compiler/rustc_ast_passes/src/feature_gate.rs

   1 use rustc_ast as ast;
   2 use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
   3 use rustc_ast::{AssocConstraint, AssocConstraintKind, NodeId};
   4 use rustc_ast::{PatKind, RangeEnd, VariantData};
   5 use rustc_errors::struct_span_err;
   6 use rustc_feature::{AttributeGate, BuiltinAttribute, 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;
  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, $help: expr) => {{
  18         let (visitor, has_feature, span, name, explain, help) =
  19             (&*$visitor, $has_feature, $span, $name, $explain, $help);
  20         let has_feature: bool = has_feature(visitor.features);
  21         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
  22         if !has_feature && !span.allows_unstable($name) {
  23             feature_err_issue(&visitor.sess.parse_sess, name, span, GateIssue::Language, explain)
  24                 .help(help)
  25                 .emit();
  26         }
  27     }};
  28     ($visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr) => {{
  29         let (visitor, has_feature, span, name, explain) =
  30             (&*$visitor, $has_feature, $span, $name, $explain);
  31         let has_feature: bool = has_feature(visitor.features);
  32         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
  33         if !has_feature && !span.allows_unstable($name) {
  34             feature_err_issue(&visitor.sess.parse_sess, name, span, GateIssue::Language, explain)
  35                 .emit();
  36         }
  37     }};
  38 }
  39
  40 macro_rules! gate_feature_post {
  41     ($visitor: expr, $feature: ident, $span: expr, $explain: expr, $help: expr) => {
  42         gate_feature_fn!($visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain, $help)
  43     };
  44     ($visitor: expr, $feature: ident, $span: expr, $explain: expr) => {
  45         gate_feature_fn!($visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain)
  46     };
  47 }
  48
  49 pub fn check_attribute(attr: &ast::Attribute, sess: &Session, features: &Features) {
  50     PostExpansionVisitor { sess, features }.visit_attribute(attr)
  51 }
  52
  53 struct PostExpansionVisitor<'a> {
  54     sess: &'a Session,
  55
  56     // `sess` contains a `Features`, but this might not be that one.
  57     features: &'a Features,
  58 }
  59
  60 impl<'a> PostExpansionVisitor<'a> {
  61     fn check_abi(&self, abi: ast::StrLit, constness: ast::Const) {
  62         let ast::StrLit { symbol_unescaped, span, .. } = abi;
  63
  64         if let ast::Const::Yes(_) = constness {
  65             match symbol_unescaped.as_str() {
  66                 // Stable
  67                 "Rust" | "C" => {}
  68                 abi => gate_feature_post!(
  69                     &self,
  70                     const_extern_fn,
  71                     span,
  72                     &format!("`{}` as a `const fn` ABI is unstable", abi)
  73                 ),
  74             }
  75         }
  76
  77         match symbol_unescaped.as_str() {
  78             // Stable
  79             "Rust" | "C" | "cdecl" | "stdcall" | "fastcall" | "aapcs" | "win64" | "sysv64"
  80             | "system" => {}
  81             "rust-intrinsic" => {
  82                 gate_feature_post!(&self, intrinsics, span, "intrinsics are subject to change");
  83             }
  84             "platform-intrinsic" => {
  85                 gate_feature_post!(
  86                     &self,
  87                     platform_intrinsics,
  88                     span,
  89                     "platform intrinsics are experimental and possibly buggy"
  90                 );
  91             }
  92             "vectorcall" => {
  93                 gate_feature_post!(
  94                     &self,
  95                     abi_vectorcall,
  96                     span,
  97                     "vectorcall is experimental and subject to change"
  98                 );
  99             }
 100             "thiscall" => {
 101                 gate_feature_post!(
 102                     &self,
 103                     abi_thiscall,
 104                     span,
 105                     "thiscall is experimental and subject to change"
 106                 );
 107             }
 108             "rust-call" => {
 109                 gate_feature_post!(
 110                     &self,
 111                     unboxed_closures,
 112                     span,
 113                     "rust-call ABI is subject to change"
 114                 );
 115             }
 116             "rust-cold" => {
 117                 gate_feature_post!(
 118                     &self,
 119                     rust_cold_cc,
 120                     span,
 121                     "rust-cold is experimental and subject to change"
 122                 );
 123             }
 124             "ptx-kernel" => {
 125                 gate_feature_post!(
 126                     &self,
 127                     abi_ptx,
 128                     span,
 129                     "PTX ABIs are experimental and subject to change"
 130                 );
 131             }
 132             "unadjusted" => {
 133                 gate_feature_post!(
 134                     &self,
 135                     abi_unadjusted,
 136                     span,
 137                     "unadjusted ABI is an implementation detail and perma-unstable"
 138                 );
 139             }
 140             "msp430-interrupt" => {
 141                 gate_feature_post!(
 142                     &self,
 143                     abi_msp430_interrupt,
 144                     span,
 145                     "msp430-interrupt ABI is experimental and subject to change"
 146                 );
 147             }
 148             "x86-interrupt" => {
 149                 gate_feature_post!(
 150                     &self,
 151                     abi_x86_interrupt,
 152                     span,
 153                     "x86-interrupt ABI is experimental and subject to change"
 154                 );
 155             }
 156             "amdgpu-kernel" => {
 157                 gate_feature_post!(
 158                     &self,
 159                     abi_amdgpu_kernel,
 160                     span,
 161                     "amdgpu-kernel ABI is experimental and subject to change"
 162                 );
 163             }
 164             "avr-interrupt" | "avr-non-blocking-interrupt" => {
 165                 gate_feature_post!(
 166                     &self,
 167                     abi_avr_interrupt,
 168                     span,
 169                     "avr-interrupt and avr-non-blocking-interrupt ABIs are experimental and subject to change"
 170                 );
 171             }
 172             "efiapi" => {
 173                 gate_feature_post!(
 174                     &self,
 175                     abi_efiapi,
 176                     span,
 177                     "efiapi ABI is experimental and subject to change"
 178                 );
 179             }
 180             "C-cmse-nonsecure-call" => {
 181                 gate_feature_post!(
 182                     &self,
 183                     abi_c_cmse_nonsecure_call,
 184                     span,
 185                     "C-cmse-nonsecure-call ABI is experimental and subject to change"
 186                 );
 187             }
 188             "C-unwind" => {
 189                 gate_feature_post!(
 190                     &self,
 191                     c_unwind,
 192                     span,
 193                     "C-unwind ABI is experimental and subject to change"
 194                 );
 195             }
 196             "stdcall-unwind" => {
 197                 gate_feature_post!(
 198                     &self,
 199                     c_unwind,
 200                     span,
 201                     "stdcall-unwind ABI is experimental and subject to change"
 202                 );
 203             }
 204             "system-unwind" => {
 205                 gate_feature_post!(
 206                     &self,
 207                     c_unwind,
 208                     span,
 209                     "system-unwind ABI is experimental and subject to change"
 210                 );
 211             }
 212             "thiscall-unwind" => {
 213                 gate_feature_post!(
 214                     &self,
 215                     c_unwind,
 216                     span,
 217                     "thiscall-unwind ABI is experimental and subject to change"
 218                 );
 219             }
 220             "cdecl-unwind" => {
 221                 gate_feature_post!(
 222                     &self,
 223                     c_unwind,
 224                     span,
 225                     "cdecl-unwind ABI is experimental and subject to change"
 226                 );
 227             }
 228             "fastcall-unwind" => {
 229                 gate_feature_post!(
 230                     &self,
 231                     c_unwind,
 232                     span,
 233                     "fastcall-unwind ABI is experimental and subject to change"
 234                 );
 235             }
 236             "vectorcall-unwind" => {
 237                 gate_feature_post!(
 238                     &self,
 239                     c_unwind,
 240                     span,
 241                     "vectorcall-unwind ABI is experimental and subject to change"
 242                 );
 243             }
 244             "aapcs-unwind" => {
 245                 gate_feature_post!(
 246                     &self,
 247                     c_unwind,
 248                     span,
 249                     "aapcs-unwind ABI is experimental and subject to change"
 250                 );
 251             }
 252             "win64-unwind" => {
 253                 gate_feature_post!(
 254                     &self,
 255                     c_unwind,
 256                     span,
 257                     "win64-unwind ABI is experimental and subject to change"
 258                 );
 259             }
 260             "sysv64-unwind" => {
 261                 gate_feature_post!(
 262                     &self,
 263                     c_unwind,
 264                     span,
 265                     "sysv64-unwind ABI is experimental and subject to change"
 266                 );
 267             }
 268             "wasm" => {
 269                 gate_feature_post!(
 270                     &self,
 271                     wasm_abi,
 272                     span,
 273                     "wasm ABI is experimental and subject to change"
 274                 );
 275             }
 276             abi => {
 277                 if self.sess.opts.pretty.map_or(true, |ppm| ppm.needs_hir()) {
 278                     self.sess.parse_sess.span_diagnostic.delay_span_bug(
 279                         span,
 280                         &format!("unrecognized ABI not caught in lowering: {}", abi),
 281                     );
 282                 }
 283             }
 284         }
 285     }
 286
 287     fn check_extern(&self, ext: ast::Extern, constness: ast::Const) {
 288         if let ast::Extern::Explicit(abi, _) = ext {
 289             self.check_abi(abi, constness);
 290         }
 291     }
 292
 293     fn maybe_report_invalid_custom_discriminants(&self, variants: &[ast::Variant]) {
 294         let has_fields = variants.iter().any(|variant| match variant.data {
 295             VariantData::Tuple(..) | VariantData::Struct(..) => true,
 296             VariantData::Unit(..) => false,
 297         });
 298
 299         let discriminant_spans = variants
 300             .iter()
 301             .filter(|variant| match variant.data {
 302                 VariantData::Tuple(..) | VariantData::Struct(..) => false,
 303                 VariantData::Unit(..) => true,
 304             })
 305             .filter_map(|variant| variant.disr_expr.as_ref().map(|c| c.value.span))
 306             .collect::<Vec<_>>();
 307
 308         if !discriminant_spans.is_empty() && has_fields {
 309             let mut err = feature_err(
 310                 &self.sess.parse_sess,
 311                 sym::arbitrary_enum_discriminant,
 312                 discriminant_spans.clone(),
 313                 "custom discriminant values are not allowed in enums with tuple or struct variants",
 314             );
 315             for sp in discriminant_spans {
 316                 err.span_label(sp, "disallowed custom discriminant");
 317             }
 318             for variant in variants.iter() {
 319                 match &variant.data {
 320                     VariantData::Struct(..) => {
 321                         err.span_label(variant.span, "struct variant defined here");
 322                     }
 323                     VariantData::Tuple(..) => {
 324                         err.span_label(variant.span, "tuple variant defined here");
 325                     }
 326                     VariantData::Unit(..) => {}
 327                 }
 328             }
 329             err.emit();
 330         }
 331     }
 332
 333     fn check_gat(&self, generics: &ast::Generics, span: Span) {
 334         if !generics.params.is_empty() {
 335             gate_feature_post!(
 336                 &self,
 337                 generic_associated_types,
 338                 span,
 339                 "generic associated types are unstable"
 340             );
 341         }
 342         if !generics.where_clause.predicates.is_empty() {
 343             gate_feature_post!(
 344                 &self,
 345                 generic_associated_types,
 346                 span,
 347                 "where clauses on associated types are unstable"
 348             );
 349         }
 350     }
 351
 352     /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
 353     fn check_impl_trait(&self, ty: &ast::Ty) {
 354         struct ImplTraitVisitor<'a> {
 355             vis: &'a PostExpansionVisitor<'a>,
 356         }
 357         impl Visitor<'_> for ImplTraitVisitor<'_> {
 358             fn visit_ty(&mut self, ty: &ast::Ty) {
 359                 if let ast::TyKind::ImplTrait(..) = ty.kind {
 360                     gate_feature_post!(
 361                         &self.vis,
 362                         type_alias_impl_trait,
 363                         ty.span,
 364                         "`impl Trait` in type aliases is unstable"
 365                     );
 366                 }
 367                 visit::walk_ty(self, ty);
 368             }
 369         }
 370         ImplTraitVisitor { vis: self }.visit_ty(ty);
 371     }
 372 }
 373
 374 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 375     fn visit_attribute(&mut self, attr: &ast::Attribute) {
 376         let attr_info = attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name));
 377         // Check feature gates for built-in attributes.
 378         if let Some(BuiltinAttribute {
 379             gate: AttributeGate::Gated(_, name, descr, has_feature),
 380             ..
 381         }) = attr_info
 382         {
 383             gate_feature_fn!(self, has_feature, attr.span, *name, descr);
 384         }
 385         // Check unstable flavors of the `#[doc]` attribute.
 386         if attr.has_name(sym::doc) {
 387             for nested_meta in attr.meta_item_list().unwrap_or_default() {
 388                 macro_rules! gate_doc { ($($name:ident => $feature:ident)*) => {
 389                     $(if nested_meta.has_name(sym::$name) {
 390                         let msg = concat!("`#[doc(", stringify!($name), ")]` is experimental");
 391                         gate_feature_post!(self, $feature, attr.span, msg);
 392                     })*
 393                 }}
 394
 395                 gate_doc!(
 396                     cfg => doc_cfg
 397                     cfg_hide => doc_cfg_hide
 398                     masked => doc_masked
 399                     notable_trait => doc_notable_trait
 400                 );
 401
 402                 if nested_meta.has_name(sym::keyword) {
 403                     let msg = "`#[doc(keyword)]` is meant for internal use only";
 404                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
 405                 }
 406
 407                 if nested_meta.has_name(sym::tuple_variadic) {
 408                     let msg = "`#[doc(tuple_variadic)]` is meant for internal use only";
 409                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
 410                 }
 411             }
 412         }
 413
 414         // Emit errors for non-staged-api crates.
 415         if !self.features.staged_api {
 416             if attr.has_name(sym::unstable)
 417                 || attr.has_name(sym::stable)
 418                 || attr.has_name(sym::rustc_const_unstable)
 419                 || attr.has_name(sym::rustc_const_stable)
 420             {
 421                 struct_span_err!(
 422                     self.sess,
 423                     attr.span,
 424                     E0734,
 425                     "stability attributes may not be used outside of the standard library",
 426                 )
 427                 .emit();
 428             }
 429         }
 430     }
 431
 432     fn visit_item(&mut self, i: &'a ast::Item) {
 433         match i.kind {
 434             ast::ItemKind::ForeignMod(ref foreign_module) => {
 435                 if let Some(abi) = foreign_module.abi {
 436                     self.check_abi(abi, ast::Const::No);
 437                 }
 438             }
 439
 440             ast::ItemKind::Fn(..) => {
 441                 if self.sess.contains_name(&i.attrs, sym::start) {
 442                     gate_feature_post!(
 443                         &self,
 444                         start,
 445                         i.span,
 446                         "`#[start]` functions are experimental \
 447                          and their signature may change \
 448                          over time"
 449                     );
 450                 }
 451             }
 452
 453             ast::ItemKind::Struct(..) => {
 454                 for attr in self.sess.filter_by_name(&i.attrs, sym::repr) {
 455                     for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
 456                         if item.has_name(sym::simd) {
 457                             gate_feature_post!(
 458                                 &self,
 459                                 repr_simd,
 460                                 attr.span,
 461                                 "SIMD types are experimental and possibly buggy"
 462                             );
 463                         }
 464                     }
 465                 }
 466             }
 467
 468             ast::ItemKind::Enum(ast::EnumDef { ref variants, .. }, ..) => {
 469                 for variant in variants {
 470                     match (&variant.data, &variant.disr_expr) {
 471                         (ast::VariantData::Unit(..), _) => {}
 472                         (_, Some(disr_expr)) => gate_feature_post!(
 473                             &self,
 474                             arbitrary_enum_discriminant,
 475                             disr_expr.value.span,
 476                             "discriminants on non-unit variants are experimental"
 477                         ),
 478                         _ => {}
 479                     }
 480                 }
 481
 482                 let has_feature = self.features.arbitrary_enum_discriminant;
 483                 if !has_feature && !i.span.allows_unstable(sym::arbitrary_enum_discriminant) {
 484                     self.maybe_report_invalid_custom_discriminants(&variants);
 485                 }
 486             }
 487
 488             ast::ItemKind::Impl(box ast::Impl { polarity, defaultness, ref of_trait, .. }) => {
 489                 if let ast::ImplPolarity::Negative(span) = polarity {
 490                     gate_feature_post!(
 491                         &self,
 492                         negative_impls,
 493                         span.to(of_trait.as_ref().map_or(span, |t| t.path.span)),
 494                         "negative trait bounds are not yet fully implemented; \
 495                          use marker types for now"
 496                     );
 497                 }
 498
 499                 if let ast::Defaultness::Default(_) = defaultness {
 500                     gate_feature_post!(&self, specialization, i.span, "specialization is unstable");
 501                 }
 502             }
 503
 504             ast::ItemKind::Trait(box ast::Trait { is_auto: ast::IsAuto::Yes, .. }) => {
 505                 gate_feature_post!(
 506                     &self,
 507                     auto_traits,
 508                     i.span,
 509                     "auto traits are experimental and possibly buggy"
 510                 );
 511             }
 512
 513             ast::ItemKind::TraitAlias(..) => {
 514                 gate_feature_post!(&self, trait_alias, i.span, "trait aliases are experimental");
 515             }
 516
 517             ast::ItemKind::MacroDef(ast::MacroDef { macro_rules: false, .. }) => {
 518                 let msg = "`macro` is experimental";
 519                 gate_feature_post!(&self, decl_macro, i.span, msg);
 520             }
 521
 522             ast::ItemKind::TyAlias(box ast::TyAlias { ty: Some(ref ty), .. }) => {
 523                 self.check_impl_trait(&ty)
 524             }
 525
 526             _ => {}
 527         }
 528
 529         visit::walk_item(self, i);
 530     }
 531
 532     fn visit_foreign_item(&mut self, i: &'a ast::ForeignItem) {
 533         match i.kind {
 534             ast::ForeignItemKind::Fn(..) | ast::ForeignItemKind::Static(..) => {
 535                 let link_name = self.sess.first_attr_value_str_by_name(&i.attrs, sym::link_name);
 536                 let links_to_llvm =
 537                     link_name.map_or(false, |val| val.as_str().starts_with("llvm."));
 538                 if links_to_llvm {
 539                     gate_feature_post!(
 540                         &self,
 541                         link_llvm_intrinsics,
 542                         i.span,
 543                         "linking to LLVM intrinsics is experimental"
 544                     );
 545                 }
 546             }
 547             ast::ForeignItemKind::TyAlias(..) => {
 548                 gate_feature_post!(&self, extern_types, i.span, "extern types are experimental");
 549             }
 550             ast::ForeignItemKind::MacCall(..) => {}
 551         }
 552
 553         visit::walk_foreign_item(self, i)
 554     }
 555
 556     fn visit_ty(&mut self, ty: &'a ast::Ty) {
 557         match ty.kind {
 558             ast::TyKind::BareFn(ref bare_fn_ty) => {
 559                 // Function pointers cannot be `const`
 560                 self.check_extern(bare_fn_ty.ext, ast::Const::No);
 561             }
 562             ast::TyKind::Never => {
 563                 gate_feature_post!(&self, never_type, ty.span, "the `!` type is experimental");
 564             }
 565             _ => {}
 566         }
 567         visit::walk_ty(self, ty)
 568     }
 569
 570     fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FnRetTy) {
 571         if let ast::FnRetTy::Ty(ref output_ty) = *ret_ty {
 572             if let ast::TyKind::Never = output_ty.kind {
 573                 // Do nothing.
 574             } else {
 575                 self.visit_ty(output_ty)
 576             }
 577         }
 578     }
 579
 580     fn visit_expr(&mut self, e: &'a ast::Expr) {
 581         match e.kind {
 582             ast::ExprKind::Box(_) => {
 583                 gate_feature_post!(
 584                     &self,
 585                     box_syntax,
 586                     e.span,
 587                     "box expression syntax is experimental; you can call `Box::new` instead"
 588                 );
 589             }
 590             ast::ExprKind::Type(..) => {
 591                 // To avoid noise about type ascription in common syntax errors, only emit if it
 592                 // is the *only* error.
 593                 if self.sess.parse_sess.span_diagnostic.err_count() == 0 {
 594                     gate_feature_post!(
 595                         &self,
 596                         type_ascription,
 597                         e.span,
 598                         "type ascription is experimental"
 599                     );
 600                 }
 601             }
 602             ast::ExprKind::TryBlock(_) => {
 603                 gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
 604             }
 605             ast::ExprKind::Block(_, Some(label)) => {
 606                 gate_feature_post!(
 607                     &self,
 608                     label_break_value,
 609                     label.ident.span,
 610                     "labels on blocks are unstable"
 611                 );
 612             }
 613             _ => {}
 614         }
 615         visit::walk_expr(self, e)
 616     }
 617
 618     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
 619         match &pattern.kind {
 620             PatKind::Slice(pats) => {
 621                 for pat in pats {
 622                     let inner_pat = match &pat.kind {
 623                         PatKind::Ident(.., Some(pat)) => pat,
 624                         _ => pat,
 625                     };
 626                     if let PatKind::Range(Some(_), None, Spanned { .. }) = inner_pat.kind {
 627                         gate_feature_post!(
 628                             &self,
 629                             half_open_range_patterns,
 630                             pat.span,
 631                             "`X..` patterns in slices are experimental"
 632                         );
 633                     }
 634                 }
 635             }
 636             PatKind::Box(..) => {
 637                 gate_feature_post!(
 638                     &self,
 639                     box_patterns,
 640                     pattern.span,
 641                     "box pattern syntax is experimental"
 642                 );
 643             }
 644             PatKind::Range(_, Some(_), Spanned { node: RangeEnd::Excluded, .. }) => {
 645                 gate_feature_post!(
 646                     &self,
 647                     exclusive_range_pattern,
 648                     pattern.span,
 649                     "exclusive range pattern syntax is experimental"
 650                 );
 651             }
 652             _ => {}
 653         }
 654         visit::walk_pat(self, pattern)
 655     }
 656
 657     fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
 658         if let Some(header) = fn_kind.header() {
 659             // Stability of const fn methods are covered in `visit_assoc_item` below.
 660             self.check_extern(header.ext, header.constness);
 661         }
 662
 663         if fn_kind.ctxt() != Some(FnCtxt::Foreign) && fn_kind.decl().c_variadic() {
 664             gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
 665         }
 666
 667         visit::walk_fn(self, fn_kind, span)
 668     }
 669
 670     fn visit_assoc_constraint(&mut self, constraint: &'a AssocConstraint) {
 671         if let AssocConstraintKind::Bound { .. } = constraint.kind {
 672             gate_feature_post!(
 673                 &self,
 674                 associated_type_bounds,
 675                 constraint.span,
 676                 "associated type bounds are unstable"
 677             )
 678         }
 679         visit::walk_assoc_constraint(self, constraint)
 680     }
 681
 682     fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
 683         let is_fn = match i.kind {
 684             ast::AssocItemKind::Fn(_) => true,
 685             ast::AssocItemKind::TyAlias(box ast::TyAlias { ref generics, ref ty, .. }) => {
 686                 if let (Some(_), AssocCtxt::Trait) = (ty, ctxt) {
 687                     gate_feature_post!(
 688                         &self,
 689                         associated_type_defaults,
 690                         i.span,
 691                         "associated type defaults are unstable"
 692                     );
 693                 }
 694                 if let Some(ty) = ty {
 695                     self.check_impl_trait(ty);
 696                 }
 697                 self.check_gat(generics, i.span);
 698                 false
 699             }
 700             _ => false,
 701         };
 702         if let ast::Defaultness::Default(_) = i.kind.defaultness() {
 703             // Limit `min_specialization` to only specializing functions.
 704             gate_feature_fn!(
 705                 &self,
 706                 |x: &Features| x.specialization || (is_fn && x.min_specialization),
 707                 i.span,
 708                 sym::specialization,
 709                 "specialization is unstable"
 710             );
 711         }
 712         visit::walk_assoc_item(self, i, ctxt)
 713     }
 714 }
 715
 716 pub fn check_crate(krate: &ast::Crate, sess: &Session) {
 717     maybe_stage_features(sess, krate);
 718     check_incompatible_features(sess);
 719     let mut visitor = PostExpansionVisitor { sess, features: &sess.features_untracked() };
 720
 721     let spans = sess.parse_sess.gated_spans.spans.borrow();
 722     macro_rules! gate_all {
 723         ($gate:ident, $msg:literal, $help:literal) => {
 724             if let Some(spans) = spans.get(&sym::$gate) {
 725                 for span in spans {
 726                     gate_feature_post!(&visitor, $gate, *span, $msg, $help);
 727                 }
 728             }
 729         };
 730         ($gate:ident, $msg:literal) => {
 731             if let Some(spans) = spans.get(&sym::$gate) {
 732                 for span in spans {
 733                     gate_feature_post!(&visitor, $gate, *span, $msg);
 734                 }
 735             }
 736         };
 737     }
 738     gate_all!(
 739         if_let_guard,
 740         "`if let` guards are experimental",
 741         "you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`"
 742     );
 743     gate_all!(let_chains, "`let` expressions in this position are unstable");
 744     gate_all!(
 745         async_closure,
 746         "async closures are unstable",
 747         "to use an async block, remove the `||`: `async {`"
 748     );
 749     gate_all!(
 750         closure_lifetime_binder,
 751         "`for<...>` binders for closures are experimental",
 752         "consider removing `for<...>`"
 753     );
 754     gate_all!(more_qualified_paths, "usage of qualified paths in this context is experimental");
 755     gate_all!(generators, "yield syntax is experimental");
 756     gate_all!(raw_ref_op, "raw address of syntax is experimental");
 757     gate_all!(const_trait_impl, "const trait impls are experimental");
 758     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
 759     gate_all!(inline_const, "inline-const is experimental");
 760     gate_all!(inline_const_pat, "inline-const in pattern position is experimental");
 761     gate_all!(associated_const_equality, "associated const equality is incomplete");
 762     gate_all!(yeet_expr, "`do yeet` expression is experimental");
 763
 764     // All uses of `gate_all!` below this point were added in #65742,
 765     // and subsequently disabled (with the non-early gating readded).
 766     macro_rules! gate_all {
 767         ($gate:ident, $msg:literal) => {
 768             // FIXME(eddyb) do something more useful than always
 769             // disabling these uses of early feature-gatings.
 770             if false {
 771                 for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
 772                     gate_feature_post!(&visitor, $gate, *span, $msg);
 773                 }
 774             }
 775         };
 776     }
 777
 778     gate_all!(trait_alias, "trait aliases are experimental");
 779     gate_all!(associated_type_bounds, "associated type bounds are unstable");
 780     gate_all!(decl_macro, "`macro` is experimental");
 781     gate_all!(box_patterns, "box pattern syntax is experimental");
 782     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
 783     gate_all!(try_blocks, "`try` blocks are unstable");
 784     gate_all!(label_break_value, "labels on blocks are unstable");
 785     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
 786     // To avoid noise about type ascription in common syntax errors,
 787     // only emit if it is the *only* error. (Also check it last.)
 788     if sess.parse_sess.span_diagnostic.err_count() == 0 {
 789         gate_all!(type_ascription, "type ascription is experimental");
 790     }
 791
 792     visit::walk_crate(&mut visitor, krate);
 793 }
 794
 795 fn maybe_stage_features(sess: &Session, krate: &ast::Crate) {
 796     // checks if `#![feature]` has been used to enable any lang feature
 797     // does not check the same for lib features unless there's at least one
 798     // declared lang feature
 799     use rustc_errors::Applicability;
 800
 801     if !sess.opts.unstable_features.is_nightly_build() {
 802         let lang_features = &sess.features_untracked().declared_lang_features;
 803         if lang_features.len() == 0 {
 804             return;
 805         }
 806         for attr in krate.attrs.iter().filter(|attr| attr.has_name(sym::feature)) {
 807             let mut err = struct_span_err!(
 808                 sess.parse_sess.span_diagnostic,
 809                 attr.span,
 810                 E0554,
 811                 "`#![feature]` may not be used on the {} release channel",
 812                 option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)")
 813             );
 814             let mut all_stable = true;
 815             for ident in
 816                 attr.meta_item_list().into_iter().flatten().flat_map(|nested| nested.ident())
 817             {
 818                 let name = ident.name;
 819                 let stable_since = lang_features
 820                     .iter()
 821                     .flat_map(|&(feature, _, since)| if feature == name { since } else { None })
 822                     .next();
 823                 if let Some(since) = stable_since {
 824                     err.help(&format!(
 825                         "the feature `{}` has been stable since {} and no longer requires \
 826                                   an attribute to enable",
 827                         name, since
 828                     ));
 829                 } else {
 830                     all_stable = false;
 831                 }
 832             }
 833             if all_stable {
 834                 err.span_suggestion(
 835                     attr.span,
 836                     "remove the attribute",
 837                     "",
 838                     Applicability::MachineApplicable,
 839                 );
 840             }
 841             err.emit();
 842         }
 843     }
 844 }
 845
 846 fn check_incompatible_features(sess: &Session) {
 847     let features = sess.features_untracked();
 848
 849     let declared_features = features
 850         .declared_lang_features
 851         .iter()
 852         .copied()
 853         .map(|(name, span, _)| (name, span))
 854         .chain(features.declared_lib_features.iter().copied());
 855
 856     for (f1, f2) in rustc_feature::INCOMPATIBLE_FEATURES
 857         .iter()
 858         .filter(|&&(f1, f2)| features.enabled(f1) && features.enabled(f2))
 859     {
 860         if let Some((f1_name, f1_span)) = declared_features.clone().find(|(name, _)| name == f1) {
 861             if let Some((f2_name, f2_span)) = declared_features.clone().find(|(name, _)| name == f2)
 862             {
 863                 let spans = vec![f1_span, f2_span];
 864                 sess.struct_span_err(
 865                     spans.clone(),
 866                     &format!(
 867                         "features `{}` and `{}` are incompatible, using them at the same time \
 868                         is not allowed",
 869                         f1_name, f2_name
 870                     ),
 871                 )
 872                 .help("remove one of these features")
 873                 .emit();
 874             }
 875         }
 876     }
 877 }