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) {
  62         let ast::StrLit { symbol_unescaped, span, .. } = abi;
  63
  64         match symbol_unescaped.as_str() {
  65             // Stable
  66             "Rust" | "C" | "cdecl" | "stdcall" | "fastcall" | "aapcs" | "win64" | "sysv64"
  67             | "system" => {}
  68             "rust-intrinsic" => {
  69                 gate_feature_post!(&self, intrinsics, span, "intrinsics are subject to change");
  70             }
  71             "platform-intrinsic" => {
  72                 gate_feature_post!(
  73                     &self,
  74                     platform_intrinsics,
  75                     span,
  76                     "platform intrinsics are experimental and possibly buggy"
  77                 );
  78             }
  79             "vectorcall" => {
  80                 gate_feature_post!(
  81                     &self,
  82                     abi_vectorcall,
  83                     span,
  84                     "vectorcall is experimental and subject to change"
  85                 );
  86             }
  87             "thiscall" => {
  88                 gate_feature_post!(
  89                     &self,
  90                     abi_thiscall,
  91                     span,
  92                     "thiscall is experimental and subject to change"
  93                 );
  94             }
  95             "rust-call" => {
  96                 gate_feature_post!(
  97                     &self,
  98                     unboxed_closures,
  99                     span,
 100                     "rust-call ABI is subject to change"
 101                 );
 102             }
 103             "ptx-kernel" => {
 104                 gate_feature_post!(
 105                     &self,
 106                     abi_ptx,
 107                     span,
 108                     "PTX ABIs are experimental and subject to change"
 109                 );
 110             }
 111             "unadjusted" => {
 112                 gate_feature_post!(
 113                     &self,
 114                     abi_unadjusted,
 115                     span,
 116                     "unadjusted ABI is an implementation detail and perma-unstable"
 117                 );
 118             }
 119             "msp430-interrupt" => {
 120                 gate_feature_post!(
 121                     &self,
 122                     abi_msp430_interrupt,
 123                     span,
 124                     "msp430-interrupt ABI is experimental and subject to change"
 125                 );
 126             }
 127             "x86-interrupt" => {
 128                 gate_feature_post!(
 129                     &self,
 130                     abi_x86_interrupt,
 131                     span,
 132                     "x86-interrupt ABI is experimental and subject to change"
 133                 );
 134             }
 135             "amdgpu-kernel" => {
 136                 gate_feature_post!(
 137                     &self,
 138                     abi_amdgpu_kernel,
 139                     span,
 140                     "amdgpu-kernel ABI is experimental and subject to change"
 141                 );
 142             }
 143             "avr-interrupt" | "avr-non-blocking-interrupt" => {
 144                 gate_feature_post!(
 145                     &self,
 146                     abi_avr_interrupt,
 147                     span,
 148                     "avr-interrupt and avr-non-blocking-interrupt ABIs are experimental and subject to change"
 149                 );
 150             }
 151             "efiapi" => {
 152                 gate_feature_post!(
 153                     &self,
 154                     abi_efiapi,
 155                     span,
 156                     "efiapi ABI is experimental and subject to change"
 157                 );
 158             }
 159             "C-cmse-nonsecure-call" => {
 160                 gate_feature_post!(
 161                     &self,
 162                     abi_c_cmse_nonsecure_call,
 163                     span,
 164                     "C-cmse-nonsecure-call ABI is experimental and subject to change"
 165                 );
 166             }
 167             "C-unwind" => {
 168                 gate_feature_post!(
 169                     &self,
 170                     c_unwind,
 171                     span,
 172                     "C-unwind ABI is experimental and subject to change"
 173                 );
 174             }
 175             "stdcall-unwind" => {
 176                 gate_feature_post!(
 177                     &self,
 178                     c_unwind,
 179                     span,
 180                     "stdcall-unwind ABI is experimental and subject to change"
 181                 );
 182             }
 183             "system-unwind" => {
 184                 gate_feature_post!(
 185                     &self,
 186                     c_unwind,
 187                     span,
 188                     "system-unwind ABI is experimental and subject to change"
 189                 );
 190             }
 191             "thiscall-unwind" => {
 192                 gate_feature_post!(
 193                     &self,
 194                     c_unwind,
 195                     span,
 196                     "thiscall-unwind ABI is experimental and subject to change"
 197                 );
 198             }
 199             "cdecl-unwind" => {
 200                 gate_feature_post!(
 201                     &self,
 202                     c_unwind,
 203                     span,
 204                     "cdecl-unwind ABI is experimental and subject to change"
 205                 );
 206             }
 207             "fastcall-unwind" => {
 208                 gate_feature_post!(
 209                     &self,
 210                     c_unwind,
 211                     span,
 212                     "fastcall-unwind ABI is experimental and subject to change"
 213                 );
 214             }
 215             "vectorcall-unwind" => {
 216                 gate_feature_post!(
 217                     &self,
 218                     c_unwind,
 219                     span,
 220                     "vectorcall-unwind ABI is experimental and subject to change"
 221                 );
 222             }
 223             "aapcs-unwind" => {
 224                 gate_feature_post!(
 225                     &self,
 226                     c_unwind,
 227                     span,
 228                     "aapcs-unwind ABI is experimental and subject to change"
 229                 );
 230             }
 231             "win64-unwind" => {
 232                 gate_feature_post!(
 233                     &self,
 234                     c_unwind,
 235                     span,
 236                     "win64-unwind ABI is experimental and subject to change"
 237                 );
 238             }
 239             "sysv64-unwind" => {
 240                 gate_feature_post!(
 241                     &self,
 242                     c_unwind,
 243                     span,
 244                     "sysv64-unwind ABI is experimental and subject to change"
 245                 );
 246             }
 247             "wasm" => {
 248                 gate_feature_post!(
 249                     &self,
 250                     wasm_abi,
 251                     span,
 252                     "wasm ABI is experimental and subject to change"
 253                 );
 254             }
 255             abi => self
 256                 .sess
 257                 .parse_sess
 258                 .span_diagnostic
 259                 .delay_span_bug(span, &format!("unrecognized ABI not caught in lowering: {}", abi)),
 260         }
 261     }
 262
 263     fn check_extern(&self, ext: ast::Extern) {
 264         if let ast::Extern::Explicit(abi) = ext {
 265             self.check_abi(abi);
 266         }
 267     }
 268
 269     fn maybe_report_invalid_custom_discriminants(&self, variants: &[ast::Variant]) {
 270         let has_fields = variants.iter().any(|variant| match variant.data {
 271             VariantData::Tuple(..) | VariantData::Struct(..) => true,
 272             VariantData::Unit(..) => false,
 273         });
 274
 275         let discriminant_spans = variants
 276             .iter()
 277             .filter(|variant| match variant.data {
 278                 VariantData::Tuple(..) | VariantData::Struct(..) => false,
 279                 VariantData::Unit(..) => true,
 280             })
 281             .filter_map(|variant| variant.disr_expr.as_ref().map(|c| c.value.span))
 282             .collect::<Vec<_>>();
 283
 284         if !discriminant_spans.is_empty() && has_fields {
 285             let mut err = feature_err(
 286                 &self.sess.parse_sess,
 287                 sym::arbitrary_enum_discriminant,
 288                 discriminant_spans.clone(),
 289                 "custom discriminant values are not allowed in enums with tuple or struct variants",
 290             );
 291             for sp in discriminant_spans {
 292                 err.span_label(sp, "disallowed custom discriminant");
 293             }
 294             for variant in variants.iter() {
 295                 match &variant.data {
 296                     VariantData::Struct(..) => {
 297                         err.span_label(variant.span, "struct variant defined here");
 298                     }
 299                     VariantData::Tuple(..) => {
 300                         err.span_label(variant.span, "tuple variant defined here");
 301                     }
 302                     VariantData::Unit(..) => {}
 303                 }
 304             }
 305             err.emit();
 306         }
 307     }
 308
 309     fn check_gat(&self, generics: &ast::Generics, span: Span) {
 310         if !generics.params.is_empty() {
 311             gate_feature_post!(
 312                 &self,
 313                 generic_associated_types,
 314                 span,
 315                 "generic associated types are unstable"
 316             );
 317         }
 318         if !generics.where_clause.predicates.is_empty() {
 319             gate_feature_post!(
 320                 &self,
 321                 generic_associated_types,
 322                 span,
 323                 "where clauses on associated types are unstable"
 324             );
 325         }
 326     }
 327
 328     /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
 329     fn check_impl_trait(&self, ty: &ast::Ty) {
 330         struct ImplTraitVisitor<'a> {
 331             vis: &'a PostExpansionVisitor<'a>,
 332         }
 333         impl Visitor<'_> for ImplTraitVisitor<'_> {
 334             fn visit_ty(&mut self, ty: &ast::Ty) {
 335                 if let ast::TyKind::ImplTrait(..) = ty.kind {
 336                     gate_feature_post!(
 337                         &self.vis,
 338                         type_alias_impl_trait,
 339                         ty.span,
 340                         "`impl Trait` in type aliases is unstable"
 341                     );
 342                 }
 343                 visit::walk_ty(self, ty);
 344             }
 345         }
 346         ImplTraitVisitor { vis: self }.visit_ty(ty);
 347     }
 348 }
 349
 350 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 351     fn visit_attribute(&mut self, attr: &ast::Attribute) {
 352         let attr_info = attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name));
 353         // Check feature gates for built-in attributes.
 354         if let Some(BuiltinAttribute {
 355             gate: AttributeGate::Gated(_, name, descr, has_feature),
 356             ..
 357         }) = attr_info
 358         {
 359             gate_feature_fn!(self, has_feature, attr.span, *name, descr);
 360         }
 361         // Check unstable flavors of the `#[doc]` attribute.
 362         if attr.has_name(sym::doc) {
 363             for nested_meta in attr.meta_item_list().unwrap_or_default() {
 364                 macro_rules! gate_doc { ($($name:ident => $feature:ident)*) => {
 365                     $(if nested_meta.has_name(sym::$name) {
 366                         let msg = concat!("`#[doc(", stringify!($name), ")]` is experimental");
 367                         gate_feature_post!(self, $feature, attr.span, msg);
 368                     })*
 369                 }}
 370
 371                 gate_doc!(
 372                     cfg => doc_cfg
 373                     cfg_hide => doc_cfg_hide
 374                     masked => doc_masked
 375                     notable_trait => doc_notable_trait
 376                 );
 377
 378                 if nested_meta.has_name(sym::keyword) {
 379                     let msg = "`#[doc(keyword)]` is meant for internal use only";
 380                     gate_feature_post!(self, rustdoc_internals, attr.span, msg);
 381                 }
 382             }
 383         }
 384
 385         // Check for unstable modifiers on `#[link(..)]` attribute
 386         if attr.has_name(sym::link) {
 387             for nested_meta in attr.meta_item_list().unwrap_or_default() {
 388                 if nested_meta.has_name(sym::modifiers) {
 389                     gate_feature_post!(
 390                         self,
 391                         native_link_modifiers,
 392                         nested_meta.span(),
 393                         "native link modifiers are experimental"
 394                     );
 395
 396                     if let Some(modifiers) = nested_meta.value_str() {
 397                         for modifier in modifiers.as_str().split(',') {
 398                             if let Some(modifier) = modifier.strip_prefix(&['+', '-']) {
 399                                 macro_rules! gate_modifier { ($($name:literal => $feature:ident)*) => {
 400                                     $(if modifier == $name {
 401                                         let msg = concat!("`#[link(modifiers=\"", $name, "\")]` is unstable");
 402                                         gate_feature_post!(
 403                                             self,
 404                                             $feature,
 405                                             nested_meta.name_value_literal_span().unwrap(),
 406                                             msg
 407                                         );
 408                                     })*
 409                                 }}
 410
 411                                 gate_modifier!(
 412                                     "bundle" => native_link_modifiers_bundle
 413                                     "verbatim" => native_link_modifiers_verbatim
 414                                     "whole-archive" => native_link_modifiers_whole_archive
 415                                     "as-needed" => native_link_modifiers_as_needed
 416                                 );
 417                             }
 418                         }
 419                     }
 420                 }
 421             }
 422         }
 423     }
 424
 425     fn visit_item(&mut self, i: &'a ast::Item) {
 426         match i.kind {
 427             ast::ItemKind::ForeignMod(ref foreign_module) => {
 428                 if let Some(abi) = foreign_module.abi {
 429                     self.check_abi(abi);
 430                 }
 431             }
 432
 433             ast::ItemKind::Fn(..) => {
 434                 if self.sess.contains_name(&i.attrs, sym::start) {
 435                     gate_feature_post!(
 436                         &self,
 437                         start,
 438                         i.span,
 439                         "`#[start]` functions are experimental \
 440                          and their signature may change \
 441                          over time"
 442                     );
 443                 }
 444             }
 445
 446             ast::ItemKind::Struct(..) => {
 447                 for attr in self.sess.filter_by_name(&i.attrs, sym::repr) {
 448                     for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
 449                         if item.has_name(sym::simd) {
 450                             gate_feature_post!(
 451                                 &self,
 452                                 repr_simd,
 453                                 attr.span,
 454                                 "SIMD types are experimental and possibly buggy"
 455                             );
 456                         }
 457                     }
 458                 }
 459             }
 460
 461             ast::ItemKind::Enum(ast::EnumDef { ref variants, .. }, ..) => {
 462                 for variant in variants {
 463                     match (&variant.data, &variant.disr_expr) {
 464                         (ast::VariantData::Unit(..), _) => {}
 465                         (_, Some(disr_expr)) => gate_feature_post!(
 466                             &self,
 467                             arbitrary_enum_discriminant,
 468                             disr_expr.value.span,
 469                             "discriminants on non-unit variants are experimental"
 470                         ),
 471                         _ => {}
 472                     }
 473                 }
 474
 475                 let has_feature = self.features.arbitrary_enum_discriminant;
 476                 if !has_feature && !i.span.allows_unstable(sym::arbitrary_enum_discriminant) {
 477                     self.maybe_report_invalid_custom_discriminants(&variants);
 478                 }
 479             }
 480
 481             ast::ItemKind::Impl(box ast::Impl { polarity, defaultness, ref of_trait, .. }) => {
 482                 if let ast::ImplPolarity::Negative(span) = polarity {
 483                     gate_feature_post!(
 484                         &self,
 485                         negative_impls,
 486                         span.to(of_trait.as_ref().map_or(span, |t| t.path.span)),
 487                         "negative trait bounds are not yet fully implemented; \
 488                          use marker types for now"
 489                     );
 490                 }
 491
 492                 if let ast::Defaultness::Default(_) = defaultness {
 493                     gate_feature_post!(&self, specialization, i.span, "specialization is unstable");
 494                 }
 495             }
 496
 497             ast::ItemKind::Trait(box ast::Trait { is_auto: ast::IsAuto::Yes, .. }) => {
 498                 gate_feature_post!(
 499                     &self,
 500                     auto_traits,
 501                     i.span,
 502                     "auto traits are experimental and possibly buggy"
 503                 );
 504             }
 505
 506             ast::ItemKind::TraitAlias(..) => {
 507                 gate_feature_post!(&self, trait_alias, i.span, "trait aliases are experimental");
 508             }
 509
 510             ast::ItemKind::MacroDef(ast::MacroDef { macro_rules: false, .. }) => {
 511                 let msg = "`macro` is experimental";
 512                 gate_feature_post!(&self, decl_macro, i.span, msg);
 513             }
 514
 515             ast::ItemKind::TyAlias(box ast::TyAlias { ty: Some(ref ty), .. }) => {
 516                 self.check_impl_trait(&ty)
 517             }
 518
 519             _ => {}
 520         }
 521
 522         visit::walk_item(self, i);
 523     }
 524
 525     fn visit_foreign_item(&mut self, i: &'a ast::ForeignItem) {
 526         match i.kind {
 527             ast::ForeignItemKind::Fn(..) | ast::ForeignItemKind::Static(..) => {
 528                 let link_name = self.sess.first_attr_value_str_by_name(&i.attrs, sym::link_name);
 529                 let links_to_llvm =
 530                     link_name.map_or(false, |val| val.as_str().starts_with("llvm."));
 531                 if links_to_llvm {
 532                     gate_feature_post!(
 533                         &self,
 534                         link_llvm_intrinsics,
 535                         i.span,
 536                         "linking to LLVM intrinsics is experimental"
 537                     );
 538                 }
 539             }
 540             ast::ForeignItemKind::TyAlias(..) => {
 541                 gate_feature_post!(&self, extern_types, i.span, "extern types are experimental");
 542             }
 543             ast::ForeignItemKind::MacCall(..) => {}
 544         }
 545
 546         visit::walk_foreign_item(self, i)
 547     }
 548
 549     fn visit_ty(&mut self, ty: &'a ast::Ty) {
 550         match ty.kind {
 551             ast::TyKind::BareFn(ref bare_fn_ty) => {
 552                 self.check_extern(bare_fn_ty.ext);
 553             }
 554             ast::TyKind::Never => {
 555                 gate_feature_post!(&self, never_type, ty.span, "the `!` type is experimental");
 556             }
 557             _ => {}
 558         }
 559         visit::walk_ty(self, ty)
 560     }
 561
 562     fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FnRetTy) {
 563         if let ast::FnRetTy::Ty(ref output_ty) = *ret_ty {
 564             if let ast::TyKind::Never = output_ty.kind {
 565                 // Do nothing.
 566             } else {
 567                 self.visit_ty(output_ty)
 568             }
 569         }
 570     }
 571
 572     fn visit_expr(&mut self, e: &'a ast::Expr) {
 573         match e.kind {
 574             ast::ExprKind::Box(_) => {
 575                 gate_feature_post!(
 576                     &self,
 577                     box_syntax,
 578                     e.span,
 579                     "box expression syntax is experimental; you can call `Box::new` instead"
 580                 );
 581             }
 582             ast::ExprKind::Type(..) => {
 583                 // To avoid noise about type ascription in common syntax errors, only emit if it
 584                 // is the *only* error.
 585                 if self.sess.parse_sess.span_diagnostic.err_count() == 0 {
 586                     gate_feature_post!(
 587                         &self,
 588                         type_ascription,
 589                         e.span,
 590                         "type ascription is experimental"
 591                     );
 592                 }
 593             }
 594             ast::ExprKind::TryBlock(_) => {
 595                 gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
 596             }
 597             ast::ExprKind::Block(_, Some(label)) => {
 598                 gate_feature_post!(
 599                     &self,
 600                     label_break_value,
 601                     label.ident.span,
 602                     "labels on blocks are unstable"
 603                 );
 604             }
 605             _ => {}
 606         }
 607         visit::walk_expr(self, e)
 608     }
 609
 610     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
 611         match &pattern.kind {
 612             PatKind::Slice(pats) => {
 613                 for pat in pats {
 614                     let inner_pat = match &pat.kind {
 615                         PatKind::Ident(.., Some(pat)) => pat,
 616                         _ => pat,
 617                     };
 618                     if let PatKind::Range(Some(_), None, Spanned { .. }) = inner_pat.kind {
 619                         gate_feature_post!(
 620                             &self,
 621                             half_open_range_patterns,
 622                             pat.span,
 623                             "`X..` patterns in slices are experimental"
 624                         );
 625                     }
 626                 }
 627             }
 628             PatKind::Box(..) => {
 629                 gate_feature_post!(
 630                     &self,
 631                     box_patterns,
 632                     pattern.span,
 633                     "box pattern syntax is experimental"
 634                 );
 635             }
 636             PatKind::Range(_, Some(_), Spanned { node: RangeEnd::Excluded, .. }) => {
 637                 gate_feature_post!(
 638                     &self,
 639                     exclusive_range_pattern,
 640                     pattern.span,
 641                     "exclusive range pattern syntax is experimental"
 642                 );
 643             }
 644             _ => {}
 645         }
 646         visit::walk_pat(self, pattern)
 647     }
 648
 649     fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
 650         if let Some(header) = fn_kind.header() {
 651             // Stability of const fn methods are covered in `visit_assoc_item` below.
 652             self.check_extern(header.ext);
 653
 654             if let (ast::Const::Yes(_), ast::Extern::Implicit)
 655             | (ast::Const::Yes(_), ast::Extern::Explicit(_)) = (header.constness, header.ext)
 656             {
 657                 gate_feature_post!(
 658                     &self,
 659                     const_extern_fn,
 660                     span,
 661                     "`const extern fn` definitions are unstable"
 662                 );
 663             }
 664         }
 665
 666         if fn_kind.ctxt() != Some(FnCtxt::Foreign) && fn_kind.decl().c_variadic() {
 667             gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
 668         }
 669
 670         visit::walk_fn(self, fn_kind, span)
 671     }
 672
 673     fn visit_assoc_constraint(&mut self, constraint: &'a AssocConstraint) {
 674         if let AssocConstraintKind::Bound { .. } = constraint.kind {
 675             gate_feature_post!(
 676                 &self,
 677                 associated_type_bounds,
 678                 constraint.span,
 679                 "associated type bounds are unstable"
 680             )
 681         }
 682         visit::walk_assoc_constraint(self, constraint)
 683     }
 684
 685     fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
 686         let is_fn = match i.kind {
 687             ast::AssocItemKind::Fn(_) => true,
 688             ast::AssocItemKind::TyAlias(box ast::TyAlias { ref generics, ref ty, .. }) => {
 689                 if let (Some(_), AssocCtxt::Trait) = (ty, ctxt) {
 690                     gate_feature_post!(
 691                         &self,
 692                         associated_type_defaults,
 693                         i.span,
 694                         "associated type defaults are unstable"
 695                     );
 696                 }
 697                 if let Some(ty) = ty {
 698                     self.check_impl_trait(ty);
 699                 }
 700                 self.check_gat(generics, i.span);
 701                 false
 702             }
 703             _ => false,
 704         };
 705         if let ast::Defaultness::Default(_) = i.kind.defaultness() {
 706             // Limit `min_specialization` to only specializing functions.
 707             gate_feature_fn!(
 708                 &self,
 709                 |x: &Features| x.specialization || (is_fn && x.min_specialization),
 710                 i.span,
 711                 sym::specialization,
 712                 "specialization is unstable"
 713             );
 714         }
 715         visit::walk_assoc_item(self, i, ctxt)
 716     }
 717
 718     fn visit_vis(&mut self, vis: &'a ast::Visibility) {
 719         if let ast::VisibilityKind::Crate(ast::CrateSugar::JustCrate) = vis.kind {
 720             gate_feature_post!(
 721                 &self,
 722                 crate_visibility_modifier,
 723                 vis.span,
 724                 "`crate` visibility modifier is experimental"
 725             );
 726         }
 727         visit::walk_vis(self, vis)
 728     }
 729 }
 730
 731 pub fn check_crate(krate: &ast::Crate, sess: &Session) {
 732     maybe_stage_features(sess, krate);
 733     check_incompatible_features(sess);
 734     let mut visitor = PostExpansionVisitor { sess, features: &sess.features_untracked() };
 735
 736     let spans = sess.parse_sess.gated_spans.spans.borrow();
 737     macro_rules! gate_all {
 738         ($gate:ident, $msg:literal, $help:literal) => {
 739             if let Some(spans) = spans.get(&sym::$gate) {
 740                 for span in spans {
 741                     gate_feature_post!(&visitor, $gate, *span, $msg, $help);
 742                 }
 743             }
 744         };
 745         ($gate:ident, $msg:literal) => {
 746             if let Some(spans) = spans.get(&sym::$gate) {
 747                 for span in spans {
 748                     gate_feature_post!(&visitor, $gate, *span, $msg);
 749                 }
 750             }
 751         };
 752     }
 753     gate_all!(
 754         if_let_guard,
 755         "`if let` guards are experimental",
 756         "you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`"
 757     );
 758     gate_all!(let_chains, "`let` expressions in this position are unstable");
 759     gate_all!(
 760         async_closure,
 761         "async closures are unstable",
 762         "to use an async block, remove the `||`: `async {`"
 763     );
 764     gate_all!(more_qualified_paths, "usage of qualified paths in this context is experimental");
 765     gate_all!(generators, "yield syntax is experimental");
 766     gate_all!(raw_ref_op, "raw address of syntax is experimental");
 767     gate_all!(const_trait_impl, "const trait impls are experimental");
 768     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
 769     gate_all!(inline_const, "inline-const is experimental");
 770     gate_all!(inline_const_pat, "inline-const in pattern position is experimental");
 771     gate_all!(associated_const_equality, "associated const equality is incomplete");
 772
 773     // All uses of `gate_all!` below this point were added in #65742,
 774     // and subsequently disabled (with the non-early gating readded).
 775     macro_rules! gate_all {
 776         ($gate:ident, $msg:literal) => {
 777             // FIXME(eddyb) do something more useful than always
 778             // disabling these uses of early feature-gatings.
 779             if false {
 780                 for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
 781                     gate_feature_post!(&visitor, $gate, *span, $msg);
 782                 }
 783             }
 784         };
 785     }
 786
 787     gate_all!(trait_alias, "trait aliases are experimental");
 788     gate_all!(associated_type_bounds, "associated type bounds are unstable");
 789     gate_all!(crate_visibility_modifier, "`crate` visibility modifier is experimental");
 790     gate_all!(decl_macro, "`macro` is experimental");
 791     gate_all!(box_patterns, "box pattern syntax is experimental");
 792     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
 793     gate_all!(try_blocks, "`try` blocks are unstable");
 794     gate_all!(label_break_value, "labels on blocks are unstable");
 795     gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
 796     // To avoid noise about type ascription in common syntax errors,
 797     // only emit if it is the *only* error. (Also check it last.)
 798     if sess.parse_sess.span_diagnostic.err_count() == 0 {
 799         gate_all!(type_ascription, "type ascription is experimental");
 800     }
 801
 802     visit::walk_crate(&mut visitor, krate);
 803 }
 804
 805 fn maybe_stage_features(sess: &Session, krate: &ast::Crate) {
 806     // checks if `#![feature]` has been used to enable any lang feature
 807     // does not check the same for lib features unless there's at least one
 808     // declared lang feature
 809     use rustc_errors::Applicability;
 810
 811     if !sess.opts.unstable_features.is_nightly_build() {
 812         let lang_features = &sess.features_untracked().declared_lang_features;
 813         if lang_features.len() == 0 {
 814             return;
 815         }
 816         for attr in krate.attrs.iter().filter(|attr| attr.has_name(sym::feature)) {
 817             let mut err = struct_span_err!(
 818                 sess.parse_sess.span_diagnostic,
 819                 attr.span,
 820                 E0554,
 821                 "`#![feature]` may not be used on the {} release channel",
 822                 option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)")
 823             );
 824             let mut all_stable = true;
 825             for ident in
 826                 attr.meta_item_list().into_iter().flatten().flat_map(|nested| nested.ident())
 827             {
 828                 let name = ident.name;
 829                 let stable_since = lang_features
 830                     .iter()
 831                     .flat_map(|&(feature, _, since)| if feature == name { since } else { None })
 832                     .next();
 833                 if let Some(since) = stable_since {
 834                     err.help(&format!(
 835                         "the feature `{}` has been stable since {} and no longer requires \
 836                                   an attribute to enable",
 837                         name, since
 838                     ));
 839                 } else {
 840                     all_stable = false;
 841                 }
 842             }
 843             if all_stable {
 844                 err.span_suggestion(
 845                     attr.span,
 846                     "remove the attribute",
 847                     String::new(),
 848                     Applicability::MachineApplicable,
 849                 );
 850             }
 851             err.emit();
 852         }
 853     }
 854 }
 855
 856 fn check_incompatible_features(sess: &Session) {
 857     let features = sess.features_untracked();
 858
 859     let declared_features = features
 860         .declared_lang_features
 861         .iter()
 862         .copied()
 863         .map(|(name, span, _)| (name, span))
 864         .chain(features.declared_lib_features.iter().copied());
 865
 866     for (f1, f2) in rustc_feature::INCOMPATIBLE_FEATURES
 867         .iter()
 868         .filter(|&&(f1, f2)| features.enabled(f1) && features.enabled(f2))
 869     {
 870         if let Some((f1_name, f1_span)) = declared_features.clone().find(|(name, _)| name == f1) {
 871             if let Some((f2_name, f2_span)) = declared_features.clone().find(|(name, _)| name == f2)
 872             {
 873                 let spans = vec![f1_span, f2_span];
 874                 sess.struct_span_err(
 875                     spans.clone(),
 876                     &format!(
 877                         "features `{}` and `{}` are incompatible, using them at the same time \
 878                         is not allowed",
 879                         f1_name, f2_name
 880                     ),
 881                 )
 882                 .help("remove one of these features")
 883                 .emit();
 884             }
 885         }
 886     }
 887 }