src/librustc_ast_passes/feature_gate.rs

   1 use rustc_errors::{struct_span_err, Handler};
   2 use rustc_feature::{AttributeGate, BUILTIN_ATTRIBUTE_MAP};
   3 use rustc_feature::{Features, GateIssue, UnstableFeatures};
   4 use rustc_span::source_map::Spanned;
   5 use rustc_span::symbol::sym;
   6 use rustc_span::Span;
   7 use syntax::ast::{self, AssocTyConstraint, AssocTyConstraintKind, NodeId};
   8 use syntax::ast::{GenericParam, GenericParamKind, PatKind, RangeEnd, VariantData};
   9 use syntax::attr;
  10 use syntax::sess::{feature_err, feature_err_issue, ParseSess};
  11 use syntax::visit::{self, FnKind, Visitor};
  12
  13 use log::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             "efiapi" => {
 125                 gate_feature_post!(
 126                     &self,
 127                     abi_efiapi,
 128                     span,
 129                     "efiapi ABI is experimental and subject to change"
 130                 );
 131             }
 132             abi => self
 133                 .parse_sess
 134                 .span_diagnostic
 135                 .delay_span_bug(span, &format!("unrecognized ABI not caught in lowering: {}", abi)),
 136         }
 137     }
 138
 139     fn check_extern(&self, ext: ast::Extern) {
 140         if let ast::Extern::Explicit(abi) = ext {
 141             self.check_abi(abi);
 142         }
 143     }
 144
 145     fn maybe_report_invalid_custom_discriminants(&self, variants: &[ast::Variant]) {
 146         let has_fields = variants.iter().any(|variant| match variant.data {
 147             VariantData::Tuple(..) | VariantData::Struct(..) => true,
 148             VariantData::Unit(..) => false,
 149         });
 150
 151         let discriminant_spans = variants
 152             .iter()
 153             .filter(|variant| match variant.data {
 154                 VariantData::Tuple(..) | VariantData::Struct(..) => false,
 155                 VariantData::Unit(..) => true,
 156             })
 157             .filter_map(|variant| variant.disr_expr.as_ref().map(|c| c.value.span))
 158             .collect::<Vec<_>>();
 159
 160         if !discriminant_spans.is_empty() && has_fields {
 161             let mut err = feature_err(
 162                 self.parse_sess,
 163                 sym::arbitrary_enum_discriminant,
 164                 discriminant_spans.clone(),
 165                 "custom discriminant values are not allowed in enums with tuple or struct variants",
 166             );
 167             for sp in discriminant_spans {
 168                 err.span_label(sp, "disallowed custom discriminant");
 169             }
 170             for variant in variants.iter() {
 171                 match &variant.data {
 172                     VariantData::Struct(..) => {
 173                         err.span_label(variant.span, "struct variant defined here");
 174                     }
 175                     VariantData::Tuple(..) => {
 176                         err.span_label(variant.span, "tuple variant defined here");
 177                     }
 178                     VariantData::Unit(..) => {}
 179                 }
 180             }
 181             err.emit();
 182         }
 183     }
 184
 185     fn check_gat(&self, generics: &ast::Generics, span: Span) {
 186         if !generics.params.is_empty() {
 187             gate_feature_post!(
 188                 &self,
 189                 generic_associated_types,
 190                 span,
 191                 "generic associated types are unstable"
 192             );
 193         }
 194         if !generics.where_clause.predicates.is_empty() {
 195             gate_feature_post!(
 196                 &self,
 197                 generic_associated_types,
 198                 span,
 199                 "where clauses on associated types are unstable"
 200             );
 201         }
 202     }
 203
 204     /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
 205     fn check_impl_trait(&self, ty: &ast::Ty) {
 206         struct ImplTraitVisitor<'a> {
 207             vis: &'a PostExpansionVisitor<'a>,
 208         }
 209         impl Visitor<'_> for ImplTraitVisitor<'_> {
 210             fn visit_ty(&mut self, ty: &ast::Ty) {
 211                 if let ast::TyKind::ImplTrait(..) = ty.kind {
 212                     gate_feature_post!(
 213                         &self.vis,
 214                         type_alias_impl_trait,
 215                         ty.span,
 216                         "`impl Trait` in type aliases is unstable"
 217                     );
 218                 }
 219                 visit::walk_ty(self, ty);
 220             }
 221         }
 222         ImplTraitVisitor { vis: self }.visit_ty(ty);
 223     }
 224 }
 225
 226 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 227     fn visit_attribute(&mut self, attr: &ast::Attribute) {
 228         let attr_info =
 229             attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name)).map(|a| **a);
 230         // Check feature gates for built-in attributes.
 231         if let Some((.., AttributeGate::Gated(_, name, descr, has_feature))) = attr_info {
 232             gate_feature_fn!(self, has_feature, attr.span, name, descr);
 233         }
 234         // Check unstable flavors of the `#[doc]` attribute.
 235         if attr.check_name(sym::doc) {
 236             for nested_meta in attr.meta_item_list().unwrap_or_default() {
 237                 macro_rules! gate_doc { ($($name:ident => $feature:ident)*) => {
 238                     $(if nested_meta.check_name(sym::$name) {
 239                         let msg = concat!("`#[doc(", stringify!($name), ")]` is experimental");
 240                         gate_feature_post!(self, $feature, attr.span, msg);
 241                     })*
 242                 }}
 243
 244                 gate_doc!(
 245                     include => external_doc
 246                     cfg => doc_cfg
 247                     masked => doc_masked
 248                     spotlight => doc_spotlight
 249                     alias => doc_alias
 250                     keyword => doc_keyword
 251                 );
 252             }
 253         }
 254     }
 255
 256     fn visit_name(&mut self, sp: Span, name: ast::Name) {
 257         if !name.as_str().is_ascii() {
 258             gate_feature_post!(
 259                 &self,
 260                 non_ascii_idents,
 261                 self.parse_sess.source_map().def_span(sp),
 262                 "non-ascii idents are not fully supported"
 263             );
 264         }
 265     }
 266
 267     fn visit_item(&mut self, i: &'a ast::Item) {
 268         match i.kind {
 269             ast::ItemKind::ForeignMod(ref foreign_module) => {
 270                 if let Some(abi) = foreign_module.abi {
 271                     self.check_abi(abi);
 272                 }
 273             }
 274
 275             ast::ItemKind::Fn(..) => {
 276                 if attr::contains_name(&i.attrs[..], sym::plugin_registrar) {
 277                     gate_feature_post!(
 278                         &self,
 279                         plugin_registrar,
 280                         i.span,
 281                         "compiler plugins are experimental and possibly buggy"
 282                     );
 283                 }
 284                 if attr::contains_name(&i.attrs[..], sym::start) {
 285                     gate_feature_post!(
 286                         &self,
 287                         start,
 288                         i.span,
 289                         "`#[start]` functions are experimental \
 290                                        and their signature may change \
 291                                        over time"
 292                     );
 293                 }
 294                 if attr::contains_name(&i.attrs[..], sym::main) {
 295                     gate_feature_post!(
 296                         &self,
 297                         main,
 298                         i.span,
 299                         "declaration of a non-standard `#[main]` \
 300                                         function may change over time, for now \
 301                                         a top-level `fn main()` is required"
 302                     );
 303                 }
 304             }
 305
 306             ast::ItemKind::Struct(..) => {
 307                 for attr in attr::filter_by_name(&i.attrs[..], sym::repr) {
 308                     for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
 309                         if item.check_name(sym::simd) {
 310                             gate_feature_post!(
 311                                 &self,
 312                                 repr_simd,
 313                                 attr.span,
 314                                 "SIMD types are experimental and possibly buggy"
 315                             );
 316                         }
 317                     }
 318                 }
 319             }
 320
 321             ast::ItemKind::Enum(ast::EnumDef { ref variants, .. }, ..) => {
 322                 for variant in variants {
 323                     match (&variant.data, &variant.disr_expr) {
 324                         (ast::VariantData::Unit(..), _) => {}
 325                         (_, Some(disr_expr)) => gate_feature_post!(
 326                             &self,
 327                             arbitrary_enum_discriminant,
 328                             disr_expr.value.span,
 329                             "discriminants on non-unit variants are experimental"
 330                         ),
 331                         _ => {}
 332                     }
 333                 }
 334
 335                 let has_feature = self.features.arbitrary_enum_discriminant;
 336                 if !has_feature && !i.span.allows_unstable(sym::arbitrary_enum_discriminant) {
 337                     self.maybe_report_invalid_custom_discriminants(&variants);
 338                 }
 339             }
 340
 341             ast::ItemKind::Impl { polarity, defaultness, .. } => {
 342                 if polarity == ast::ImplPolarity::Negative {
 343                     gate_feature_post!(
 344                         &self,
 345                         optin_builtin_traits,
 346                         i.span,
 347                         "negative trait bounds are not yet fully implemented; \
 348                                         use marker types for now"
 349                     );
 350                 }
 351
 352                 if let ast::Defaultness::Default = defaultness {
 353                     gate_feature_post!(&self, specialization, i.span, "specialization is unstable");
 354                 }
 355             }
 356
 357             ast::ItemKind::Trait(ast::IsAuto::Yes, ..) => {
 358                 gate_feature_post!(
 359                     &self,
 360                     optin_builtin_traits,
 361                     i.span,
 362                     "auto traits are experimental and possibly buggy"
 363                 );
 364             }
 365
 366             ast::ItemKind::TraitAlias(..) => {
 367                 gate_feature_post!(&self, trait_alias, i.span, "trait aliases are experimental");
 368             }
 369
 370             ast::ItemKind::MacroDef(ast::MacroDef { legacy: false, .. }) => {
 371                 let msg = "`macro` is experimental";
 372                 gate_feature_post!(&self, decl_macro, i.span, msg);
 373             }
 374
 375             ast::ItemKind::TyAlias(ref ty, ..) => self.check_impl_trait(&ty),
 376
 377             _ => {}
 378         }
 379
 380         visit::walk_item(self, i);
 381     }
 382
 383     fn visit_foreign_item(&mut self, i: &'a ast::ForeignItem) {
 384         match i.kind {
 385             ast::ForeignItemKind::Fn(..) | ast::ForeignItemKind::Static(..) => {
 386                 let link_name = attr::first_attr_value_str_by_name(&i.attrs, sym::link_name);
 387                 let links_to_llvm = match link_name {
 388                     Some(val) => val.as_str().starts_with("llvm."),
 389                     _ => false,
 390                 };
 391                 if links_to_llvm {
 392                     gate_feature_post!(
 393                         &self,
 394                         link_llvm_intrinsics,
 395                         i.span,
 396                         "linking to LLVM intrinsics is experimental"
 397                     );
 398                 }
 399             }
 400             ast::ForeignItemKind::Ty => {
 401                 gate_feature_post!(&self, extern_types, i.span, "extern types are experimental");
 402             }
 403             ast::ForeignItemKind::Macro(..) => {}
 404         }
 405
 406         visit::walk_foreign_item(self, i)
 407     }
 408
 409     fn visit_ty(&mut self, ty: &'a ast::Ty) {
 410         match ty.kind {
 411             ast::TyKind::BareFn(ref bare_fn_ty) => {
 412                 self.check_extern(bare_fn_ty.ext);
 413             }
 414             ast::TyKind::Never => {
 415                 gate_feature_post!(&self, never_type, ty.span, "the `!` type is experimental");
 416             }
 417             _ => {}
 418         }
 419         visit::walk_ty(self, ty)
 420     }
 421
 422     fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FunctionRetTy) {
 423         if let ast::FunctionRetTy::Ty(ref output_ty) = *ret_ty {
 424             if let ast::TyKind::Never = output_ty.kind {
 425                 // Do nothing.
 426             } else {
 427                 self.visit_ty(output_ty)
 428             }
 429         }
 430     }
 431
 432     fn visit_expr(&mut self, e: &'a ast::Expr) {
 433         match e.kind {
 434             ast::ExprKind::Box(_) => {
 435                 gate_feature_post!(
 436                     &self,
 437                     box_syntax,
 438                     e.span,
 439                     "box expression syntax is experimental; you can call `Box::new` instead"
 440                 );
 441             }
 442             ast::ExprKind::Type(..) => {
 443                 // To avoid noise about type ascription in common syntax errors, only emit if it
 444                 // is the *only* error.
 445                 if self.parse_sess.span_diagnostic.err_count() == 0 {
 446                     gate_feature_post!(
 447                         &self,
 448                         type_ascription,
 449                         e.span,
 450                         "type ascription is experimental"
 451                     );
 452                 }
 453             }
 454             ast::ExprKind::TryBlock(_) => {
 455                 gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
 456             }
 457             ast::ExprKind::Block(_, opt_label) => {
 458                 if let Some(label) = opt_label {
 459                     gate_feature_post!(
 460                         &self,
 461                         label_break_value,
 462                         label.ident.span,
 463                         "labels on blocks are unstable"
 464                     );
 465                 }
 466             }
 467             _ => {}
 468         }
 469         visit::walk_expr(self, e)
 470     }
 471
 472     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
 473         match &pattern.kind {
 474             PatKind::Box(..) => {
 475                 gate_feature_post!(
 476                     &self,
 477                     box_patterns,
 478                     pattern.span,
 479                     "box pattern syntax is experimental"
 480                 );
 481             }
 482             PatKind::Range(_, _, Spanned { node: RangeEnd::Excluded, .. }) => {
 483                 gate_feature_post!(
 484                     &self,
 485                     exclusive_range_pattern,
 486                     pattern.span,
 487                     "exclusive range pattern syntax is experimental"
 488                 );
 489             }
 490             _ => {}
 491         }
 492         visit::walk_pat(self, pattern)
 493     }
 494
 495     fn visit_fn(
 496         &mut self,
 497         fn_kind: FnKind<'a>,
 498         fn_decl: &'a ast::FnDecl,
 499         span: Span,
 500         _node_id: NodeId,
 501     ) {
 502         if let Some(header) = fn_kind.header() {
 503             // Stability of const fn methods are covered in
 504             // `visit_trait_item` and `visit_impl_item` below; this is
 505             // because default methods don't pass through this point.
 506             self.check_extern(header.ext);
 507         }
 508
 509         if fn_decl.c_variadic() {
 510             gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
 511         }
 512
 513         visit::walk_fn(self, fn_kind, fn_decl, span)
 514     }
 515
 516     fn visit_generic_param(&mut self, param: &'a GenericParam) {
 517         match param.kind {
 518             GenericParamKind::Const { .. } => gate_feature_post!(
 519                 &self,
 520                 const_generics,
 521                 param.ident.span,
 522                 "const generics are unstable"
 523             ),
 524             _ => {}
 525         }
 526         visit::walk_generic_param(self, param)
 527     }
 528
 529     fn visit_assoc_ty_constraint(&mut self, constraint: &'a AssocTyConstraint) {
 530         match constraint.kind {
 531             AssocTyConstraintKind::Bound { .. } => gate_feature_post!(
 532                 &self,
 533                 associated_type_bounds,
 534                 constraint.span,
 535                 "associated type bounds are unstable"
 536             ),
 537             _ => {}
 538         }
 539         visit::walk_assoc_ty_constraint(self, constraint)
 540     }
 541
 542     fn visit_trait_item(&mut self, ti: &'a ast::AssocItem) {
 543         match ti.kind {
 544             ast::AssocItemKind::Fn(ref sig, ref block) => {
 545                 if block.is_none() {
 546                     self.check_extern(sig.header.ext);
 547                 }
 548                 if sig.header.constness.node == ast::Constness::Const {
 549                     gate_feature_post!(&self, const_fn, ti.span, "const fn is unstable");
 550                 }
 551             }
 552             ast::AssocItemKind::TyAlias(_, ref default) => {
 553                 if let Some(_) = default {
 554                     gate_feature_post!(
 555                         &self,
 556                         associated_type_defaults,
 557                         ti.span,
 558                         "associated type defaults are unstable"
 559                     );
 560                 }
 561             }
 562             _ => {}
 563         }
 564         visit::walk_trait_item(self, ti)
 565     }
 566
 567     fn visit_assoc_item(&mut self, ii: &'a ast::AssocItem) {
 568         if ii.defaultness == ast::Defaultness::Default {
 569             gate_feature_post!(&self, specialization, ii.span, "specialization is unstable");
 570         }
 571
 572         match ii.kind {
 573             ast::AssocItemKind::Fn(ref sig, _) => {
 574                 if sig.decl.c_variadic() {
 575                     gate_feature_post!(
 576                         &self,
 577                         c_variadic,
 578                         ii.span,
 579                         "C-variadic functions are unstable"
 580                     );
 581                 }
 582             }
 583             ast::AssocItemKind::TyAlias(_, ref ty) => {
 584                 if let Some(ty) = ty {
 585                     self.check_impl_trait(ty);
 586                 }
 587                 self.check_gat(&ii.generics, ii.span);
 588             }
 589             _ => {}
 590         }
 591         visit::walk_assoc_item(self, ii)
 592     }
 593
 594     fn visit_vis(&mut self, vis: &'a ast::Visibility) {
 595         if let ast::VisibilityKind::Crate(ast::CrateSugar::JustCrate) = vis.node {
 596             gate_feature_post!(
 597                 &self,
 598                 crate_visibility_modifier,
 599                 vis.span,
 600                 "`crate` visibility modifier is experimental"
 601             );
 602         }
 603         visit::walk_vis(self, vis)
 604     }
 605 }
 606
 607 pub fn check_crate(
 608     krate: &ast::Crate,
 609     parse_sess: &ParseSess,
 610     features: &Features,
 611     unstable: UnstableFeatures,
 612 ) {
 613     maybe_stage_features(&parse_sess.span_diagnostic, krate, unstable);
 614     let mut visitor = PostExpansionVisitor { parse_sess, features };
 615
 616     let spans = parse_sess.gated_spans.spans.borrow();
 617     macro_rules! gate_all {
 618         ($gate:ident, $msg:literal) => {
 619             for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
 620                 gate_feature_post!(&visitor, $gate, *span, $msg);
 621             }
 622         };
 623     }
 624     gate_all!(let_chains, "`let` expressions in this position are experimental");
 625     gate_all!(async_closure, "async closures are unstable");
 626     gate_all!(generators, "yield syntax is experimental");
 627     gate_all!(or_patterns, "or-patterns syntax is experimental");
 628     gate_all!(const_extern_fn, "`const extern fn` definitions are unstable");
 629     gate_all!(raw_ref_op, "raw address of syntax is experimental");
 630     gate_all!(const_trait_bound_opt_out, "`?const` on trait bounds is experimental");
 631     gate_all!(const_trait_impl, "const trait impls are experimental");
 632     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
 633
 634     // All uses of `gate_all!` below this point were added in #65742,
 635     // and subsequently disabled (with the non-early gating readded).
 636     macro_rules! gate_all {
 637         ($gate:ident, $msg:literal) => {
 638             // FIXME(eddyb) do something more useful than always
 639             // disabling these uses of early feature-gatings.
 640             if false {
 641                 for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
 642                     gate_feature_post!(&visitor, $gate, *span, $msg);
 643                 }
 644             }
 645         };
 646     }
 647
 648     gate_all!(trait_alias, "trait aliases are experimental");
 649     gate_all!(associated_type_bounds, "associated type bounds are unstable");
 650     gate_all!(crate_visibility_modifier, "`crate` visibility modifier is experimental");
 651     gate_all!(const_generics, "const generics are unstable");
 652     gate_all!(decl_macro, "`macro` is experimental");
 653     gate_all!(box_patterns, "box pattern syntax is experimental");
 654     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
 655     gate_all!(try_blocks, "`try` blocks are unstable");
 656     gate_all!(label_break_value, "labels on blocks are unstable");
 657     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
 658     // To avoid noise about type ascription in common syntax errors,
 659     // only emit if it is the *only* error. (Also check it last.)
 660     if parse_sess.span_diagnostic.err_count() == 0 {
 661         gate_all!(type_ascription, "type ascription is experimental");
 662     }
 663
 664     visit::walk_crate(&mut visitor, krate);
 665 }
 666
 667 fn maybe_stage_features(span_handler: &Handler, krate: &ast::Crate, unstable: UnstableFeatures) {
 668     if !unstable.is_nightly_build() {
 669         for attr in krate.attrs.iter().filter(|attr| attr.check_name(sym::feature)) {
 670             struct_span_err!(
 671                 span_handler,
 672                 attr.span,
 673                 E0554,
 674                 "`#![feature]` may not be used on the {} release channel",
 675                 option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)")
 676             )
 677             .emit();
 678         }
 679     }
 680 }