src/librustc_ast_passes/feature_gate.rs

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