src/librustc_ast_passes/ast_validation.rs

   1 // Validate AST before lowering it to HIR.
   2 //
   3 // This pass is supposed to catch things that fit into AST data structures,
   4 // but not permitted by the language. It runs after expansion when AST is frozen,
   5 // so it can check for erroneous constructions produced by syntax extensions.
   6 // This pass is supposed to perform only simple checks not requiring name resolution
   7 // or type checking or some other kind of complex analysis.
   8
   9 use itertools::{Either, Itertools};
  10 use rustc_ast::ast::*;
  11 use rustc_ast::attr;
  12 use rustc_ast::expand::is_proc_macro_attr;
  13 use rustc_ast::ptr::P;
  14 use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
  15 use rustc_ast::walk_list;
  16 use rustc_ast_pretty::pprust;
  17 use rustc_data_structures::fx::FxHashMap;
  18 use rustc_errors::{error_code, pluralize, struct_span_err, Applicability};
  19 use rustc_parse::validate_attr;
  20 use rustc_session::lint::builtin::PATTERNS_IN_FNS_WITHOUT_BODY;
  21 use rustc_session::lint::LintBuffer;
  22 use rustc_session::Session;
  23 use rustc_span::symbol::{kw, sym};
  24 use rustc_span::Span;
  25 use std::mem;
  26 use std::ops::DerefMut;
  27
  28 const MORE_EXTERN: &str =
  29     "for more information, visit https://doc.rust-lang.org/std/keyword.extern.html";
  30
  31 /// Is `self` allowed semantically as the first parameter in an `FnDecl`?
  32 enum SelfSemantic {
  33     Yes,
  34     No,
  35 }
  36
  37 /// A syntactic context that disallows certain kinds of bounds (e.g., `?Trait` or `?const Trait`).
  38 #[derive(Clone, Copy)]
  39 enum BoundContext {
  40     ImplTrait,
  41     TraitBounds,
  42     TraitObject,
  43 }
  44
  45 impl BoundContext {
  46     fn description(&self) -> &'static str {
  47         match self {
  48             Self::ImplTrait => "`impl Trait`",
  49             Self::TraitBounds => "supertraits",
  50             Self::TraitObject => "trait objects",
  51         }
  52     }
  53 }
  54
  55 struct AstValidator<'a> {
  56     session: &'a Session,
  57
  58     /// The span of the `extern` in an `extern { ... }` block, if any.
  59     extern_mod: Option<&'a Item>,
  60
  61     /// Are we inside a trait impl?
  62     in_trait_impl: bool,
  63
  64     has_proc_macro_decls: bool,
  65
  66     /// Used to ban nested `impl Trait`, e.g., `impl Into<impl Debug>`.
  67     /// Nested `impl Trait` _is_ allowed in associated type position,
  68     /// e.g., `impl Iterator<Item = impl Debug>`.
  69     outer_impl_trait: Option<Span>,
  70
  71     /// Keeps track of the `BoundContext` as we recurse.
  72     ///
  73     /// This is used to forbid `?const Trait` bounds in, e.g.,
  74     /// `impl Iterator<Item = Box<dyn ?const Trait>`.
  75     bound_context: Option<BoundContext>,
  76
  77     /// Used to ban `impl Trait` in path projections like `<impl Iterator>::Item`
  78     /// or `Foo::Bar<impl Trait>`
  79     is_impl_trait_banned: bool,
  80
  81     /// Used to ban associated type bounds (i.e., `Type<AssocType: Bounds>`) in
  82     /// certain positions.
  83     is_assoc_ty_bound_banned: bool,
  84
  85     lint_buffer: &'a mut LintBuffer,
  86 }
  87
  88 impl<'a> AstValidator<'a> {
  89     fn with_in_trait_impl(&mut self, is_in: bool, f: impl FnOnce(&mut Self)) {
  90         let old = mem::replace(&mut self.in_trait_impl, is_in);
  91         f(self);
  92         self.in_trait_impl = old;
  93     }
  94
  95     fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
  96         let old = mem::replace(&mut self.is_impl_trait_banned, true);
  97         f(self);
  98         self.is_impl_trait_banned = old;
  99     }
 100
 101     fn with_banned_assoc_ty_bound(&mut self, f: impl FnOnce(&mut Self)) {
 102         let old = mem::replace(&mut self.is_assoc_ty_bound_banned, true);
 103         f(self);
 104         self.is_assoc_ty_bound_banned = old;
 105     }
 106
 107     fn with_impl_trait(&mut self, outer: Option<Span>, f: impl FnOnce(&mut Self)) {
 108         let old = mem::replace(&mut self.outer_impl_trait, outer);
 109         if outer.is_some() {
 110             self.with_bound_context(BoundContext::ImplTrait, |this| f(this));
 111         } else {
 112             f(self)
 113         }
 114         self.outer_impl_trait = old;
 115     }
 116
 117     fn with_bound_context(&mut self, ctx: BoundContext, f: impl FnOnce(&mut Self)) {
 118         let old = self.bound_context.replace(ctx);
 119         f(self);
 120         self.bound_context = old;
 121     }
 122
 123     fn visit_assoc_ty_constraint_from_generic_args(&mut self, constraint: &'a AssocTyConstraint) {
 124         match constraint.kind {
 125             AssocTyConstraintKind::Equality { .. } => {}
 126             AssocTyConstraintKind::Bound { .. } => {
 127                 if self.is_assoc_ty_bound_banned {
 128                     self.err_handler().span_err(
 129                         constraint.span,
 130                         "associated type bounds are not allowed within structs, enums, or unions",
 131                     );
 132                 }
 133             }
 134         }
 135         self.visit_assoc_ty_constraint(constraint);
 136     }
 137
 138     // Mirrors `visit::walk_ty`, but tracks relevant state.
 139     fn walk_ty(&mut self, t: &'a Ty) {
 140         match t.kind {
 141             TyKind::ImplTrait(..) => {
 142                 self.with_impl_trait(Some(t.span), |this| visit::walk_ty(this, t))
 143             }
 144             TyKind::TraitObject(..) => {
 145                 self.with_bound_context(BoundContext::TraitObject, |this| visit::walk_ty(this, t));
 146             }
 147             TyKind::Path(ref qself, ref path) => {
 148                 // We allow these:
 149                 //  - `Option<impl Trait>`
 150                 //  - `option::Option<impl Trait>`
 151                 //  - `option::Option<T>::Foo<impl Trait>
 152                 //
 153                 // But not these:
 154                 //  - `<impl Trait>::Foo`
 155                 //  - `option::Option<impl Trait>::Foo`.
 156                 //
 157                 // To implement this, we disallow `impl Trait` from `qself`
 158                 // (for cases like `<impl Trait>::Foo>`)
 159                 // but we allow `impl Trait` in `GenericArgs`
 160                 // iff there are no more PathSegments.
 161                 if let Some(ref qself) = *qself {
 162                     // `impl Trait` in `qself` is always illegal
 163                     self.with_banned_impl_trait(|this| this.visit_ty(&qself.ty));
 164                 }
 165
 166                 // Note that there should be a call to visit_path here,
 167                 // so if any logic is added to process `Path`s a call to it should be
 168                 // added both in visit_path and here. This code mirrors visit::walk_path.
 169                 for (i, segment) in path.segments.iter().enumerate() {
 170                     // Allow `impl Trait` iff we're on the final path segment
 171                     if i == path.segments.len() - 1 {
 172                         self.visit_path_segment(path.span, segment);
 173                     } else {
 174                         self.with_banned_impl_trait(|this| {
 175                             this.visit_path_segment(path.span, segment)
 176                         });
 177                     }
 178                 }
 179             }
 180             _ => visit::walk_ty(self, t),
 181         }
 182     }
 183
 184     fn err_handler(&self) -> &rustc_errors::Handler {
 185         &self.session.diagnostic()
 186     }
 187
 188     fn check_lifetime(&self, ident: Ident) {
 189         let valid_names = [kw::UnderscoreLifetime, kw::StaticLifetime, kw::Invalid];
 190         if !valid_names.contains(&ident.name) && ident.without_first_quote().is_reserved() {
 191             self.err_handler().span_err(ident.span, "lifetimes cannot use keyword names");
 192         }
 193     }
 194
 195     fn check_label(&self, ident: Ident) {
 196         if ident.without_first_quote().is_reserved() {
 197             self.err_handler()
 198                 .span_err(ident.span, &format!("invalid label name `{}`", ident.name));
 199         }
 200     }
 201
 202     fn invalid_visibility(&self, vis: &Visibility, note: Option<&str>) {
 203         if let VisibilityKind::Inherited = vis.node {
 204             return;
 205         }
 206
 207         let mut err =
 208             struct_span_err!(self.session, vis.span, E0449, "unnecessary visibility qualifier");
 209         if vis.node.is_pub() {
 210             err.span_label(vis.span, "`pub` not permitted here because it's implied");
 211         }
 212         if let Some(note) = note {
 213             err.note(note);
 214         }
 215         err.emit();
 216     }
 217
 218     fn check_decl_no_pat(decl: &FnDecl, mut report_err: impl FnMut(Span, bool)) {
 219         for Param { pat, .. } in &decl.inputs {
 220             match pat.kind {
 221                 PatKind::Ident(BindingMode::ByValue(Mutability::Not), _, None) | PatKind::Wild => {}
 222                 PatKind::Ident(BindingMode::ByValue(Mutability::Mut), _, None) => {
 223                     report_err(pat.span, true)
 224                 }
 225                 _ => report_err(pat.span, false),
 226             }
 227         }
 228     }
 229
 230     fn check_trait_fn_not_async(&self, fn_span: Span, asyncness: Async) {
 231         if let Async::Yes { span, .. } = asyncness {
 232             struct_span_err!(
 233                 self.session,
 234                 fn_span,
 235                 E0706,
 236                 "functions in traits cannot be declared `async`"
 237             )
 238             .span_label(span, "`async` because of this")
 239             .note("`async` trait functions are not currently supported")
 240             .note("consider using the `async-trait` crate: https://crates.io/crates/async-trait")
 241             .emit();
 242         }
 243     }
 244
 245     fn check_trait_fn_not_const(&self, constness: Const) {
 246         if let Const::Yes(span) = constness {
 247             struct_span_err!(
 248                 self.session,
 249                 span,
 250                 E0379,
 251                 "functions in traits cannot be declared const"
 252             )
 253             .span_label(span, "functions in traits cannot be const")
 254             .emit();
 255         }
 256     }
 257
 258     // FIXME(ecstaticmorse): Instead, use `bound_context` to check this in `visit_param_bound`.
 259     fn no_questions_in_bounds(&self, bounds: &GenericBounds, where_: &str, is_trait: bool) {
 260         for bound in bounds {
 261             if let GenericBound::Trait(ref poly, TraitBoundModifier::Maybe) = *bound {
 262                 let mut err = self.err_handler().struct_span_err(
 263                     poly.span,
 264                     &format!("`?Trait` is not permitted in {}", where_),
 265                 );
 266                 if is_trait {
 267                     let path_str = pprust::path_to_string(&poly.trait_ref.path);
 268                     err.note(&format!("traits are `?{}` by default", path_str));
 269                 }
 270                 err.emit();
 271             }
 272         }
 273     }
 274
 275     /// Matches `'-' lit | lit (cf. parser::Parser::parse_literal_maybe_minus)`,
 276     /// or paths for ranges.
 277     //
 278     // FIXME: do we want to allow `expr -> pattern` conversion to create path expressions?
 279     // That means making this work:
 280     //
 281     // ```rust,ignore (FIXME)
 282     // struct S;
 283     // macro_rules! m {
 284     //     ($a:expr) => {
 285     //         let $a = S;
 286     //     }
 287     // }
 288     // m!(S);
 289     // ```
 290     fn check_expr_within_pat(&self, expr: &Expr, allow_paths: bool) {
 291         match expr.kind {
 292             ExprKind::Lit(..) | ExprKind::Err => {}
 293             ExprKind::Path(..) if allow_paths => {}
 294             ExprKind::Unary(UnOp::Neg, ref inner) if matches!(inner.kind, ExprKind::Lit(_)) => {}
 295             _ => self.err_handler().span_err(
 296                 expr.span,
 297                 "arbitrary expressions aren't allowed \
 298                                                          in patterns",
 299             ),
 300         }
 301     }
 302
 303     fn check_late_bound_lifetime_defs(&self, params: &[GenericParam]) {
 304         // Check only lifetime parameters are present and that the lifetime
 305         // parameters that are present have no bounds.
 306         let non_lt_param_spans: Vec<_> = params
 307             .iter()
 308             .filter_map(|param| match param.kind {
 309                 GenericParamKind::Lifetime { .. } => {
 310                     if !param.bounds.is_empty() {
 311                         let spans: Vec<_> = param.bounds.iter().map(|b| b.span()).collect();
 312                         self.err_handler()
 313                             .span_err(spans, "lifetime bounds cannot be used in this context");
 314                     }
 315                     None
 316                 }
 317                 _ => Some(param.ident.span),
 318             })
 319             .collect();
 320         if !non_lt_param_spans.is_empty() {
 321             self.err_handler().span_err(
 322                 non_lt_param_spans,
 323                 "only lifetime parameters can be used in this context",
 324             );
 325         }
 326     }
 327
 328     fn check_fn_decl(&self, fn_decl: &FnDecl, self_semantic: SelfSemantic) {
 329         self.check_decl_cvaradic_pos(fn_decl);
 330         self.check_decl_attrs(fn_decl);
 331         self.check_decl_self_param(fn_decl, self_semantic);
 332     }
 333
 334     fn check_decl_cvaradic_pos(&self, fn_decl: &FnDecl) {
 335         match &*fn_decl.inputs {
 336             [Param { ty, span, .. }] => {
 337                 if let TyKind::CVarArgs = ty.kind {
 338                     self.err_handler().span_err(
 339                         *span,
 340                         "C-variadic function must be declared with at least one named argument",
 341                     );
 342                 }
 343             }
 344             [ps @ .., _] => {
 345                 for Param { ty, span, .. } in ps {
 346                     if let TyKind::CVarArgs = ty.kind {
 347                         self.err_handler().span_err(
 348                             *span,
 349                             "`...` must be the last argument of a C-variadic function",
 350                         );
 351                     }
 352                 }
 353             }
 354             _ => {}
 355         }
 356     }
 357
 358     fn check_decl_attrs(&self, fn_decl: &FnDecl) {
 359         fn_decl
 360             .inputs
 361             .iter()
 362             .flat_map(|i| i.attrs.as_ref())
 363             .filter(|attr| {
 364                 let arr = [sym::allow, sym::cfg, sym::cfg_attr, sym::deny, sym::forbid, sym::warn];
 365                 !arr.contains(&attr.name_or_empty()) && rustc_attr::is_builtin_attr(attr)
 366             })
 367             .for_each(|attr| {
 368                 if attr.is_doc_comment() {
 369                     self.err_handler()
 370                         .struct_span_err(
 371                             attr.span,
 372                             "documentation comments cannot be applied to function parameters",
 373                         )
 374                         .span_label(attr.span, "doc comments are not allowed here")
 375                         .emit();
 376                 } else {
 377                     self.err_handler().span_err(
 378                         attr.span,
 379                         "allow, cfg, cfg_attr, deny, \
 380                 forbid, and warn are the only allowed built-in attributes in function parameters",
 381                     )
 382                 }
 383             });
 384     }
 385
 386     fn check_decl_self_param(&self, fn_decl: &FnDecl, self_semantic: SelfSemantic) {
 387         if let (SelfSemantic::No, [param, ..]) = (self_semantic, &*fn_decl.inputs) {
 388             if param.is_self() {
 389                 self.err_handler()
 390                     .struct_span_err(
 391                         param.span,
 392                         "`self` parameter is only allowed in associated functions",
 393                     )
 394                     .span_label(param.span, "not semantically valid as function parameter")
 395                     .note("associated functions are those in `impl` or `trait` definitions")
 396                     .emit();
 397             }
 398         }
 399     }
 400
 401     fn check_defaultness(&self, span: Span, defaultness: Defaultness) {
 402         if let Defaultness::Default(def_span) = defaultness {
 403             let span = self.session.source_map().guess_head_span(span);
 404             self.err_handler()
 405                 .struct_span_err(span, "`default` is only allowed on items in `impl` definitions")
 406                 .span_label(def_span, "`default` because of this")
 407                 .emit();
 408         }
 409     }
 410
 411     fn error_item_without_body(&self, sp: Span, ctx: &str, msg: &str, sugg: &str) {
 412         self.err_handler()
 413             .struct_span_err(sp, msg)
 414             .span_suggestion(
 415                 self.session.source_map().end_point(sp),
 416                 &format!("provide a definition for the {}", ctx),
 417                 sugg.to_string(),
 418                 Applicability::HasPlaceholders,
 419             )
 420             .emit();
 421     }
 422
 423     fn check_impl_item_provided<T>(&self, sp: Span, body: &Option<T>, ctx: &str, sugg: &str) {
 424         if body.is_none() {
 425             let msg = format!("associated {} in `impl` without body", ctx);
 426             self.error_item_without_body(sp, ctx, &msg, sugg);
 427         }
 428     }
 429
 430     fn check_type_no_bounds(&self, bounds: &[GenericBound], ctx: &str) {
 431         let span = match bounds {
 432             [] => return,
 433             [b0] => b0.span(),
 434             [b0, .., bl] => b0.span().to(bl.span()),
 435         };
 436         self.err_handler()
 437             .struct_span_err(span, &format!("bounds on `type`s in {} have no effect", ctx))
 438             .emit();
 439     }
 440
 441     fn check_foreign_ty_genericless(&self, generics: &Generics) {
 442         let cannot_have = |span, descr, remove_descr| {
 443             self.err_handler()
 444                 .struct_span_err(
 445                     span,
 446                     &format!("`type`s inside `extern` blocks cannot have {}", descr),
 447                 )
 448                 .span_suggestion(
 449                     span,
 450                     &format!("remove the {}", remove_descr),
 451                     String::new(),
 452                     Applicability::MaybeIncorrect,
 453                 )
 454                 .span_label(self.current_extern_span(), "`extern` block begins here")
 455                 .note(MORE_EXTERN)
 456                 .emit();
 457         };
 458
 459         if !generics.params.is_empty() {
 460             cannot_have(generics.span, "generic parameters", "generic parameters");
 461         }
 462
 463         if !generics.where_clause.predicates.is_empty() {
 464             cannot_have(generics.where_clause.span, "`where` clauses", "`where` clause");
 465         }
 466     }
 467
 468     fn check_foreign_kind_bodyless(&self, ident: Ident, kind: &str, body: Option<Span>) {
 469         let body = match body {
 470             None => return,
 471             Some(body) => body,
 472         };
 473         self.err_handler()
 474             .struct_span_err(ident.span, &format!("incorrect `{}` inside `extern` block", kind))
 475             .span_label(ident.span, "cannot have a body")
 476             .span_label(body, "the invalid body")
 477             .span_label(
 478                 self.current_extern_span(),
 479                 format!(
 480                     "`extern` blocks define existing foreign {0}s and {0}s \
 481                     inside of them cannot have a body",
 482                     kind
 483                 ),
 484             )
 485             .note(MORE_EXTERN)
 486             .emit();
 487     }
 488
 489     /// An `fn` in `extern { ... }` cannot have a body `{ ... }`.
 490     fn check_foreign_fn_bodyless(&self, ident: Ident, body: Option<&Block>) {
 491         let body = match body {
 492             None => return,
 493             Some(body) => body,
 494         };
 495         self.err_handler()
 496             .struct_span_err(ident.span, "incorrect function inside `extern` block")
 497             .span_label(ident.span, "cannot have a body")
 498             .span_suggestion(
 499                 body.span,
 500                 "remove the invalid body",
 501                 ";".to_string(),
 502                 Applicability::MaybeIncorrect,
 503             )
 504             .help(
 505                 "you might have meant to write a function accessible through FFI, \
 506                 which can be done by writing `extern fn` outside of the `extern` block",
 507             )
 508             .span_label(
 509                 self.current_extern_span(),
 510                 "`extern` blocks define existing foreign functions and functions \
 511                 inside of them cannot have a body",
 512             )
 513             .note(MORE_EXTERN)
 514             .emit();
 515     }
 516
 517     fn current_extern_span(&self) -> Span {
 518         self.session.source_map().guess_head_span(self.extern_mod.unwrap().span)
 519     }
 520
 521     /// An `fn` in `extern { ... }` cannot have qualfiers, e.g. `async fn`.
 522     fn check_foreign_fn_headerless(&self, ident: Ident, span: Span, header: FnHeader) {
 523         if header.has_qualifiers() {
 524             self.err_handler()
 525                 .struct_span_err(ident.span, "functions in `extern` blocks cannot have qualifiers")
 526                 .span_label(self.current_extern_span(), "in this `extern` block")
 527                 .span_suggestion(
 528                     span.until(ident.span.shrink_to_lo()),
 529                     "remove the qualifiers",
 530                     "fn ".to_string(),
 531                     Applicability::MaybeIncorrect,
 532                 )
 533                 .emit();
 534         }
 535     }
 536
 537     /// Reject C-varadic type unless the function is foreign,
 538     /// or free and `unsafe extern "C"` semantically.
 539     fn check_c_varadic_type(&self, fk: FnKind<'a>) {
 540         match (fk.ctxt(), fk.header()) {
 541             (Some(FnCtxt::Foreign), _) => return,
 542             (Some(FnCtxt::Free), Some(header)) => match header.ext {
 543                 Extern::Explicit(StrLit { symbol_unescaped: sym::C, .. }) | Extern::Implicit
 544                     if matches!(header.unsafety, Unsafe::Yes(_)) =>
 545                 {
 546                     return;
 547                 }
 548                 _ => {}
 549             },
 550             _ => {}
 551         };
 552
 553         for Param { ty, span, .. } in &fk.decl().inputs {
 554             if let TyKind::CVarArgs = ty.kind {
 555                 self.err_handler()
 556                     .struct_span_err(
 557                         *span,
 558                         "only foreign or `unsafe extern \"C\" functions may be C-variadic",
 559                     )
 560                     .emit();
 561             }
 562         }
 563     }
 564
 565     fn check_item_named(&self, ident: Ident, kind: &str) {
 566         if ident.name != kw::Underscore {
 567             return;
 568         }
 569         self.err_handler()
 570             .struct_span_err(ident.span, &format!("`{}` items in this context need a name", kind))
 571             .span_label(ident.span, format!("`_` is not a valid name for this `{}` item", kind))
 572             .emit();
 573     }
 574
 575     fn deny_generic_params(&self, generics: &Generics, ident_span: Span) {
 576         if !generics.params.is_empty() {
 577             struct_span_err!(
 578                 self.session,
 579                 generics.span,
 580                 E0567,
 581                 "auto traits cannot have generic parameters"
 582             )
 583             .span_label(ident_span, "auto trait cannot have generic parameters")
 584             .span_suggestion(
 585                 generics.span,
 586                 "remove the parameters",
 587                 String::new(),
 588                 Applicability::MachineApplicable,
 589             )
 590             .emit();
 591         }
 592     }
 593
 594     fn deny_super_traits(&self, bounds: &GenericBounds, ident_span: Span) {
 595         if let [first @ last] | [first, .., last] = &bounds[..] {
 596             let span = first.span().to(last.span());
 597             struct_span_err!(self.session, span, E0568, "auto traits cannot have super traits")
 598                 .span_label(ident_span, "auto trait cannot have super traits")
 599                 .span_suggestion(
 600                     span,
 601                     "remove the super traits",
 602                     String::new(),
 603                     Applicability::MachineApplicable,
 604                 )
 605                 .emit();
 606         }
 607     }
 608
 609     fn deny_items(&self, trait_items: &[P<AssocItem>], ident_span: Span) {
 610         if !trait_items.is_empty() {
 611             let spans: Vec<_> = trait_items.iter().map(|i| i.ident.span).collect();
 612             struct_span_err!(
 613                 self.session,
 614                 spans,
 615                 E0380,
 616                 "auto traits cannot have methods or associated items"
 617             )
 618             .span_label(ident_span, "auto trait cannot have items")
 619             .emit();
 620         }
 621     }
 622
 623     fn correct_generic_order_suggestion(&self, data: &AngleBracketedArgs) -> String {
 624         // Lifetimes always come first.
 625         let lt_sugg = data.args.iter().filter_map(|arg| match arg {
 626             AngleBracketedArg::Arg(lt @ GenericArg::Lifetime(_)) => {
 627                 Some(pprust::to_string(|s| s.print_generic_arg(lt)))
 628             }
 629             _ => None,
 630         });
 631         let args_sugg = data.args.iter().filter_map(|a| match a {
 632             AngleBracketedArg::Arg(GenericArg::Lifetime(_)) | AngleBracketedArg::Constraint(_) => {
 633                 None
 634             }
 635             AngleBracketedArg::Arg(arg) => Some(pprust::to_string(|s| s.print_generic_arg(arg))),
 636         });
 637         // Constraints always come last.
 638         let constraint_sugg = data.args.iter().filter_map(|a| match a {
 639             AngleBracketedArg::Arg(_) => None,
 640             AngleBracketedArg::Constraint(c) => {
 641                 Some(pprust::to_string(|s| s.print_assoc_constraint(c)))
 642             }
 643         });
 644         format!(
 645             "<{}>",
 646             lt_sugg.chain(args_sugg).chain(constraint_sugg).collect::<Vec<String>>().join(", ")
 647         )
 648     }
 649
 650     /// Enforce generic args coming before constraints in `<...>` of a path segment.
 651     fn check_generic_args_before_constraints(&self, data: &AngleBracketedArgs) {
 652         // Early exit in case it's partitioned as it should be.
 653         if data.args.iter().is_partitioned(|arg| matches!(arg, AngleBracketedArg::Arg(_))) {
 654             return;
 655         }
 656         // Find all generic argument coming after the first constraint...
 657         let (constraint_spans, arg_spans): (Vec<Span>, Vec<Span>) =
 658             data.args.iter().partition_map(|arg| match arg {
 659                 AngleBracketedArg::Constraint(c) => Either::Left(c.span),
 660                 AngleBracketedArg::Arg(a) => Either::Right(a.span()),
 661             });
 662         let args_len = arg_spans.len();
 663         let constraint_len = constraint_spans.len();
 664         // ...and then error:
 665         self.err_handler()
 666             .struct_span_err(
 667                 arg_spans.clone(),
 668                 "generic arguments must come before the first constraint",
 669             )
 670             .span_label(constraint_spans[0], &format!("constraint{}", pluralize!(constraint_len)))
 671             .span_label(
 672                 *arg_spans.iter().last().unwrap(),
 673                 &format!("generic argument{}", pluralize!(args_len)),
 674             )
 675             .span_labels(constraint_spans, "")
 676             .span_labels(arg_spans, "")
 677             .span_suggestion_verbose(
 678                 data.span,
 679                 &format!(
 680                     "move the constraint{} after the generic argument{}",
 681                     pluralize!(constraint_len),
 682                     pluralize!(args_len)
 683                 ),
 684                 self.correct_generic_order_suggestion(&data),
 685                 Applicability::MachineApplicable,
 686             )
 687             .emit();
 688     }
 689 }
 690
 691 /// Checks that generic parameters are in the correct order,
 692 /// which is lifetimes, then types and then consts. (`<'a, T, const N: usize>`)
 693 fn validate_generic_param_order<'a>(
 694     sess: &Session,
 695     handler: &rustc_errors::Handler,
 696     generics: impl Iterator<Item = (ParamKindOrd, Option<&'a [GenericBound]>, Span, Option<String>)>,
 697     span: Span,
 698 ) {
 699     let mut max_param: Option<ParamKindOrd> = None;
 700     let mut out_of_order = FxHashMap::default();
 701     let mut param_idents = vec![];
 702
 703     for (kind, bounds, span, ident) in generics {
 704         if let Some(ident) = ident {
 705             param_idents.push((kind, bounds, param_idents.len(), ident));
 706         }
 707         let max_param = &mut max_param;
 708         match max_param {
 709             Some(max_param) if *max_param > kind => {
 710                 let entry = out_of_order.entry(kind).or_insert((*max_param, vec![]));
 711                 entry.1.push(span);
 712             }
 713             Some(_) | None => *max_param = Some(kind),
 714         };
 715     }
 716
 717     let mut ordered_params = "<".to_string();
 718     if !out_of_order.is_empty() {
 719         param_idents.sort_by_key(|&(po, _, i, _)| (po, i));
 720         let mut first = true;
 721         for (_, bounds, _, ident) in param_idents {
 722             if !first {
 723                 ordered_params += ", ";
 724             }
 725             ordered_params += &ident;
 726             if let Some(bounds) = bounds {
 727                 if !bounds.is_empty() {
 728                     ordered_params += ": ";
 729                     ordered_params += &pprust::bounds_to_string(&bounds);
 730                 }
 731             }
 732             first = false;
 733         }
 734     }
 735     ordered_params += ">";
 736
 737     for (param_ord, (max_param, spans)) in &out_of_order {
 738         let mut err =
 739             handler.struct_span_err(
 740                 spans.clone(),
 741                 &format!(
 742                     "{} parameters must be declared prior to {} parameters",
 743                     param_ord, max_param,
 744                 ),
 745             );
 746         err.span_suggestion(
 747             span,
 748             &format!(
 749                 "reorder the parameters: lifetimes, then types{}",
 750                 if sess.features_untracked().const_generics { ", then consts" } else { "" },
 751             ),
 752             ordered_params.clone(),
 753             Applicability::MachineApplicable,
 754         );
 755         err.emit();
 756     }
 757 }
 758
 759 impl<'a> Visitor<'a> for AstValidator<'a> {
 760     fn visit_attribute(&mut self, attr: &Attribute) {
 761         validate_attr::check_meta(&self.session.parse_sess, attr);
 762     }
 763
 764     fn visit_expr(&mut self, expr: &'a Expr) {
 765         match &expr.kind {
 766             ExprKind::LlvmInlineAsm(..) if !self.session.target.target.options.allow_asm => {
 767                 struct_span_err!(
 768                     self.session,
 769                     expr.span,
 770                     E0472,
 771                     "llvm_asm! is unsupported on this target"
 772                 )
 773                 .emit();
 774             }
 775             _ => {}
 776         }
 777
 778         visit::walk_expr(self, expr);
 779     }
 780
 781     fn visit_ty(&mut self, ty: &'a Ty) {
 782         match ty.kind {
 783             TyKind::BareFn(ref bfty) => {
 784                 self.check_fn_decl(&bfty.decl, SelfSemantic::No);
 785                 Self::check_decl_no_pat(&bfty.decl, |span, _| {
 786                     struct_span_err!(
 787                         self.session,
 788                         span,
 789                         E0561,
 790                         "patterns aren't allowed in function pointer types"
 791                     )
 792                     .emit();
 793                 });
 794                 self.check_late_bound_lifetime_defs(&bfty.generic_params);
 795             }
 796             TyKind::TraitObject(ref bounds, ..) => {
 797                 let mut any_lifetime_bounds = false;
 798                 for bound in bounds {
 799                     if let GenericBound::Outlives(ref lifetime) = *bound {
 800                         if any_lifetime_bounds {
 801                             struct_span_err!(
 802                                 self.session,
 803                                 lifetime.ident.span,
 804                                 E0226,
 805                                 "only a single explicit lifetime bound is permitted"
 806                             )
 807                             .emit();
 808                             break;
 809                         }
 810                         any_lifetime_bounds = true;
 811                     }
 812                 }
 813                 self.no_questions_in_bounds(bounds, "trait object types", false);
 814             }
 815             TyKind::ImplTrait(_, ref bounds) => {
 816                 if self.is_impl_trait_banned {
 817                     struct_span_err!(
 818                         self.session,
 819                         ty.span,
 820                         E0667,
 821                         "`impl Trait` is not allowed in path parameters"
 822                     )
 823                     .emit();
 824                 }
 825
 826                 if let Some(outer_impl_trait_sp) = self.outer_impl_trait {
 827                     struct_span_err!(
 828                         self.session,
 829                         ty.span,
 830                         E0666,
 831                         "nested `impl Trait` is not allowed"
 832                     )
 833                     .span_label(outer_impl_trait_sp, "outer `impl Trait`")
 834                     .span_label(ty.span, "nested `impl Trait` here")
 835                     .emit();
 836                 }
 837
 838                 if !bounds
 839                     .iter()
 840                     .any(|b| if let GenericBound::Trait(..) = *b { true } else { false })
 841                 {
 842                     self.err_handler().span_err(ty.span, "at least one trait must be specified");
 843                 }
 844
 845                 self.walk_ty(ty);
 846                 return;
 847             }
 848             _ => {}
 849         }
 850
 851         self.walk_ty(ty)
 852     }
 853
 854     fn visit_label(&mut self, label: &'a Label) {
 855         self.check_label(label.ident);
 856         visit::walk_label(self, label);
 857     }
 858
 859     fn visit_lifetime(&mut self, lifetime: &'a Lifetime) {
 860         self.check_lifetime(lifetime.ident);
 861         visit::walk_lifetime(self, lifetime);
 862     }
 863
 864     fn visit_item(&mut self, item: &'a Item) {
 865         if item.attrs.iter().any(|attr| is_proc_macro_attr(attr)) {
 866             self.has_proc_macro_decls = true;
 867         }
 868
 869         match item.kind {
 870             ItemKind::Impl {
 871                 unsafety,
 872                 polarity,
 873                 defaultness: _,
 874                 constness: _,
 875                 generics: _,
 876                 of_trait: Some(ref t),
 877                 ref self_ty,
 878                 items: _,
 879             } => {
 880                 self.with_in_trait_impl(true, |this| {
 881                     this.invalid_visibility(&item.vis, None);
 882                     if let TyKind::Err = self_ty.kind {
 883                         this.err_handler()
 884                             .struct_span_err(
 885                                 item.span,
 886                                 "`impl Trait for .. {}` is an obsolete syntax",
 887                             )
 888                             .help("use `auto trait Trait {}` instead")
 889                             .emit();
 890                     }
 891                     if let (Unsafe::Yes(span), ImplPolarity::Negative(sp)) = (unsafety, polarity) {
 892                         struct_span_err!(
 893                             this.session,
 894                             sp.to(t.path.span),
 895                             E0198,
 896                             "negative impls cannot be unsafe"
 897                         )
 898                         .span_label(sp, "negative because of this")
 899                         .span_label(span, "unsafe because of this")
 900                         .emit();
 901                     }
 902
 903                     visit::walk_item(this, item);
 904                 });
 905                 return; // Avoid visiting again.
 906             }
 907             ItemKind::Impl {
 908                 unsafety,
 909                 polarity,
 910                 defaultness,
 911                 constness,
 912                 generics: _,
 913                 of_trait: None,
 914                 ref self_ty,
 915                 items: _,
 916             } => {
 917                 let error = |annotation_span, annotation| {
 918                     let mut err = self.err_handler().struct_span_err(
 919                         self_ty.span,
 920                         &format!("inherent impls cannot be {}", annotation),
 921                     );
 922                     err.span_label(annotation_span, &format!("{} because of this", annotation));
 923                     err.span_label(self_ty.span, "inherent impl for this type");
 924                     err
 925                 };
 926
 927                 self.invalid_visibility(
 928                     &item.vis,
 929                     Some("place qualifiers on individual impl items instead"),
 930                 );
 931                 if let Unsafe::Yes(span) = unsafety {
 932                     error(span, "unsafe").code(error_code!(E0197)).emit();
 933                 }
 934                 if let ImplPolarity::Negative(span) = polarity {
 935                     error(span, "negative").emit();
 936                 }
 937                 if let Defaultness::Default(def_span) = defaultness {
 938                     error(def_span, "`default`")
 939                         .note("only trait implementations may be annotated with `default`")
 940                         .emit();
 941                 }
 942                 if let Const::Yes(span) = constness {
 943                     error(span, "`const`")
 944                         .note("only trait implementations may be annotated with `const`")
 945                         .emit();
 946                 }
 947             }
 948             ItemKind::Fn(def, _, _, ref body) => {
 949                 self.check_defaultness(item.span, def);
 950
 951                 if body.is_none() {
 952                     let msg = "free function without a body";
 953                     self.error_item_without_body(item.span, "function", msg, " { <body> }");
 954                 }
 955             }
 956             ItemKind::ForeignMod(_) => {
 957                 let old_item = mem::replace(&mut self.extern_mod, Some(item));
 958                 self.invalid_visibility(
 959                     &item.vis,
 960                     Some("place qualifiers on individual foreign items instead"),
 961                 );
 962                 visit::walk_item(self, item);
 963                 self.extern_mod = old_item;
 964                 return; // Avoid visiting again.
 965             }
 966             ItemKind::Enum(ref def, _) => {
 967                 for variant in &def.variants {
 968                     self.invalid_visibility(&variant.vis, None);
 969                     for field in variant.data.fields() {
 970                         self.invalid_visibility(&field.vis, None);
 971                     }
 972                 }
 973             }
 974             ItemKind::Trait(is_auto, _, ref generics, ref bounds, ref trait_items) => {
 975                 if is_auto == IsAuto::Yes {
 976                     // Auto traits cannot have generics, super traits nor contain items.
 977                     self.deny_generic_params(generics, item.ident.span);
 978                     self.deny_super_traits(bounds, item.ident.span);
 979                     self.deny_items(trait_items, item.ident.span);
 980                 }
 981                 self.no_questions_in_bounds(bounds, "supertraits", true);
 982
 983                 // Equivalent of `visit::walk_item` for `ItemKind::Trait` that inserts a bound
 984                 // context for the supertraits.
 985                 self.visit_vis(&item.vis);
 986                 self.visit_ident(item.ident);
 987                 self.visit_generics(generics);
 988                 self.with_bound_context(BoundContext::TraitBounds, |this| {
 989                     walk_list!(this, visit_param_bound, bounds);
 990                 });
 991                 walk_list!(self, visit_assoc_item, trait_items, AssocCtxt::Trait);
 992                 walk_list!(self, visit_attribute, &item.attrs);
 993                 return;
 994             }
 995             ItemKind::Mod(_) => {
 996                 // Ensure that `path` attributes on modules are recorded as used (cf. issue #35584).
 997                 attr::first_attr_value_str_by_name(&item.attrs, sym::path);
 998             }
 999             ItemKind::Union(ref vdata, _) => {
1000                 if let VariantData::Tuple(..) | VariantData::Unit(..) = vdata {
1001                     self.err_handler()
1002                         .span_err(item.span, "tuple and unit unions are not permitted");
1003                 }
1004                 if vdata.fields().is_empty() {
1005                     self.err_handler().span_err(item.span, "unions cannot have zero fields");
1006                 }
1007             }
1008             ItemKind::Const(def, .., None) => {
1009                 self.check_defaultness(item.span, def);
1010                 let msg = "free constant item without body";
1011                 self.error_item_without_body(item.span, "constant", msg, " = <expr>;");
1012             }
1013             ItemKind::Static(.., None) => {
1014                 let msg = "free static item without body";
1015                 self.error_item_without_body(item.span, "static", msg, " = <expr>;");
1016             }
1017             ItemKind::TyAlias(def, _, ref bounds, ref body) => {
1018                 self.check_defaultness(item.span, def);
1019                 if body.is_none() {
1020                     let msg = "free type alias without body";
1021                     self.error_item_without_body(item.span, "type", msg, " = <type>;");
1022                 }
1023                 self.check_type_no_bounds(bounds, "this context");
1024             }
1025             _ => {}
1026         }
1027
1028         visit::walk_item(self, item)
1029     }
1030
1031     fn visit_foreign_item(&mut self, fi: &'a ForeignItem) {
1032         match &fi.kind {
1033             ForeignItemKind::Fn(def, sig, _, body) => {
1034                 self.check_defaultness(fi.span, *def);
1035                 self.check_foreign_fn_bodyless(fi.ident, body.as_deref());
1036                 self.check_foreign_fn_headerless(fi.ident, fi.span, sig.header);
1037             }
1038             ForeignItemKind::TyAlias(def, generics, bounds, body) => {
1039                 self.check_defaultness(fi.span, *def);
1040                 self.check_foreign_kind_bodyless(fi.ident, "type", body.as_ref().map(|b| b.span));
1041                 self.check_type_no_bounds(bounds, "`extern` blocks");
1042                 self.check_foreign_ty_genericless(generics);
1043             }
1044             ForeignItemKind::Static(_, _, body) => {
1045                 self.check_foreign_kind_bodyless(fi.ident, "static", body.as_ref().map(|b| b.span));
1046             }
1047             ForeignItemKind::MacCall(..) => {}
1048         }
1049
1050         visit::walk_foreign_item(self, fi)
1051     }
1052
1053     // Mirrors `visit::walk_generic_args`, but tracks relevant state.
1054     fn visit_generic_args(&mut self, _: Span, generic_args: &'a GenericArgs) {
1055         match *generic_args {
1056             GenericArgs::AngleBracketed(ref data) => {
1057                 self.check_generic_args_before_constraints(data);
1058
1059                 for arg in &data.args {
1060                     match arg {
1061                         AngleBracketedArg::Arg(arg) => self.visit_generic_arg(arg),
1062                         // Type bindings such as `Item = impl Debug` in `Iterator<Item = Debug>`
1063                         // are allowed to contain nested `impl Trait`.
1064                         AngleBracketedArg::Constraint(constraint) => {
1065                             self.with_impl_trait(None, |this| {
1066                                 this.visit_assoc_ty_constraint_from_generic_args(constraint);
1067                             });
1068                         }
1069                     }
1070                 }
1071             }
1072             GenericArgs::Parenthesized(ref data) => {
1073                 walk_list!(self, visit_ty, &data.inputs);
1074                 if let FnRetTy::Ty(ty) = &data.output {
1075                     // `-> Foo` syntax is essentially an associated type binding,
1076                     // so it is also allowed to contain nested `impl Trait`.
1077                     self.with_impl_trait(None, |this| this.visit_ty(ty));
1078                 }
1079             }
1080         }
1081     }
1082
1083     fn visit_generics(&mut self, generics: &'a Generics) {
1084         let mut prev_ty_default = None;
1085         for param in &generics.params {
1086             if let GenericParamKind::Type { ref default, .. } = param.kind {
1087                 if default.is_some() {
1088                     prev_ty_default = Some(param.ident.span);
1089                 } else if let Some(span) = prev_ty_default {
1090                     self.err_handler()
1091                         .span_err(span, "type parameters with a default must be trailing");
1092                     break;
1093                 }
1094             }
1095         }
1096
1097         validate_generic_param_order(
1098             self.session,
1099             self.err_handler(),
1100             generics.params.iter().map(|param| {
1101                 let ident = Some(param.ident.to_string());
1102                 let (kind, ident) = match &param.kind {
1103                     GenericParamKind::Lifetime { .. } => (ParamKindOrd::Lifetime, ident),
1104                     GenericParamKind::Type { .. } => (ParamKindOrd::Type, ident),
1105                     GenericParamKind::Const { ref ty } => {
1106                         let ty = pprust::ty_to_string(ty);
1107                         (ParamKindOrd::Const, Some(format!("const {}: {}", param.ident, ty)))
1108                     }
1109                 };
1110                 (kind, Some(&*param.bounds), param.ident.span, ident)
1111             }),
1112             generics.span,
1113         );
1114
1115         for predicate in &generics.where_clause.predicates {
1116             if let WherePredicate::EqPredicate(ref predicate) = *predicate {
1117                 deny_equality_constraints(self, predicate, generics);
1118             }
1119         }
1120
1121         visit::walk_generics(self, generics)
1122     }
1123
1124     fn visit_generic_param(&mut self, param: &'a GenericParam) {
1125         if let GenericParamKind::Lifetime { .. } = param.kind {
1126             self.check_lifetime(param.ident);
1127         }
1128         visit::walk_generic_param(self, param);
1129     }
1130
1131     fn visit_param_bound(&mut self, bound: &'a GenericBound) {
1132         match bound {
1133             GenericBound::Trait(_, TraitBoundModifier::MaybeConst) => {
1134                 if let Some(ctx) = self.bound_context {
1135                     let msg = format!("`?const` is not permitted in {}", ctx.description());
1136                     self.err_handler().span_err(bound.span(), &msg);
1137                 }
1138             }
1139
1140             GenericBound::Trait(_, TraitBoundModifier::MaybeConstMaybe) => {
1141                 self.err_handler()
1142                     .span_err(bound.span(), "`?const` and `?` are mutually exclusive");
1143             }
1144
1145             _ => {}
1146         }
1147
1148         visit::walk_param_bound(self, bound)
1149     }
1150
1151     fn visit_pat(&mut self, pat: &'a Pat) {
1152         match pat.kind {
1153             PatKind::Lit(ref expr) => {
1154                 self.check_expr_within_pat(expr, false);
1155             }
1156             PatKind::Range(ref start, ref end, _) => {
1157                 if let Some(expr) = start {
1158                     self.check_expr_within_pat(expr, true);
1159                 }
1160                 if let Some(expr) = end {
1161                     self.check_expr_within_pat(expr, true);
1162                 }
1163             }
1164             _ => {}
1165         }
1166
1167         visit::walk_pat(self, pat)
1168     }
1169
1170     fn visit_where_predicate(&mut self, p: &'a WherePredicate) {
1171         if let &WherePredicate::BoundPredicate(ref bound_predicate) = p {
1172             // A type binding, eg `for<'c> Foo: Send+Clone+'c`
1173             self.check_late_bound_lifetime_defs(&bound_predicate.bound_generic_params);
1174         }
1175         visit::walk_where_predicate(self, p);
1176     }
1177
1178     fn visit_poly_trait_ref(&mut self, t: &'a PolyTraitRef, m: &'a TraitBoundModifier) {
1179         self.check_late_bound_lifetime_defs(&t.bound_generic_params);
1180         visit::walk_poly_trait_ref(self, t, m);
1181     }
1182
1183     fn visit_variant_data(&mut self, s: &'a VariantData) {
1184         self.with_banned_assoc_ty_bound(|this| visit::walk_struct_def(this, s))
1185     }
1186
1187     fn visit_enum_def(
1188         &mut self,
1189         enum_definition: &'a EnumDef,
1190         generics: &'a Generics,
1191         item_id: NodeId,
1192         _: Span,
1193     ) {
1194         self.with_banned_assoc_ty_bound(|this| {
1195             visit::walk_enum_def(this, enum_definition, generics, item_id)
1196         })
1197     }
1198
1199     fn visit_fn(&mut self, fk: FnKind<'a>, span: Span, id: NodeId) {
1200         // Only associated `fn`s can have `self` parameters.
1201         let self_semantic = match fk.ctxt() {
1202             Some(FnCtxt::Assoc(_)) => SelfSemantic::Yes,
1203             _ => SelfSemantic::No,
1204         };
1205         self.check_fn_decl(fk.decl(), self_semantic);
1206
1207         self.check_c_varadic_type(fk);
1208
1209         // Functions cannot both be `const async`
1210         if let Some(FnHeader {
1211             constness: Const::Yes(cspan),
1212             asyncness: Async::Yes { span: aspan, .. },
1213             ..
1214         }) = fk.header()
1215         {
1216             self.err_handler()
1217                 .struct_span_err(
1218                     vec![*cspan, *aspan],
1219                     "functions cannot be both `const` and `async`",
1220                 )
1221                 .span_label(*cspan, "`const` because of this")
1222                 .span_label(*aspan, "`async` because of this")
1223                 .span_label(span, "") // Point at the fn header.
1224                 .emit();
1225         }
1226
1227         // Functions without bodies cannot have patterns.
1228         if let FnKind::Fn(ctxt, _, sig, _, None) = fk {
1229             Self::check_decl_no_pat(&sig.decl, |span, mut_ident| {
1230                 let (code, msg, label) = match ctxt {
1231                     FnCtxt::Foreign => (
1232                         error_code!(E0130),
1233                         "patterns aren't allowed in foreign function declarations",
1234                         "pattern not allowed in foreign function",
1235                     ),
1236                     _ => (
1237                         error_code!(E0642),
1238                         "patterns aren't allowed in functions without bodies",
1239                         "pattern not allowed in function without body",
1240                     ),
1241                 };
1242                 if mut_ident && matches!(ctxt, FnCtxt::Assoc(_)) {
1243                     self.lint_buffer.buffer_lint(PATTERNS_IN_FNS_WITHOUT_BODY, id, span, msg);
1244                 } else {
1245                     self.err_handler()
1246                         .struct_span_err(span, msg)
1247                         .span_label(span, label)
1248                         .code(code)
1249                         .emit();
1250                 }
1251             });
1252         }
1253
1254         visit::walk_fn(self, fk, span);
1255     }
1256
1257     fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
1258         if ctxt == AssocCtxt::Trait || !self.in_trait_impl {
1259             self.check_defaultness(item.span, item.kind.defaultness());
1260         }
1261
1262         if ctxt == AssocCtxt::Impl {
1263             match &item.kind {
1264                 AssocItemKind::Const(_, _, body) => {
1265                     self.check_impl_item_provided(item.span, body, "constant", " = <expr>;");
1266                 }
1267                 AssocItemKind::Fn(_, _, _, body) => {
1268                     self.check_impl_item_provided(item.span, body, "function", " { <body> }");
1269                 }
1270                 AssocItemKind::TyAlias(_, _, bounds, body) => {
1271                     self.check_impl_item_provided(item.span, body, "type", " = <type>;");
1272                     self.check_type_no_bounds(bounds, "`impl`s");
1273                 }
1274                 _ => {}
1275             }
1276         }
1277
1278         if ctxt == AssocCtxt::Trait || self.in_trait_impl {
1279             self.invalid_visibility(&item.vis, None);
1280             if let AssocItemKind::Fn(_, sig, _, _) = &item.kind {
1281                 self.check_trait_fn_not_const(sig.header.constness);
1282                 self.check_trait_fn_not_async(item.span, sig.header.asyncness);
1283             }
1284         }
1285
1286         if let AssocItemKind::Const(..) = item.kind {
1287             self.check_item_named(item.ident, "const");
1288         }
1289
1290         self.with_in_trait_impl(false, |this| visit::walk_assoc_item(this, item, ctxt));
1291     }
1292 }
1293
1294 fn deny_equality_constraints(
1295     this: &mut AstValidator<'_>,
1296     predicate: &WhereEqPredicate,
1297     generics: &Generics,
1298 ) {
1299     let mut err = this.err_handler().struct_span_err(
1300         predicate.span,
1301         "equality constraints are not yet supported in `where` clauses",
1302     );
1303     err.span_label(predicate.span, "not supported");
1304
1305     // Given `<A as Foo>::Bar = RhsTy`, suggest `A: Foo<Bar = RhsTy>`.
1306     if let TyKind::Path(Some(qself), full_path) = &predicate.lhs_ty.kind {
1307         if let TyKind::Path(None, path) = &qself.ty.kind {
1308             match &path.segments[..] {
1309                 [PathSegment { ident, args: None, .. }] => {
1310                     for param in &generics.params {
1311                         if param.ident == *ident {
1312                             let param = ident;
1313                             match &full_path.segments[qself.position..] {
1314                                 [PathSegment { ident, .. }] => {
1315                                     // Make a new `Path` from `foo::Bar` to `Foo<Bar = RhsTy>`.
1316                                     let mut assoc_path = full_path.clone();
1317                                     // Remove `Bar` from `Foo::Bar`.
1318                                     assoc_path.segments.pop();
1319                                     let len = assoc_path.segments.len() - 1;
1320                                     // Build `<Bar = RhsTy>`.
1321                                     let arg = AngleBracketedArg::Constraint(AssocTyConstraint {
1322                                         id: rustc_ast::node_id::DUMMY_NODE_ID,
1323                                         ident: *ident,
1324                                         kind: AssocTyConstraintKind::Equality {
1325                                             ty: predicate.rhs_ty.clone(),
1326                                         },
1327                                         span: ident.span,
1328                                     });
1329                                     // Add `<Bar = RhsTy>` to `Foo`.
1330                                     match &mut assoc_path.segments[len].args {
1331                                         Some(args) => match args.deref_mut() {
1332                                             GenericArgs::Parenthesized(_) => continue,
1333                                             GenericArgs::AngleBracketed(args) => {
1334                                                 args.args.push(arg);
1335                                             }
1336                                         },
1337                                         empty_args => {
1338                                             *empty_args = AngleBracketedArgs {
1339                                                 span: ident.span,
1340                                                 args: vec![arg],
1341                                             }
1342                                             .into();
1343                                         }
1344                                     }
1345                                     err.span_suggestion_verbose(
1346                                         predicate.span,
1347                                         &format!(
1348                                             "if `{}` is an associated type you're trying to set, \
1349                                             use the associated type binding syntax",
1350                                             ident
1351                                         ),
1352                                         format!(
1353                                             "{}: {}",
1354                                             param,
1355                                             pprust::path_to_string(&assoc_path)
1356                                         ),
1357                                         Applicability::MaybeIncorrect,
1358                                     );
1359                                 }
1360                                 _ => {}
1361                             };
1362                         }
1363                     }
1364                 }
1365                 _ => {}
1366             }
1367         }
1368     }
1369     err.note(
1370         "see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information",
1371     );
1372     err.emit();
1373 }
1374
1375 pub fn check_crate(session: &Session, krate: &Crate, lints: &mut LintBuffer) -> bool {
1376     let mut validator = AstValidator {
1377         session,
1378         extern_mod: None,
1379         in_trait_impl: false,
1380         has_proc_macro_decls: false,
1381         outer_impl_trait: None,
1382         bound_context: None,
1383         is_impl_trait_banned: false,
1384         is_assoc_ty_bound_banned: false,
1385         lint_buffer: lints,
1386     };
1387     visit::walk_crate(&mut validator, krate);
1388
1389     validator.has_proc_macro_decls
1390 }