src/librustc_ast_passes/feature_gate.rs

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