounds,
 535                 constraint.span,
 536                 "associated type bounds are unstable"
 537             )
 538         }
 539         visit::walk_assoc_ty_constraint(self, constraint)
 540     }
 541
 542     fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
 543         let is_fn = match i.kind {
 544             ast::AssocItemKind::Fn(_, ref sig, _, _) => {
 545                 if let (ast::Const::Yes(_), AssocCtxt::Trait) = (sig.header.constness, ctxt) {
 546                     gate_feature_post!(&self, const_fn, i.span, "const fn is unstable");
 547                 }
 548                 true
 549             }
 550             ast::AssocItemKind::TyAlias(_, ref generics, _, ref ty) => {
 551                 if let (Some(_), AssocCtxt::Trait) = (ty, ctxt) {
 552                     gate_feature_post!(
 553                         &self,
 554                         associated_type_defaults,
 555                         i.span,
 556                         "associated type defaults are unstable"
 557                     );
 558                 }
 559                 if let Some(ty) = ty {
 560                     self.check_impl_trait(ty);
 561                 }
 562                 self.check_gat(generics, i.span);
 563                 false
 564             }
 565             _ => false,
 566         };
 567         if let ast::Defaultness::Default(_) = i.kind.defaultness() {
 568             // Limit `min_specialization` to only specializing functions.
 569             gate_feature_fn!(
 570                 &self,
 571                 |x: &Features| x.specialization || (is_fn && x.min_specialization),
 572                 i.span,
 573                 sym::specialization,
 574                 "specialization is unstable"
 575             );
 576         }
 577         visit::walk_assoc_item(self, i, ctxt)
 578     }
 579
 580     fn visit_vis(&mut self, vis: &'a ast::Visibility) {
 581         if let ast::VisibilityKind::Crate(ast::CrateSugar::JustCrate) = vis.kind {
 582             gate_feature_post!(
 583                 &self,
 584                 crate_visibility_modifier,
 585                 vis.span,
 586                 "`crate` visibility modifier is experimental"
 587             );
 588         }
 589         visit::walk_vis(self, vis)
 590     }
 591 }
 592
 593 pub fn check_crate(krate: &ast::Crate, sess: &Session) {
 594     maybe_stage_features(sess, krate);
 595     check_incompatible_features(sess);
 596     let mut visitor = PostExpansionVisitor { sess, features: &sess.features_untracked() };
 597
 598     let spans = sess.parse_sess.gated_spans.spans.borrow();
 599     macro_rules! gate_all {
 600         ($gate:ident, $msg:literal) => {
 601             if let Some(spans) = spans.get(&sym::$gate) {
 602                 for span in spans {
 603                     gate_feature_post!(&visitor, $gate, *span, $msg);
 604                 }
 605             }
 606         };
 607     }
 608     gate_all!(if_let_guard, "`if let` guards are experimental");
 609     gate_all!(let_chains, "`let` expressions in this position are experimental");
 610     gate_all!(async_closure, "async closures are unstable");
 611     gate_all!(generators, "yield syntax is experimental");
 612     gate_all!(or_patterns, "or-patterns syntax is experimental");
 613     gate_all!(raw_ref_op, "raw address of syntax is experimental");
 614     gate_all!(const_trait_bound_opt_out, "`?const` on trait bounds is experimental");
 615     gate_all!(const_trait_impl, "const trait impls are experimental");
 616     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
 617     gate_all!(inline_const, "inline-const is experimental");
 618     gate_all!(
 619         extended_key_value_attributes,
 620         "arbitrary expressions in key-value attributes are unstable"
 621     );
 622     gate_all!(
 623         const_generics_defaults,
 624         "default values for const generic parameters are experimental"
 625     );
 626     if sess.parse_sess.span_diagnostic.err_count() == 0 {
 627         // Errors for `destructuring_assignment` can get quite noisy, especially where `_` is
 628         // involved, so we only emit errors where there are no other parsing errors.
 629         gate_all!(destructuring_assignment, "destructuring assignments are unstable");
 630     }
 631
 632     // All uses of `gate_all!` below this point were added in #65742,
 633     // and subsequently disabled (with the non-early gating readded).
 634     macro_rules! gate_all {
 635         ($gate:ident, $msg:literal) => {
 636             // FIXME(eddyb) do something more useful than always
 637             // disabling these uses of early feature-gatings.
 638             if false {
 639                 for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
 640                     gate_feature_post!(&visitor, $gate, *span, $msg);
 641                 }
 642             }
 643         };
 644     }
 645
 646     gate_all!(trait_alias, "trait aliases are experimental");
 647     gate_all!(associated_type_bounds, "associated type bounds are unstable");
 648     gate_all!(crate_visibility_modifier, "`crate` visibility modifier is experimental");
 649     gate_all!(const_generics, "const generics are unstable");
 650     gate_all!(decl_macro, "`macro` is experimental");
 651     gate_all!(box_patterns, "box pattern syntax is experimental");
 652     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
 653     gate_all!(try_blocks, "`try` blocks are unstable");
 654     gate_all!(label_break_value, "labels on blocks are unstable");
 655     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
 656     // To avoid noise about type ascription in common syntax errors,
 657     // only emit if it is the *only* error. (Also check it last.)
 658     if sess.parse_sess.span_diagnostic.err_count() == 0 {
 659         gate_all!(type_ascription, "type ascription is experimental");
 660     }
 661
 662     visit::walk_crate(&mut visitor, krate);
 663 }
 664
 665 fn maybe_stage_features(sess: &Session, krate: &ast::Crate) {
 666     if !sess.opts.unstable_features.is_nightly_build() {
 667         for attr in krate.attrs.iter().filter(|attr| sess.check_name(attr, sym::feature)) {
 668             struct_span_err!(
 669                 sess.parse_sess.span_diagnostic,
 670                 attr.span,
 671                 E0554,
 672                 "`#![feature]` may not be used on the {} release channel",
 673                 option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)")
 674             )
 675             .emit();
 676         }
 677     }
 678 }
 679
 680 fn check_incompatible_features(sess: &Session) {
 681     let features = sess.features_untracked();
 682
 683     let declared_features = features
 684         .declared_lang_features
 685         .iter()
 686         .copied()
 687         .map(|(name, span, _)| (name, span))
 688         .chain(features.declared_lib_features.iter().copied());
 689
 690     for (f1, f2) in rustc_feature::INCOMPATIBLE_FEATURES
 691         .iter()
 692         .filter(|&&(f1, f2)| features.enabled(f1) && features.enabled(f2))
 693     {
 694         if let Some((f1_name, f1_span)) = declared_features.clone().find(|(name, _)| name == f1) {
 695             if let Some((f2_name, f2_span)) = declared_features.clone().find(|(name, _)| name == f2)
 696             {
 697                 let spans = vec![f1_span, f2_span];
 698                 sess.struct_span_err(
 699                     spans.clone(),
 700                     &format!(
 701                         "features `{}` and `{}` are incompatible, using them at the same time \
 702                         is not allowed",
 703                         f1_name, f2_name
 704                     ),
 705                 )
 706                 .help("remove one of these features")
 707                 .emit();
 708             }
 709         }
 710     }
 711 }