src/librustc_ast_passes/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::{GenericParam, GenericParamKind, 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                     alias => doc_alias
 264                     keyword => doc_keyword
 265                 );
 266             }
 267         }
 268     }
 269
 270     fn visit_name(&mut self, sp: Span, name: Symbol) {
 271         if !name.as_str().is_ascii() {
 272             gate_feature_post!(
 273                 &self,
 274                 non_ascii_idents,
 275                 self.sess.parse_sess.source_map().guess_head_span(sp),
 276                 "non-ascii idents are not fully supported"
 277             );
 278         }
 279     }
 280
 281     fn visit_item(&mut self, i: &'a ast::Item) {
 282         match i.kind {
 283             ast::ItemKind::ForeignMod(ref foreign_module) => {
 284                 if let Some(abi) = foreign_module.abi {
 285                     self.check_abi(abi);
 286                 }
 287             }
 288
 289             ast::ItemKind::Fn(..) => {
 290                 if self.sess.contains_name(&i.attrs[..], sym::plugin_registrar) {
 291                     gate_feature_post!(
 292                         &self,
 293                         plugin_registrar,
 294                         i.span,
 295                         "compiler plugins are experimental and possibly buggy"
 296                     );
 297                 }
 298                 if self.sess.contains_name(&i.attrs[..], sym::start) {
 299                     gate_feature_post!(
 300                         &self,
 301                         start,
 302                         i.span,
 303                         "`#[start]` functions are experimental \
 304                          and their signature may change \
 305                          over time"
 306                     );
 307                 }
 308                 if self.sess.contains_name(&i.attrs[..], sym::main) {
 309                     gate_feature_post!(
 310                         &self,
 311                         main,
 312                         i.span,
 313                         "declaration of a non-standard `#[main]` \
 314                          function may change over time, for now \
 315                          a top-level `fn main()` is required"
 316                     );
 317                 }
 318             }
 319
 320             ast::ItemKind::Struct(..) => {
 321                 for attr in self.sess.filter_by_name(&i.attrs[..], sym::repr) {
 322                     for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
 323                         if item.has_name(sym::simd) {
 324                             gate_feature_post!(
 325                                 &self,
 326                                 repr_simd,
 327                                 attr.span,
 328                                 "SIMD types are experimental and possibly buggy"
 329                             );
 330                         }
 331                     }
 332                 }
 333             }
 334
 335             ast::ItemKind::Enum(ast::EnumDef { ref variants, .. }, ..) => {
 336                 for variant in variants {
 337                     match (&variant.data, &variant.disr_expr) {
 338                         (ast::VariantData::Unit(..), _) => {}
 339                         (_, Some(disr_expr)) => gate_feature_post!(
 340                             &self,
 341                             arbitrary_enum_discriminant,
 342                             disr_expr.value.span,
 343                             "discriminants on non-unit variants are experimental"
 344                         ),
 345                         _ => {}
 346                     }
 347                 }
 348
 349                 let has_feature = self.features.arbitrary_enum_discriminant;
 350                 if !has_feature && !i.span.allows_unstable(sym::arbitrary_enum_discriminant) {
 351                     self.maybe_report_invalid_custom_discriminants(&variants);
 352                 }
 353             }
 354
 355             ast::ItemKind::Impl { polarity, defaultness, ref of_trait, .. } => {
 356                 if let ast::ImplPolarity::Negative(span) = polarity {
 357                     gate_feature_post!(
 358                         &self,
 359                         negative_impls,
 360                         span.to(of_trait.as_ref().map(|t| t.path.span).unwrap_or(span)),
 361                         "negative trait bounds are not yet fully implemented; \
 362                          use marker types for now"
 363                     );
 364                 }
 365
 366                 if let ast::Defaultness::Default(_) = defaultness {
 367                     gate_feature_post!(&self, specialization, i.span, "specialization is unstable");
 368                 }
 369             }
 370
 371             ast::ItemKind::Trait(ast::IsAuto::Yes, ..) => {
 372                 gate_feature_post!(
 373                     &self,
 374                     optin_builtin_traits,
 375                     i.span,
 376                     "auto traits are experimental and possibly buggy"
 377                 );
 378             }
 379
 380             ast::ItemKind::TraitAlias(..) => {
 381                 gate_feature_post!(&self, trait_alias, i.span, "trait aliases are experimental");
 382             }
 383
 384             ast::ItemKind::MacroDef(ast::MacroDef { macro_rules: false, .. }) => {
 385                 let msg = "`macro` is experimental";
 386                 gate_feature_post!(&self, decl_macro, i.span, msg);
 387             }
 388
 389             ast::ItemKind::TyAlias(_, _, _, Some(ref ty)) => self.check_impl_trait(&ty),
 390
 391             _ => {}
 392         }
 393
 394         visit::walk_item(self, i);
 395     }
 396
 397     fn visit_foreign_item(&mut self, i: &'a ast::ForeignItem) {
 398         match i.kind {
 399             ast::ForeignItemKind::Fn(..) | ast::ForeignItemKind::Static(..) => {
 400                 let link_name = self.sess.first_attr_value_str_by_name(&i.attrs, sym::link_name);
 401                 let links_to_llvm = match link_name {
 402                     Some(val) => val.as_str().starts_with("llvm."),
 403                     _ => false,
 404                 };
 405                 if links_to_llvm {
 406                     gate_feature_post!(
 407                         &self,
 408                         link_llvm_intrinsics,
 409                         i.span,
 410                         "linking to LLVM intrinsics is experimental"
 411                     );
 412                 }
 413             }
 414             ast::ForeignItemKind::TyAlias(..) => {
 415                 gate_feature_post!(&self, extern_types, i.span, "extern types are experimental");
 416             }
 417             ast::ForeignItemKind::MacCall(..) => {}
 418         }
 419
 420         visit::walk_foreign_item(self, i)
 421     }
 422
 423     fn visit_ty(&mut self, ty: &'a ast::Ty) {
 424         match ty.kind {
 425             ast::TyKind::BareFn(ref bare_fn_ty) => {
 426                 self.check_extern(bare_fn_ty.ext);
 427             }
 428             ast::TyKind::Never => {
 429                 gate_feature_post!(&self, never_type, ty.span, "the `!` type is experimental");
 430             }
 431             _ => {}
 432         }
 433         visit::walk_ty(self, ty)
 434     }
 435
 436     fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FnRetTy) {
 437         if let ast::FnRetTy::Ty(ref output_ty) = *ret_ty {
 438             if let ast::TyKind::Never = output_ty.kind {
 439                 // Do nothing.
 440             } else {
 441                 self.visit_ty(output_ty)
 442             }
 443         }
 444     }
 445
 446     fn visit_expr(&mut self, e: &'a ast::Expr) {
 447         match e.kind {
 448             ast::ExprKind::Box(_) => {
 449                 gate_feature_post!(
 450                     &self,
 451                     box_syntax,
 452                     e.span,
 453                     "box expression syntax is experimental; you can call `Box::new` instead"
 454                 );
 455             }
 456             ast::ExprKind::Type(..) => {
 457                 // To avoid noise about type ascription in common syntax errors, only emit if it
 458                 // is the *only* error.
 459                 if self.sess.parse_sess.span_diagnostic.err_count() == 0 {
 460                     gate_feature_post!(
 461                         &self,
 462                         type_ascription,
 463                         e.span,
 464                         "type ascription is experimental"
 465                     );
 466                 }
 467             }
 468             ast::ExprKind::TryBlock(_) => {
 469                 gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
 470             }
 471             ast::ExprKind::Block(_, opt_label) => {
 472                 if let Some(label) = opt_label {
 473                     gate_feature_post!(
 474                         &self,
 475                         label_break_value,
 476                         label.ident.span,
 477                         "labels on blocks are unstable"
 478                     );
 479                 }
 480             }
 481             _ => {}
 482         }
 483         visit::walk_expr(self, e)
 484     }
 485
 486     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
 487         match &pattern.kind {
 488             PatKind::Box(..) => {
 489                 gate_feature_post!(
 490                     &self,
 491                     box_patterns,
 492                     pattern.span,
 493                     "box pattern syntax is experimental"
 494                 );
 495             }
 496             PatKind::Range(_, _, Spanned { node: RangeEnd::Excluded, .. }) => {
 497                 gate_feature_post!(
 498                     &self,
 499                     exclusive_range_pattern,
 500                     pattern.span,
 501                     "exclusive range pattern syntax is experimental"
 502                 );
 503             }
 504             _ => {}
 505         }
 506         visit::walk_pat(self, pattern)
 507     }
 508
 509     fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
 510         if let Some(header) = fn_kind.header() {
 511             // Stability of const fn methods are covered in `visit_assoc_item` below.
 512             self.check_extern(header.ext);
 513
 514             if let (ast::Const::Yes(_), ast::Extern::Implicit)
 515             | (ast::Const::Yes(_), ast::Extern::Explicit(_)) = (header.constness, header.ext)
 516             {
 517                 gate_feature_post!(
 518                     &self,
 519                     const_extern_fn,
 520                     span,
 521                     "`const extern fn` definitions are unstable"
 522                 );
 523             }
 524         }
 525
 526         if fn_kind.ctxt() != Some(FnCtxt::Foreign) && fn_kind.decl().c_variadic() {
 527             gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
 528         }
 529
 530         visit::walk_fn(self, fn_kind, span)
 531     }
 532
 533     fn visit_generic_param(&mut self, param: &'a GenericParam) {
 534         if let GenericParamKind::Const { .. } = param.kind {
 535             gate_feature_fn!(
 536                 &self,
 537                 |x: &Features| x.const_generics || x.min_const_generics,
 538                 param.ident.span,
 539                 sym::min_const_generics,
 540                 "const generics are unstable"
 541             );
 542         }
 543         visit::walk_generic_param(self, param)
 544     }
 545
 546     fn visit_assoc_ty_constraint(&mut self, constraint: &'a AssocTyConstraint) {
 547         if let AssocTyConstraintKind::Bound { .. } = constraint.kind {
 548             gate_feature_post!(
 549                 &self,
 550                 associated_type_bounds,
 551                 constraint.span,
 552                 "associated type bounds are unstable"
 553             )
 554         }
 555         visit::walk_assoc_ty_constraint(self, constraint)
 556     }
 557
 558     fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
 559         let is_fn = match i.kind {
 560             ast::AssocItemKind::Fn(_, ref sig, _, _) => {
 561                 if let (ast::Const::Yes(_), AssocCtxt::Trait) = (sig.header.constness, ctxt) {
 562                     gate_feature_post!(&self, const_fn, i.span, "const fn is unstable");
 563                 }
 564                 true
 565             }
 566             ast::AssocItemKind::TyAlias(_, ref generics, _, ref ty) => {
 567                 if let (Some(_), AssocCtxt::Trait) = (ty, ctxt) {
 568                     gate_feature_post!(
 569                         &self,
 570                         associated_type_defaults,
 571                         i.span,
 572                         "associated type defaults are unstable"
 573                     );
 574                 }
 575                 if let Some(ty) = ty {
 576                     self.check_impl_trait(ty);
 577                 }
 578                 self.check_gat(generics, i.span);
 579                 false
 580             }
 581             _ => false,
 582         };
 583         if let ast::Defaultness::Default(_) = i.kind.defaultness() {
 584             // Limit `min_specialization` to only specializing functions.
 585             gate_feature_fn!(
 586                 &self,
 587                 |x: &Features| x.specialization || (is_fn && x.min_specialization),
 588                 i.span,
 589                 sym::specialization,
 590                 "specialization is unstable"
 591             );
 592         }
 593         visit::walk_assoc_item(self, i, ctxt)
 594     }
 595
 596     fn visit_vis(&mut self, vis: &'a ast::Visibility) {
 597         if let ast::VisibilityKind::Crate(ast::CrateSugar::JustCrate) = vis.node {
 598             gate_feature_post!(
 599                 &self,
 600                 crate_visibility_modifier,
 601                 vis.span,
 602                 "`crate` visibility modifier is experimental"
 603             );
 604         }
 605         visit::walk_vis(self, vis)
 606     }
 607 }
 608
 609 pub fn check_crate(krate: &ast::Crate, sess: &Session) {
 610     maybe_stage_features(sess, krate);
 611     let mut visitor = PostExpansionVisitor { sess, features: &sess.features_untracked() };
 612
 613     let spans = sess.parse_sess.gated_spans.spans.borrow();
 614     macro_rules! gate_all {
 615         ($gate:ident, $msg:literal) => {
 616             for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
 617                 gate_feature_post!(&visitor, $gate, *span, $msg);
 618             }
 619         };
 620     }
 621     gate_all!(let_chains, "`let` expressions in this position are experimental");
 622     gate_all!(async_closure, "async closures are unstable");
 623     gate_all!(generators, "yield syntax is experimental");
 624     gate_all!(or_patterns, "or-patterns syntax is experimental");
 625     gate_all!(raw_ref_op, "raw address of syntax is experimental");
 626     gate_all!(const_trait_bound_opt_out, "`?const` on trait bounds is experimental");
 627     gate_all!(const_trait_impl, "const trait impls are experimental");
 628     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
 629
 630     // All uses of `gate_all!` below this point were added in #65742,
 631     // and subsequently disabled (with the non-early gating readded).
 632     macro_rules! gate_all {
 633         ($gate:ident, $msg:literal) => {
 634             // FIXME(eddyb) do something more useful than always
 635             // disabling these uses of early feature-gatings.
 636             if false {
 637                 for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
 638                     gate_feature_post!(&visitor, $gate, *span, $msg);
 639                 }
 640             }
 641         };
 642     }
 643
 644     gate_all!(trait_alias, "trait aliases are experimental");
 645     gate_all!(associated_type_bounds, "associated type bounds are unstable");
 646     gate_all!(crate_visibility_modifier, "`crate` visibility modifier is experimental");
 647     gate_all!(const_generics, "const generics are unstable");
 648     gate_all!(decl_macro, "`macro` is experimental");
 649     gate_all!(box_patterns, "box pattern syntax is experimental");
 650     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
 651     gate_all!(try_blocks, "`try` blocks are unstable");
 652     gate_all!(label_break_value, "labels on blocks are unstable");
 653     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
 654     // To avoid noise about type ascription in common syntax errors,
 655     // only emit if it is the *only* error. (Also check it last.)
 656     if sess.parse_sess.span_diagnostic.err_count() == 0 {
 657         gate_all!(type_ascription, "type ascription is experimental");
 658     }
 659
 660     visit::walk_crate(&mut visitor, krate);
 661 }
 662
 663 fn maybe_stage_features(sess: &Session, krate: &ast::Crate) {
 664     if !sess.opts.unstable_features.is_nightly_build() {
 665         for attr in krate.attrs.iter().filter(|attr| sess.check_name(attr, sym::feature)) {
 666             struct_span_err!(
 667                 sess.parse_sess.span_diagnostic,
 668                 attr.span,
 669                 E0554,
 670                 "`#![feature]` may not be used on the {} release channel",
 671                 option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)")
 672             )
 673             .emit();
 674         }
 675     }
 676 }