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                 self.sess.parse_sess.span_diagnostic.delay_span_bug(
 278                     span,
 279                     &format!("unrecognized ABI not caught in lowering: {}", abi),
 280                 );
 281             }
 282         }
 283     }
 284
 285     fn check_extern(&self, ext: ast::Extern, constness: ast::Const) {
 286         if let ast::Extern::Explicit(abi) = ext {
 287             self.check_abi(abi, constness);
 288         }
 289     }
 290
 291     fn maybe_report_invalid_custom_discriminants(&self, variants: &[ast::Variant]) {
 292         let has_fields = variants.iter().any(|variant| match variant.data {
 293             VariantData::Tuple(..) | VariantData::Struct(..) => true,
 294             VariantData::Unit(..) => false,
 295         });
 296
 297         let discriminant_spans = variants
 298             .iter()
 299             .filter(|variant| match variant.data {
 300                 VariantData::Tuple(..) | VariantData::Struct(..) => false,
 301                 VariantData::Unit(..) => true,
 302             })
 303             .filter_map(|variant| variant.disr_expr.as_ref().map(|c| c.value.span))
 304             .collect::<Vec<_>>();
 305
 306         if !discriminant_spans.is_empty() && has_fields {
 307             let mut err = feature_err(
 308                 &self.sess.parse_sess,
 309                 sym::arbitrary_enum_discriminant,
 310                 discriminant_spans.clone(),
 311                 "custom discriminant values are not allowed in enums with tuple or struct variants",
 312             );
 313             for sp in discriminant_spans {
 314                 err.span_label(sp, "disallowed custom discriminant");
 315             }
 316             for variant in variants.iter() {
 317                 match &variant.data {
 318                     VariantData::Struct(..) => {
 319                         err.span_label(variant.span, "struct variant defined here");
 320                     }
 321                     VariantData::Tuple(..) => {
 322                         err.span_label(variant.span, "tuple variant defined here");
 323                     }
 324                     VariantData::Unit(..) => {}
 325                 }
 326             }
 327             err.emit();
 328         }
 329     }
 330
 331     fn check_gat(&self, generics: &ast::Generics, span: Span) {
 332         if !generics.params.is_empty() {
 333             gate_feature_post!(
 334                 &self,
 335                 generic_associated_types,
 336                 span,
 337                 "generic associated types are unstable"
 338             );
 339         }
 340         if !generics.where_clause.predicates.is_empty() {
 341             gate_feature_post!(
 342                 &self,
 343                 generic_associated_types,
 344                 span,
 345                 "where clauses on associated types are unstable"
 346             );
 347         }
 348     }
 349
 350     /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
 351     fn check_impl_trait(&self, ty: &ast::Ty) {
 352         struct ImplTraitVisitor<'a> {
 353             vis: &'a PostExpansionVisitor<'a>,
 354         }
 355         impl Visitor<'_> for ImplTraitVisitor<'_> {
 356             fn visit_ty(&mut self, ty: &ast::Ty) {
 357                 if let ast::TyKind::ImplTrait(..) = ty.kind {
 358                     gate_feature_post!(
 359                         &self.vis,
 360                         type_alias_impl_trait,
 361                         ty.span,
 362                         "`impl Trait` in type aliases is unstable"
 363                     );
 364                 }
 365                 visit::walk_ty(self, ty);
 366             }
 367         }
 368         ImplTraitVisitor { vis: self }.visit_ty(ty);
 369     }
 370 }
 371
 372 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 373     fn visit_attribute(&mut self, attr: &ast::Attribute) {
 374         let attr_info = attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name));
 375         // Check feature gates for built-in attributes.
 376         if let Some(BuiltinAttribute {
 377             gate: AttributeGate::Gated(_, name, descr, has_feature),
 378             ..
 379         }) = attr_info
 380         {
 381             gate_feature_fn!(self, has_feature, attr.span, *name, descr);
 382         }
 383         // Check unstable flavors of the `#[doc]` attribute.
 384         if attr.has_name(sym::doc) {
 385             for nested_meta in attr.meta_item_list().unwrap_or_default() {
 386                 macro_rules! gate_doc { ($($name:ident => $feature:ident)*) => {
 387                     $(if nested_meta.has_name(sym::$name) {
 388                         let msg = concat!("`#[doc(", stringify!($name), ")]` is experimental");
 389                         gate_feature_post!(self, $feature, attr.span, msg);
 390                     })*
 391                 }}
 392
 393                 gate_doc!(
 394                     cfg => doc_cfg
 395                     cfg_hide => doc_cfg_hide
 396                     masked => doc_masked
 397                     notable_trait => doc_notable_trait
 398                 );
 399
 400                 if nested_meta.has_name(sym::keyword) {
 401                     let msg = "`#[doc(keyword)]` is meant for internal use only";
 402                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
 403                 }
 404
 405                 if nested_meta.has_name(sym::tuple_variadic) {
 406                     let msg = "`#[doc(tuple_variadic)]` is meant for internal use only";
 407                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
 408                 }
 409             }
 410         }
 411
 412         // Emit errors for non-staged-api crates.
 413         if !self.features.staged_api {
 414             if attr.has_name(sym::unstable)
 415                 || attr.has_name(sym::stable)
 416                 || attr.has_name(sym::rustc_const_unstable)
 417                 || attr.has_name(sym::rustc_const_stable)
 418             {
 419                 struct_span_err!(
 420                     self.sess,
 421                     attr.span,
 422                     E0734,
 423                     "stability attributes may not be used outside of the standard library",
 424                 )
 425                 .emit();
 426             }
 427         }
 428     }
 429
 430     fn visit_item(&mut self, i: &'a ast::Item) {
 431         match i.kind {
 432             ast::ItemKind::ForeignMod(ref foreign_module) => {
 433                 if let Some(abi) = foreign_module.abi {
 434                     self.check_abi(abi, ast::Const::No);
 435                 }
 436             }
 437
 438             ast::ItemKind::Fn(..) => {
 439                 if self.sess.contains_name(&i.attrs, sym::start) {
 440                     gate_feature_post!(
 441                         &self,
 442                         start,
 443                         i.span,
 444                         "`#[start]` functions are experimental \
 445                          and their signature may change \
 446                          over time"
 447                     );
 448                 }
 449             }
 450
 451             ast::ItemKind::Struct(..) => {
 452                 for attr in self.sess.filter_by_name(&i.attrs, sym::repr) {
 453                     for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
 454                         if item.has_name(sym::simd) {
 455                             gate_feature_post!(
 456                                 &self,
 457                                 repr_simd,
 458                                 attr.span,
 459                                 "SIMD types are experimental and possibly buggy"
 460                             );
 461                         }
 462                     }
 463                 }
 464             }
 465
 466             ast::ItemKind::Enum(ast::EnumDef { ref variants, .. }, ..) => {
 467                 for variant in variants {
 468                     match (&variant.data, &variant.disr_expr) {
 469                         (ast::VariantData::Unit(..), _) => {}
 470                         (_, Some(disr_expr)) => gate_feature_post!(
 471                             &self,
 472                             arbitrary_enum_discriminant,
 473                             disr_expr.value.span,
 474                             "discriminants on non-unit variants are experimental"
 475                         ),
 476                         _ => {}
 477                     }
 478                 }
 479
 480                 let has_feature = self.features.arbitrary_enum_discriminant;
 481                 if !has_feature && !i.span.allows_unstable(sym::arbitrary_enum_discriminant) {
 482                     self.maybe_report_invalid_custom_discriminants(&variants);
 483                 }
 484             }
 485
 486             ast::ItemKind::Impl(box ast::Impl { polarity, defaultness, ref of_trait, .. }) => {
 487                 if let ast::ImplPolarity::Negative(span) = polarity {
 488                     gate_feature_post!(
 489                         &self,
 490                         negative_impls,
 491                         span.to(of_trait.as_ref().map_or(span, |t| t.path.span)),
 492                         "negative trait bounds are not yet fully implemented; \
 493                          use marker types for now"
 494                     );
 495                 }
 496
 497                 if let ast::Defaultness::Default(_) = defaultness {
 498                     gate_feature_post!(&self, specialization, i.span, "specialization is unstable");
 499                 }
 500             }
 501
 502             ast::ItemKind::Trait(box ast::Trait { is_auto: ast::IsAuto::Yes, .. }) => {
 503                 gate_feature_post!(
 504                     &self,
 505                     auto_traits,
 506                     i.span,
 507                     "auto traits are experimental and possibly buggy"
 508                 );
 509             }
 510
 511             ast::ItemKind::TraitAlias(..) => {
 512                 gate_feature_post!(&self, trait_alias, i.span, "trait aliases are experimental");
 513             }
 514
 515             ast::ItemKind::MacroDef(ast::MacroDef { macro_rules: false, .. }) => {
 516                 let msg = "`macro` is experimental";
 517                 gate_feature_post!(&self, decl_macro, i.span, msg);
 518             }
 519
 520             ast::ItemKind::TyAlias(box ast::TyAlias { ty: Some(ref ty), .. }) => {
 521                 self.check_impl_trait(&ty)
 522             }
 523
 524             _ => {}
 525         }
 526
 527         visit::walk_item(self, i);
 528     }
 529
 530     fn visit_foreign_item(&mut self, i: &'a ast::ForeignItem) {
 531         match i.kind {
 532             ast::ForeignItemKind::Fn(..) | ast::ForeignItemKind::Static(..) => {
 533                 let link_name = self.sess.first_attr_value_str_by_name(&i.attrs, sym::link_name);
 534                 let links_to_llvm =
 535                     link_name.map_or(false, |val| val.as_str().starts_with("llvm."));
 536                 if links_to_llvm {
 537                     gate_feature_post!(
 538                         &self,
 539                         link_llvm_intrinsics,
 540                         i.span,
 541                         "linking to LLVM intrinsics is experimental"
 542                     );
 543                 }
 544             }
 545             ast::ForeignItemKind::TyAlias(..) => {
 546                 gate_feature_post!(&self, extern_types, i.span, "extern types are experimental");
 547             }
 548             ast::ForeignItemKind::MacCall(..) => {}
 549         }
 550
 551         visit::walk_foreign_item(self, i)
 552     }
 553
 554     fn visit_ty(&mut self, ty: &'a ast::Ty) {
 555         match ty.kind {
 556             ast::TyKind::BareFn(ref bare_fn_ty) => {
 557                 // Function pointers cannot be `const`
 558                 self.check_extern(bare_fn_ty.ext, ast::Const::No);
 559             }
 560             ast::TyKind::Never => {
 561                 gate_feature_post!(&self, never_type, ty.span, "the `!` type is experimental");
 562             }
 563             _ => {}
 564         }
 565         visit::walk_ty(self, ty)
 566     }
 567
 568     fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FnRetTy) {
 569         if let ast::FnRetTy::Ty(ref output_ty) = *ret_ty {
 570             if let ast::TyKind::Never = output_ty.kind {
 571                 // Do nothing.
 572             } else {
 573                 self.visit_ty(output_ty)
 574             }
 575         }
 576     }
 577
 578     fn visit_expr(&mut self, e: &'a ast::Expr) {
 579         match e.kind {
 580             ast::ExprKind::Box(_) => {
 581                 gate_feature_post!(
 582                     &self,
 583                     box_syntax,
 584                     e.span,
 585                     "box expression syntax is experimental; you can call `Box::new` instead"
 586                 );
 587             }
 588             ast::ExprKind::Type(..) => {
 589                 // To avoid noise about type ascription in common syntax errors, only emit if it
 590                 // is the *only* error.
 591                 if self.sess.parse_sess.span_diagnostic.err_count() == 0 {
 592                     gate_feature_post!(
 593                         &self,
 594                         type_ascription,
 595                         e.span,
 596                         "type ascription is experimental"
 597                     );
 598                 }
 599             }
 600             ast::ExprKind::TryBlock(_) => {
 601                 gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
 602             }
 603             ast::ExprKind::Block(_, Some(label)) => {
 604                 gate_feature_post!(
 605                     &self,
 606                     label_break_value,
 607                     label.ident.span,
 608                     "labels on blocks are unstable"
 609                 );
 610             }
 611             _ => {}
 612         }
 613         visit::walk_expr(self, e)
 614     }
 615
 616     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
 617         match &pattern.kind {
 618             PatKind::Slice(pats) => {
 619                 for pat in pats {
 620                     let inner_pat = match &pat.kind {
 621                         PatKind::Ident(.., Some(pat)) => pat,
 622                         _ => pat,
 623                     };
 624                     if let PatKind::Range(Some(_), None, Spanned { .. }) = inner_pat.kind {
 625                         gate_feature_post!(
 626                             &self,
 627                             half_open_range_patterns,
 628                             pat.span,
 629                             "`X..` patterns in slices are experimental"
 630                         );
 631                     }
 632                 }
 633             }
 634             PatKind::Box(..) => {
 635                 gate_feature_post!(
 636                     &self,
 637                     box_patterns,
 638                     pattern.span,
 639                     "box pattern syntax is experimental"
 640                 );
 641             }
 642             PatKind::Range(_, Some(_), Spanned { node: RangeEnd::Excluded, .. }) => {
 643                 gate_feature_post!(
 644                     &self,
 645                     exclusive_range_pattern,
 646                     pattern.span,
 647                     "exclusive range pattern syntax is experimental"
 648                 );
 649             }
 650             _ => {}
 651         }
 652         visit::walk_pat(self, pattern)
 653     }
 654
 655     fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
 656         if let Some(header) = fn_kind.header() {
 657             // Stability of const fn methods are covered in `visit_assoc_item` below.
 658             self.check_extern(header.ext, header.constness);
 659         }
 660
 661         if fn_kind.ctxt() != Some(FnCtxt::Foreign) && fn_kind.decl().c_variadic() {
 662             gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
 663         }
 664
 665         visit::walk_fn(self, fn_kind, span)
 666     }
 667
 668     fn visit_assoc_constraint(&mut self, constraint: &'a AssocConstraint) {
 669         if let AssocConstraintKind::Bound { .. } = constraint.kind {
 670             gate_feature_post!(
 671                 &self,
 672                 associated_type_bounds,
 673                 constraint.span,
 674                 "associated type bounds are unstable"
 675             )
 676         }
 677         visit::walk_assoc_constraint(self, constraint)
 678     }
 679
 680     fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
 681         let is_fn = match i.kind {
 682             ast::AssocItemKind::Fn(_) => true,
 683             ast::AssocItemKind::TyAlias(box ast::TyAlias { ref generics, ref ty, .. }) => {
 684                 if let (Some(_), AssocCtxt::Trait) = (ty, ctxt) {
 685                     gate_feature_post!(
 686                         &self,
 687                         associated_type_defaults,
 688                         i.span,
 689                         "associated type defaults are unstable"
 690                     );
 691                 }
 692                 if let Some(ty) = ty {
 693                     self.check_impl_trait(ty);
 694                 }
 695                 self.check_gat(generics, i.span);
 696                 false
 697             }
 698             _ => false,
 699         };
 700         if let ast::Defaultness::Default(_) = i.kind.defaultness() {
 701             // Limit `min_specialization` to only specializing functions.
 702             gate_feature_fn!(
 703                 &self,
 704                 |x: &Features| x.specialization || (is_fn && x.min_specialization),
 705                 i.span,
 706                 sym::specialization,
 707                 "specialization is unstable"
 708             );
 709         }
 710         visit::walk_assoc_item(self, i, ctxt)
 711     }
 712 }
 713
 714 pub fn check_crate(krate: &ast::Crate, sess: &Session) {
 715     maybe_stage_features(sess, krate);
 716     check_incompatible_features(sess);
 717     let mut visitor = PostExpansionVisitor { sess, features: &sess.features_untracked() };
 718
 719     let spans = sess.parse_sess.gated_spans.spans.borrow();
 720     macro_rules! gate_all {
 721         ($gate:ident, $msg:literal, $help:literal) => {
 722             if let Some(spans) = spans.get(&sym::$gate) {
 723                 for span in spans {
 724                     gate_feature_post!(&visitor, $gate, *span, $msg, $help);
 725                 }
 726             }
 727         };
 728         ($gate:ident, $msg:literal) => {
 729             if let Some(spans) = spans.get(&sym::$gate) {
 730                 for span in spans {
 731                     gate_feature_post!(&visitor, $gate, *span, $msg);
 732                 }
 733             }
 734         };
 735     }
 736     gate_all!(
 737         if_let_guard,
 738         "`if let` guards are experimental",
 739         "you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`"
 740     );
 741     gate_all!(let_chains, "`let` expressions in this position are unstable");
 742     gate_all!(
 743         async_closure,
 744         "async closures are unstable",
 745         "to use an async block, remove the `||`: `async {`"
 746     );
 747     gate_all!(more_qualified_paths, "usage of qualified paths in this context is experimental");
 748     gate_all!(generators, "yield syntax is experimental");
 749     gate_all!(raw_ref_op, "raw address of syntax is experimental");
 750     gate_all!(const_trait_impl, "const trait impls are experimental");
 751     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
 752     gate_all!(inline_const, "inline-const is experimental");
 753     gate_all!(inline_const_pat, "inline-const in pattern position is experimental");
 754     gate_all!(associated_const_equality, "associated const equality is incomplete");
 755     gate_all!(yeet_expr, "`do yeet` expression is experimental");
 756
 757     // All uses of `gate_all!` below this point were added in #65742,
 758     // and subsequently disabled (with the non-early gating readded).
 759     macro_rules! gate_all {
 760         ($gate:ident, $msg:literal) => {
 761             // FIXME(eddyb) do something more useful than always
 762             // disabling these uses of early feature-gatings.
 763             if false {
 764                 for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
 765                     gate_feature_post!(&visitor, $gate, *span, $msg);
 766                 }
 767             }
 768         };
 769     }
 770
 771     gate_all!(trait_alias, "trait aliases are experimental");
 772     gate_all!(associated_type_bounds, "associated type bounds are unstable");
 773     gate_all!(decl_macro, "`macro` is experimental");
 774     gate_all!(box_patterns, "box pattern syntax is experimental");
 775     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
 776     gate_all!(try_blocks, "`try` blocks are unstable");
 777     gate_all!(label_break_value, "labels on blocks are unstable");
 778     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
 779     // To avoid noise about type ascription in common syntax errors,
 780     // only emit if it is the *only* error. (Also check it last.)
 781     if sess.parse_sess.span_diagnostic.err_count() == 0 {
 782         gate_all!(type_ascription, "type ascription is experimental");
 783     }
 784
 785     visit::walk_crate(&mut visitor, krate);
 786 }
 787
 788 fn maybe_stage_features(sess: &Session, krate: &ast::Crate) {
 789     // checks if `#![feature]` has been used to enable any lang feature
 790     // does not check the same for lib features unless there's at least one
 791     // declared lang feature
 792     use rustc_errors::Applicability;
 793
 794     if !sess.opts.unstable_features.is_nightly_build() {
 795         let lang_features = &sess.features_untracked().declared_lang_features;
 796         if lang_features.len() == 0 {
 797             return;
 798         }
 799         for attr in krate.attrs.iter().filter(|attr| attr.has_name(sym::feature)) {
 800             let mut err = struct_span_err!(
 801                 sess.parse_sess.span_diagnostic,
 802                 attr.span,
 803                 E0554,
 804                 "`#![feature]` may not be used on the {} release channel",
 805                 option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)")
 806             );
 807             let mut all_stable = true;
 808             for ident in
 809                 attr.meta_item_list().into_iter().flatten().flat_map(|nested| nested.ident())
 810             {
 811                 let name = ident.name;
 812                 let stable_since = lang_features
 813                     .iter()
 814                     .flat_map(|&(feature, _, since)| if feature == name { since } else { None })
 815                     .next();
 816                 if let Some(since) = stable_since {
 817                     err.help(&format!(
 818                         "the feature `{}` has been stable since {} and no longer requires \
 819                                   an attribute to enable",
 820                         name, since
 821                     ));
 822                 } else {
 823                     all_stable = false;
 824                 }
 825             }
 826             if all_stable {
 827                 err.span_suggestion(
 828                     attr.span,
 829                     "remove the attribute",
 830                     "",
 831                     Applicability::MachineApplicable,
 832                 );
 833             }
 834             err.emit();
 835         }
 836     }
 837 }
 838
 839 fn check_incompatible_features(sess: &Session) {
 840     let features = sess.features_untracked();
 841
 842     let declared_features = features
 843         .declared_lang_features
 844         .iter()
 845         .copied()
 846         .map(|(name, span, _)| (name, span))
 847         .chain(features.declared_lib_features.iter().copied());
 848
 849     for (f1, f2) in rustc_feature::INCOMPATIBLE_FEATURES
 850         .iter()
 851         .filter(|&&(f1, f2)| features.enabled(f1) && features.enabled(f2))
 852     {
 853         if let Some((f1_name, f1_span)) = declared_features.clone().find(|(name, _)| name == f1) {
 854             if let Some((f2_name, f2_span)) = declared_features.clone().find(|(name, _)| name == f2)
 855             {
 856                 let spans = vec![f1_span, f2_span];
 857                 sess.struct_span_err(
 858                     spans.clone(),
 859                     &format!(
 860                         "features `{}` and `{}` are incompatible, using them at the same time \
 861                         is not allowed",
 862                         f1_name, f2_name
 863                     ),
 864                 )
 865                 .help("remove one of these features")
 866                 .emit();
 867             }
 868         }
 869     }
 870 }