src/librustc_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 errors::{struct_span_err, Applicability, FatalError};
  10 use rustc::lint;
  11 use rustc::session::Session;
  12 use rustc_data_structures::fx::FxHashMap;
  13 use rustc_parse::validate_attr;
  14 use rustc_span::source_map::Spanned;
  15 use rustc_span::symbol::{kw, sym};
  16 use rustc_span::Span;
  17 use std::mem;
  18 use syntax::ast::*;
  19 use syntax::attr;
  20 use syntax::expand::is_proc_macro_attr;
  21 use syntax::print::pprust;
  22 use syntax::visit::{self, Visitor};
  23 use syntax::walk_list;
  24
  25 use rustc_error_codes::*;
  26
  27 /// A syntactic context that disallows certain kinds of bounds (e.g., `?Trait` or `?const Trait`).
  28 #[derive(Clone, Copy)]
  29 enum BoundContext {
  30     ImplTrait,
  31     TraitBounds,
  32     TraitObject,
  33 }
  34
  35 impl BoundContext {
  36     fn description(&self) -> &'static str {
  37         match self {
  38             Self::ImplTrait => "`impl Trait`",
  39             Self::TraitBounds => "supertraits",
  40             Self::TraitObject => "trait objects",
  41         }
  42     }
  43 }
  44
  45 struct AstValidator<'a> {
  46     session: &'a Session,
  47     has_proc_macro_decls: bool,
  48
  49     /// Used to ban nested `impl Trait`, e.g., `impl Into<impl Debug>`.
  50     /// Nested `impl Trait` _is_ allowed in associated type position,
  51     /// e.g., `impl Iterator<Item = impl Debug>`.
  52     outer_impl_trait: Option<Span>,
  53
  54     /// Keeps track of the `BoundContext` as we recurse.
  55     ///
  56     /// This is used to forbid `?const Trait` bounds in, e.g.,
  57     /// `impl Iterator<Item = Box<dyn ?const Trait>`.
  58     bound_context: Option<BoundContext>,
  59
  60     /// Used to ban `impl Trait` in path projections like `<impl Iterator>::Item`
  61     /// or `Foo::Bar<impl Trait>`
  62     is_impl_trait_banned: bool,
  63
  64     /// Used to ban associated type bounds (i.e., `Type<AssocType: Bounds>`) in
  65     /// certain positions.
  66     is_assoc_ty_bound_banned: bool,
  67
  68     lint_buffer: &'a mut lint::LintBuffer,
  69 }
  70
  71 impl<'a> AstValidator<'a> {
  72     fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
  73         let old = mem::replace(&mut self.is_impl_trait_banned, true);
  74         f(self);
  75         self.is_impl_trait_banned = old;
  76     }
  77
  78     fn with_banned_assoc_ty_bound(&mut self, f: impl FnOnce(&mut Self)) {
  79         let old = mem::replace(&mut self.is_assoc_ty_bound_banned, true);
  80         f(self);
  81         self.is_assoc_ty_bound_banned = old;
  82     }
  83
  84     fn with_impl_trait(&mut self, outer: Option<Span>, f: impl FnOnce(&mut Self)) {
  85         let old = mem::replace(&mut self.outer_impl_trait, outer);
  86         if outer.is_some() {
  87             self.with_bound_context(BoundContext::ImplTrait, |this| f(this));
  88         } else {
  89             f(self)
  90         }
  91         self.outer_impl_trait = old;
  92     }
  93
  94     fn with_bound_context(&mut self, ctx: BoundContext, f: impl FnOnce(&mut Self)) {
  95         let old = self.bound_context.replace(ctx);
  96         f(self);
  97         self.bound_context = old;
  98     }
  99
 100     fn visit_assoc_ty_constraint_from_generic_args(&mut self, constraint: &'a AssocTyConstraint) {
 101         match constraint.kind {
 102             AssocTyConstraintKind::Equality { .. } => {}
 103             AssocTyConstraintKind::Bound { .. } => {
 104                 if self.is_assoc_ty_bound_banned {
 105                     self.err_handler().span_err(
 106                         constraint.span,
 107                         "associated type bounds are not allowed within structs, enums, or unions",
 108                     );
 109                 }
 110             }
 111         }
 112         self.visit_assoc_ty_constraint(constraint);
 113     }
 114
 115     // Mirrors `visit::walk_ty`, but tracks relevant state.
 116     fn walk_ty(&mut self, t: &'a Ty) {
 117         match t.kind {
 118             TyKind::ImplTrait(..) => {
 119                 self.with_impl_trait(Some(t.span), |this| visit::walk_ty(this, t))
 120             }
 121             TyKind::TraitObject(..) => {
 122                 self.with_bound_context(BoundContext::TraitObject, |this| visit::walk_ty(this, t));
 123             }
 124             TyKind::Path(ref qself, ref path) => {
 125                 // We allow these:
 126                 //  - `Option<impl Trait>`
 127                 //  - `option::Option<impl Trait>`
 128                 //  - `option::Option<T>::Foo<impl Trait>
 129                 //
 130                 // But not these:
 131                 //  - `<impl Trait>::Foo`
 132                 //  - `option::Option<impl Trait>::Foo`.
 133                 //
 134                 // To implement this, we disallow `impl Trait` from `qself`
 135                 // (for cases like `<impl Trait>::Foo>`)
 136                 // but we allow `impl Trait` in `GenericArgs`
 137                 // iff there are no more PathSegments.
 138                 if let Some(ref qself) = *qself {
 139                     // `impl Trait` in `qself` is always illegal
 140                     self.with_banned_impl_trait(|this| this.visit_ty(&qself.ty));
 141                 }
 142
 143                 // Note that there should be a call to visit_path here,
 144                 // so if any logic is added to process `Path`s a call to it should be
 145                 // added both in visit_path and here. This code mirrors visit::walk_path.
 146                 for (i, segment) in path.segments.iter().enumerate() {
 147                     // Allow `impl Trait` iff we're on the final path segment
 148                     if i == path.segments.len() - 1 {
 149                         self.visit_path_segment(path.span, segment);
 150                     } else {
 151                         self.with_banned_impl_trait(|this| {
 152                             this.visit_path_segment(path.span, segment)
 153                         });
 154                     }
 155                 }
 156             }
 157             _ => visit::walk_ty(self, t),
 158         }
 159     }
 160
 161     fn err_handler(&self) -> &errors::Handler {
 162         &self.session.diagnostic()
 163     }
 164
 165     fn check_lifetime(&self, ident: Ident) {
 166         let valid_names = [kw::UnderscoreLifetime, kw::StaticLifetime, kw::Invalid];
 167         if !valid_names.contains(&ident.name) && ident.without_first_quote().is_reserved() {
 168             self.err_handler().span_err(ident.span, "lifetimes cannot use keyword names");
 169         }
 170     }
 171
 172     fn check_label(&self, ident: Ident) {
 173         if ident.without_first_quote().is_reserved() {
 174             self.err_handler()
 175                 .span_err(ident.span, &format!("invalid label name `{}`", ident.name));
 176         }
 177     }
 178
 179     fn invalid_visibility(&self, vis: &Visibility, note: Option<&str>) {
 180         if let VisibilityKind::Inherited = vis.node {
 181             return;
 182         }
 183
 184         let mut err =
 185             struct_span_err!(self.session, vis.span, E0449, "unnecessary visibility qualifier");
 186         if vis.node.is_pub() {
 187             err.span_label(vis.span, "`pub` not permitted here because it's implied");
 188         }
 189         if let Some(note) = note {
 190             err.note(note);
 191         }
 192         err.emit();
 193     }
 194
 195     fn check_decl_no_pat(decl: &FnDecl, mut report_err: impl FnMut(Span, bool)) {
 196         for Param { pat, .. } in &decl.inputs {
 197             match pat.kind {
 198                 PatKind::Ident(BindingMode::ByValue(Mutability::Not), _, None) | PatKind::Wild => {}
 199                 PatKind::Ident(BindingMode::ByValue(Mutability::Mut), _, None) => {
 200                     report_err(pat.span, true)
 201                 }
 202                 _ => report_err(pat.span, false),
 203             }
 204         }
 205     }
 206
 207     fn check_trait_fn_not_async(&self, span: Span, asyncness: IsAsync) {
 208         if asyncness.is_async() {
 209             struct_span_err!(self.session, span, E0706, "trait fns cannot be declared `async`")
 210                 .note("`async` trait functions are not currently supported")
 211                 .note(
 212                     "consider using the `async-trait` crate: \
 213                        https://crates.io/crates/async-trait",
 214                 )
 215                 .emit();
 216         }
 217     }
 218
 219     fn check_trait_fn_not_const(&self, constness: Spanned<Constness>) {
 220         if constness.node == Constness::Const {
 221             struct_span_err!(
 222                 self.session,
 223                 constness.span,
 224                 E0379,
 225                 "trait fns cannot be declared const"
 226             )
 227             .span_label(constness.span, "trait fns cannot be const")
 228             .emit();
 229         }
 230     }
 231
 232     // FIXME(ecstaticmorse): Instead, use `bound_context` to check this in `visit_param_bound`.
 233     fn no_questions_in_bounds(&self, bounds: &GenericBounds, where_: &str, is_trait: bool) {
 234         for bound in bounds {
 235             if let GenericBound::Trait(ref poly, TraitBoundModifier::Maybe) = *bound {
 236                 let mut err = self.err_handler().struct_span_err(
 237                     poly.span,
 238                     &format!("`?Trait` is not permitted in {}", where_),
 239                 );
 240                 if is_trait {
 241                     let path_str = pprust::path_to_string(&poly.trait_ref.path);
 242                     err.note(&format!("traits are `?{}` by default", path_str));
 243                 }
 244                 err.emit();
 245             }
 246         }
 247     }
 248
 249     /// Matches `'-' lit | lit (cf. parser::Parser::parse_literal_maybe_minus)`,
 250     /// or paths for ranges.
 251     //
 252     // FIXME: do we want to allow `expr -> pattern` conversion to create path expressions?
 253     // That means making this work:
 254     //
 255     // ```rust,ignore (FIXME)
 256     // struct S;
 257     // macro_rules! m {
 258     //     ($a:expr) => {
 259     //         let $a = S;
 260     //     }
 261     // }
 262     // m!(S);
 263     // ```
 264     fn check_expr_within_pat(&self, expr: &Expr, allow_paths: bool) {
 265         match expr.kind {
 266             ExprKind::Lit(..) | ExprKind::Err => {}
 267             ExprKind::Path(..) if allow_paths => {}
 268             ExprKind::Unary(UnOp::Neg, ref inner)
 269                 if match inner.kind {
 270                     ExprKind::Lit(_) => true,
 271                     _ => false,
 272                 } => {}
 273             _ => self.err_handler().span_err(
 274                 expr.span,
 275                 "arbitrary expressions aren't allowed \
 276                                                          in patterns",
 277             ),
 278         }
 279     }
 280
 281     fn check_late_bound_lifetime_defs(&self, params: &[GenericParam]) {
 282         // Check only lifetime parameters are present and that the lifetime
 283         // parameters that are present have no bounds.
 284         let non_lt_param_spans: Vec<_> = params
 285             .iter()
 286             .filter_map(|param| match param.kind {
 287                 GenericParamKind::Lifetime { .. } => {
 288                     if !param.bounds.is_empty() {
 289                         let spans: Vec<_> = param.bounds.iter().map(|b| b.span()).collect();
 290                         self.err_handler()
 291                             .span_err(spans, "lifetime bounds cannot be used in this context");
 292                     }
 293                     None
 294                 }
 295                 _ => Some(param.ident.span),
 296             })
 297             .collect();
 298         if !non_lt_param_spans.is_empty() {
 299             self.err_handler().span_err(
 300                 non_lt_param_spans,
 301                 "only lifetime parameters can be used in this context",
 302             );
 303         }
 304     }
 305
 306     fn check_fn_decl(&self, fn_decl: &FnDecl) {
 307         match &*fn_decl.inputs {
 308             [Param { ty, span, .. }] => {
 309                 if let TyKind::CVarArgs = ty.kind {
 310                     self.err_handler().span_err(
 311                         *span,
 312                         "C-variadic function must be declared with at least one named argument",
 313                     );
 314                 }
 315             }
 316             [ps @ .., _] => {
 317                 for Param { ty, span, .. } in ps {
 318                     if let TyKind::CVarArgs = ty.kind {
 319                         self.err_handler().span_err(
 320                             *span,
 321                             "`...` must be the last argument of a C-variadic function",
 322                         );
 323                     }
 324                 }
 325             }
 326             _ => {}
 327         }
 328
 329         fn_decl
 330             .inputs
 331             .iter()
 332             .flat_map(|i| i.attrs.as_ref())
 333             .filter(|attr| {
 334                 let arr = [sym::allow, sym::cfg, sym::cfg_attr, sym::deny, sym::forbid, sym::warn];
 335                 !arr.contains(&attr.name_or_empty()) && attr::is_builtin_attr(attr)
 336             })
 337             .for_each(|attr| {
 338                 if attr.is_doc_comment() {
 339                     self.err_handler()
 340                         .struct_span_err(
 341                             attr.span,
 342                             "documentation comments cannot be applied to function parameters",
 343                         )
 344                         .span_label(attr.span, "doc comments are not allowed here")
 345                         .emit();
 346                 } else {
 347                     self.err_handler().span_err(
 348                         attr.span,
 349                         "allow, cfg, cfg_attr, deny, \
 350                 forbid, and warn are the only allowed built-in attributes in function parameters",
 351                     )
 352                 }
 353             });
 354     }
 355
 356     fn check_defaultness(&self, span: Span, defaultness: Defaultness) {
 357         if let Defaultness::Default = defaultness {
 358             self.err_handler()
 359                 .struct_span_err(span, "`default` is only allowed on items in `impl` definitions")
 360                 .emit();
 361         }
 362     }
 363
 364     fn check_impl_item_provided<T>(&self, sp: Span, body: &Option<T>, ctx: &str, sugg: &str) {
 365         if body.is_some() {
 366             return;
 367         }
 368
 369         self.err_handler()
 370             .struct_span_err(sp, &format!("associated {} in `impl` without body", ctx))
 371             .span_suggestion(
 372                 self.session.source_map().end_point(sp),
 373                 &format!("provide a definition for the {}", ctx),
 374                 sugg.to_string(),
 375                 Applicability::HasPlaceholders,
 376             )
 377             .emit();
 378     }
 379
 380     fn check_impl_assoc_type_no_bounds(&self, bounds: &[GenericBound]) {
 381         let span = match bounds {
 382             [] => return,
 383             [b0] => b0.span(),
 384             [b0, .., bl] => b0.span().to(bl.span()),
 385         };
 386         self.err_handler()
 387             .struct_span_err(span, "bounds on associated `type`s in `impl`s have no effect")
 388             .emit();
 389     }
 390
 391     fn check_c_varadic_type(&self, decl: &FnDecl) {
 392         for Param { ty, span, .. } in &decl.inputs {
 393             if let TyKind::CVarArgs = ty.kind {
 394                 self.err_handler()
 395                     .struct_span_err(
 396                         *span,
 397                         "only foreign or `unsafe extern \"C\" functions may be C-variadic",
 398                     )
 399                     .emit();
 400             }
 401         }
 402     }
 403 }
 404
 405 enum GenericPosition {
 406     Param,
 407     Arg,
 408 }
 409
 410 fn validate_generics_order<'a>(
 411     sess: &Session,
 412     handler: &errors::Handler,
 413     generics: impl Iterator<Item = (ParamKindOrd, Option<&'a [GenericBound]>, Span, Option<String>)>,
 414     pos: GenericPosition,
 415     span: Span,
 416 ) {
 417     let mut max_param: Option<ParamKindOrd> = None;
 418     let mut out_of_order = FxHashMap::default();
 419     let mut param_idents = vec![];
 420     let mut found_type = false;
 421     let mut found_const = false;
 422
 423     for (kind, bounds, span, ident) in generics {
 424         if let Some(ident) = ident {
 425             param_idents.push((kind, bounds, param_idents.len(), ident));
 426         }
 427         let max_param = &mut max_param;
 428         match max_param {
 429             Some(max_param) if *max_param > kind => {
 430                 let entry = out_of_order.entry(kind).or_insert((*max_param, vec![]));
 431                 entry.1.push(span);
 432             }
 433             Some(_) | None => *max_param = Some(kind),
 434         };
 435         match kind {
 436             ParamKindOrd::Type => found_type = true,
 437             ParamKindOrd::Const => found_const = true,
 438             _ => {}
 439         }
 440     }
 441
 442     let mut ordered_params = "<".to_string();
 443     if !out_of_order.is_empty() {
 444         param_idents.sort_by_key(|&(po, _, i, _)| (po, i));
 445         let mut first = true;
 446         for (_, bounds, _, ident) in param_idents {
 447             if !first {
 448                 ordered_params += ", ";
 449             }
 450             ordered_params += &ident;
 451             if let Some(bounds) = bounds {
 452                 if !bounds.is_empty() {
 453                     ordered_params += ": ";
 454                     ordered_params += &pprust::bounds_to_string(&bounds);
 455                 }
 456             }
 457             first = false;
 458         }
 459     }
 460     ordered_params += ">";
 461
 462     let pos_str = match pos {
 463         GenericPosition::Param => "parameter",
 464         GenericPosition::Arg => "argument",
 465     };
 466
 467     for (param_ord, (max_param, spans)) in &out_of_order {
 468         let mut err = handler.struct_span_err(
 469             spans.clone(),
 470             &format!(
 471                 "{} {pos}s must be declared prior to {} {pos}s",
 472                 param_ord,
 473                 max_param,
 474                 pos = pos_str,
 475             ),
 476         );
 477         if let GenericPosition::Param = pos {
 478             err.span_suggestion(
 479                 span,
 480                 &format!(
 481                     "reorder the {}s: lifetimes, then types{}",
 482                     pos_str,
 483                     if sess.features_untracked().const_generics { ", then consts" } else { "" },
 484                 ),
 485                 ordered_params.clone(),
 486                 Applicability::MachineApplicable,
 487             );
 488         }
 489         err.emit();
 490     }
 491
 492     // FIXME(const_generics): we shouldn't have to abort here at all, but we currently get ICEs
 493     // if we don't. Const parameters and type parameters can currently conflict if they
 494     // are out-of-order.
 495     if !out_of_order.is_empty() && found_type && found_const {
 496         FatalError.raise();
 497     }
 498 }
 499
 500 impl<'a> Visitor<'a> for AstValidator<'a> {
 501     fn visit_attribute(&mut self, attr: &Attribute) {
 502         validate_attr::check_meta(&self.session.parse_sess, attr);
 503     }
 504
 505     fn visit_expr(&mut self, expr: &'a Expr) {
 506         match &expr.kind {
 507             ExprKind::Closure(_, _, _, fn_decl, _, _) => {
 508                 self.check_fn_decl(fn_decl);
 509             }
 510             ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
 511                 struct_span_err!(
 512                     self.session,
 513                     expr.span,
 514                     E0472,
 515                     "asm! is unsupported on this target"
 516                 )
 517                 .emit();
 518             }
 519             _ => {}
 520         }
 521
 522         visit::walk_expr(self, expr);
 523     }
 524
 525     fn visit_ty(&mut self, ty: &'a Ty) {
 526         match ty.kind {
 527             TyKind::BareFn(ref bfty) => {
 528                 self.check_fn_decl(&bfty.decl);
 529                 Self::check_decl_no_pat(&bfty.decl, |span, _| {
 530                     struct_span_err!(
 531                         self.session,
 532                         span,
 533                         E0561,
 534                         "patterns aren't allowed in function pointer types"
 535                     )
 536                     .emit();
 537                 });
 538                 self.check_late_bound_lifetime_defs(&bfty.generic_params);
 539             }
 540             TyKind::TraitObject(ref bounds, ..) => {
 541                 let mut any_lifetime_bounds = false;
 542                 for bound in bounds {
 543                     if let GenericBound::Outlives(ref lifetime) = *bound {
 544                         if any_lifetime_bounds {
 545                             struct_span_err!(
 546                                 self.session,
 547                                 lifetime.ident.span,
 548                                 E0226,
 549                                 "only a single explicit lifetime bound is permitted"
 550                             )
 551                             .emit();
 552                             break;
 553                         }
 554                         any_lifetime_bounds = true;
 555                     }
 556                 }
 557                 self.no_questions_in_bounds(bounds, "trait object types", false);
 558             }
 559             TyKind::ImplTrait(_, ref bounds) => {
 560                 if self.is_impl_trait_banned {
 561                     struct_span_err!(
 562                         self.session,
 563                         ty.span,
 564                         E0667,
 565                         "`impl Trait` is not allowed in path parameters"
 566                     )
 567                     .emit();
 568                 }
 569
 570                 if let Some(outer_impl_trait_sp) = self.outer_impl_trait {
 571                     struct_span_err!(
 572                         self.session,
 573                         ty.span,
 574                         E0666,
 575                         "nested `impl Trait` is not allowed"
 576                     )
 577                     .span_label(outer_impl_trait_sp, "outer `impl Trait`")
 578                     .span_label(ty.span, "nested `impl Trait` here")
 579                     .emit();
 580                 }
 581
 582                 if !bounds
 583                     .iter()
 584                     .any(|b| if let GenericBound::Trait(..) = *b { true } else { false })
 585                 {
 586                     self.err_handler().span_err(ty.span, "at least one trait must be specified");
 587                 }
 588
 589                 self.walk_ty(ty);
 590                 return;
 591             }
 592             _ => {}
 593         }
 594
 595         self.walk_ty(ty)
 596     }
 597
 598     fn visit_label(&mut self, label: &'a Label) {
 599         self.check_label(label.ident);
 600         visit::walk_label(self, label);
 601     }
 602
 603     fn visit_lifetime(&mut self, lifetime: &'a Lifetime) {
 604         self.check_lifetime(lifetime.ident);
 605         visit::walk_lifetime(self, lifetime);
 606     }
 607
 608     fn visit_item(&mut self, item: &'a Item) {
 609         if item.attrs.iter().any(|attr| is_proc_macro_attr(attr)) {
 610             self.has_proc_macro_decls = true;
 611         }
 612
 613         match item.kind {
 614             ItemKind::Impl(unsafety, polarity, _, _, Some(..), ref ty, ref impl_items) => {
 615                 self.invalid_visibility(&item.vis, None);
 616                 if let TyKind::Err = ty.kind {
 617                     self.err_handler()
 618                         .struct_span_err(item.span, "`impl Trait for .. {}` is an obsolete syntax")
 619                         .help("use `auto trait Trait {}` instead")
 620                         .emit();
 621                 }
 622                 if unsafety == Unsafety::Unsafe && polarity == ImplPolarity::Negative {
 623                     struct_span_err!(
 624                         self.session,
 625                         item.span,
 626                         E0198,
 627                         "negative impls cannot be unsafe"
 628                     )
 629                     .emit();
 630                 }
 631                 for impl_item in impl_items {
 632                     self.invalid_visibility(&impl_item.vis, None);
 633                     if let AssocItemKind::Fn(ref sig, _) = impl_item.kind {
 634                         self.check_trait_fn_not_const(sig.header.constness);
 635                         self.check_trait_fn_not_async(impl_item.span, sig.header.asyncness.node);
 636                     }
 637                 }
 638             }
 639             ItemKind::Impl(unsafety, polarity, defaultness, _, None, _, _) => {
 640                 self.invalid_visibility(
 641                     &item.vis,
 642                     Some("place qualifiers on individual impl items instead"),
 643                 );
 644                 if unsafety == Unsafety::Unsafe {
 645                     struct_span_err!(
 646                         self.session,
 647                         item.span,
 648                         E0197,
 649                         "inherent impls cannot be unsafe"
 650                     )
 651                     .emit();
 652                 }
 653                 if polarity == ImplPolarity::Negative {
 654                     self.err_handler().span_err(item.span, "inherent impls cannot be negative");
 655                 }
 656                 if defaultness == Defaultness::Default {
 657                     self.err_handler()
 658                         .struct_span_err(item.span, "inherent impls cannot be default")
 659                         .note("only trait implementations may be annotated with default")
 660                         .emit();
 661                 }
 662             }
 663             ItemKind::Fn(ref sig, ref generics, _) => {
 664                 self.visit_fn_header(&sig.header);
 665                 self.check_fn_decl(&sig.decl);
 666                 // We currently do not permit const generics in `const fn`, as
 667                 // this is tantamount to allowing compile-time dependent typing.
 668                 if sig.header.constness.node == Constness::Const {
 669                     // Look for const generics and error if we find any.
 670                     for param in &generics.params {
 671                         match param.kind {
 672                             GenericParamKind::Const { .. } => {
 673                                 self.err_handler()
 674                                     .struct_span_err(
 675                                         item.span,
 676                                         "const parameters are not permitted in `const fn`",
 677                                     )
 678                                     .emit();
 679                             }
 680                             _ => {}
 681                         }
 682                     }
 683                 }
 684                 // Reject C-varadic type unless the function is `unsafe extern "C"` semantically.
 685                 match sig.header.ext {
 686                     Extern::Explicit(StrLit { symbol_unescaped: sym::C, .. })
 687                     | Extern::Implicit
 688                         if sig.header.unsafety == Unsafety::Unsafe => {}
 689                     _ => self.check_c_varadic_type(&sig.decl),
 690                 }
 691             }
 692             ItemKind::ForeignMod(..) => {
 693                 self.invalid_visibility(
 694                     &item.vis,
 695                     Some("place qualifiers on individual foreign items instead"),
 696                 );
 697             }
 698             ItemKind::Enum(ref def, _) => {
 699                 for variant in &def.variants {
 700                     self.invalid_visibility(&variant.vis, None);
 701                     for field in variant.data.fields() {
 702                         self.invalid_visibility(&field.vis, None);
 703                     }
 704                 }
 705             }
 706             ItemKind::Trait(is_auto, _, ref generics, ref bounds, ref trait_items) => {
 707                 if is_auto == IsAuto::Yes {
 708                     // Auto traits cannot have generics, super traits nor contain items.
 709                     if !generics.params.is_empty() {
 710                         struct_span_err!(
 711                             self.session,
 712                             item.span,
 713                             E0567,
 714                             "auto traits cannot have generic parameters"
 715                         )
 716                         .emit();
 717                     }
 718                     if !bounds.is_empty() {
 719                         struct_span_err!(
 720                             self.session,
 721                             item.span,
 722                             E0568,
 723                             "auto traits cannot have super traits"
 724                         )
 725                         .emit();
 726                     }
 727                     if !trait_items.is_empty() {
 728                         struct_span_err!(
 729                             self.session,
 730                             item.span,
 731                             E0380,
 732                             "auto traits cannot have methods or associated items"
 733                         )
 734                         .emit();
 735                     }
 736                 }
 737                 self.no_questions_in_bounds(bounds, "supertraits", true);
 738
 739                 // Equivalent of `visit::walk_item` for `ItemKind::Trait` that inserts a bound
 740                 // context for the supertraits.
 741                 self.visit_vis(&item.vis);
 742                 self.visit_ident(item.ident);
 743                 self.visit_generics(generics);
 744                 self.with_bound_context(BoundContext::TraitBounds, |this| {
 745                     walk_list!(this, visit_param_bound, bounds);
 746                 });
 747                 walk_list!(self, visit_trait_item, trait_items);
 748                 walk_list!(self, visit_attribute, &item.attrs);
 749                 return;
 750             }
 751             ItemKind::Mod(_) => {
 752                 // Ensure that `path` attributes on modules are recorded as used (cf. issue #35584).
 753                 attr::first_attr_value_str_by_name(&item.attrs, sym::path);
 754             }
 755             ItemKind::Union(ref vdata, _) => {
 756                 if let VariantData::Tuple(..) | VariantData::Unit(..) = vdata {
 757                     self.err_handler()
 758                         .span_err(item.span, "tuple and unit unions are not permitted");
 759                 }
 760                 if vdata.fields().is_empty() {
 761                     self.err_handler().span_err(item.span, "unions cannot have zero fields");
 762                 }
 763             }
 764             _ => {}
 765         }
 766
 767         visit::walk_item(self, item)
 768     }
 769
 770     fn visit_foreign_item(&mut self, fi: &'a ForeignItem) {
 771         match fi.kind {
 772             ForeignItemKind::Fn(ref decl, _) => {
 773                 self.check_fn_decl(decl);
 774                 Self::check_decl_no_pat(decl, |span, _| {
 775                     struct_span_err!(
 776                         self.session,
 777                         span,
 778                         E0130,
 779                         "patterns aren't allowed in foreign function declarations"
 780                     )
 781                     .span_label(span, "pattern not allowed in foreign function")
 782                     .emit();
 783                 });
 784             }
 785             ForeignItemKind::Static(..) | ForeignItemKind::Ty | ForeignItemKind::Macro(..) => {}
 786         }
 787
 788         visit::walk_foreign_item(self, fi)
 789     }
 790
 791     // Mirrors `visit::walk_generic_args`, but tracks relevant state.
 792     fn visit_generic_args(&mut self, _: Span, generic_args: &'a GenericArgs) {
 793         match *generic_args {
 794             GenericArgs::AngleBracketed(ref data) => {
 795                 walk_list!(self, visit_generic_arg, &data.args);
 796                 validate_generics_order(
 797                     self.session,
 798                     self.err_handler(),
 799                     data.args.iter().map(|arg| {
 800                         (
 801                             match arg {
 802                                 GenericArg::Lifetime(..) => ParamKindOrd::Lifetime,
 803                                 GenericArg::Type(..) => ParamKindOrd::Type,
 804                                 GenericArg::Const(..) => ParamKindOrd::Const,
 805                             },
 806                             None,
 807                             arg.span(),
 808                             None,
 809                         )
 810                     }),
 811                     GenericPosition::Arg,
 812                     generic_args.span(),
 813                 );
 814
 815                 // Type bindings such as `Item = impl Debug` in `Iterator<Item = Debug>`
 816                 // are allowed to contain nested `impl Trait`.
 817                 self.with_impl_trait(None, |this| {
 818                     walk_list!(
 819                         this,
 820                         visit_assoc_ty_constraint_from_generic_args,
 821                         &data.constraints
 822                     );
 823                 });
 824             }
 825             GenericArgs::Parenthesized(ref data) => {
 826                 walk_list!(self, visit_ty, &data.inputs);
 827                 if let FunctionRetTy::Ty(ty) = &data.output {
 828                     // `-> Foo` syntax is essentially an associated type binding,
 829                     // so it is also allowed to contain nested `impl Trait`.
 830                     self.with_impl_trait(None, |this| this.visit_ty(ty));
 831                 }
 832             }
 833         }
 834     }
 835
 836     fn visit_generics(&mut self, generics: &'a Generics) {
 837         let mut prev_ty_default = None;
 838         for param in &generics.params {
 839             if let GenericParamKind::Type { ref default, .. } = param.kind {
 840                 if default.is_some() {
 841                     prev_ty_default = Some(param.ident.span);
 842                 } else if let Some(span) = prev_ty_default {
 843                     self.err_handler()
 844                         .span_err(span, "type parameters with a default must be trailing");
 845                     break;
 846                 }
 847             }
 848         }
 849
 850         validate_generics_order(
 851             self.session,
 852             self.err_handler(),
 853             generics.params.iter().map(|param| {
 854                 let ident = Some(param.ident.to_string());
 855                 let (kind, ident) = match &param.kind {
 856                     GenericParamKind::Lifetime { .. } => (ParamKindOrd::Lifetime, ident),
 857                     GenericParamKind::Type { .. } => (ParamKindOrd::Type, ident),
 858                     GenericParamKind::Const { ref ty } => {
 859                         let ty = pprust::ty_to_string(ty);
 860                         (ParamKindOrd::Const, Some(format!("const {}: {}", param.ident, ty)))
 861                     }
 862                 };
 863                 (kind, Some(&*param.bounds), param.ident.span, ident)
 864             }),
 865             GenericPosition::Param,
 866             generics.span,
 867         );
 868
 869         for predicate in &generics.where_clause.predicates {
 870             if let WherePredicate::EqPredicate(ref predicate) = *predicate {
 871                 self.err_handler()
 872                     .struct_span_err(
 873                         predicate.span,
 874                         "equality constraints are not yet supported in `where` clauses",
 875                     )
 876                     .span_label(predicate.span, "not supported")
 877                     .note(
 878                         "for more information, see https://github.com/rust-lang/rust/issues/20041",
 879                     )
 880                     .emit();
 881             }
 882         }
 883
 884         visit::walk_generics(self, generics)
 885     }
 886
 887     fn visit_generic_param(&mut self, param: &'a GenericParam) {
 888         if let GenericParamKind::Lifetime { .. } = param.kind {
 889             self.check_lifetime(param.ident);
 890         }
 891         visit::walk_generic_param(self, param);
 892     }
 893
 894     fn visit_param_bound(&mut self, bound: &'a GenericBound) {
 895         if let GenericBound::Trait(poly, maybe_bound) = bound {
 896             match poly.trait_ref.constness {
 897                 Some(Constness::NotConst) => {
 898                     if *maybe_bound == TraitBoundModifier::Maybe {
 899                         self.err_handler()
 900                             .span_err(bound.span(), "`?const` and `?` are mutually exclusive");
 901                     }
 902
 903                     if let Some(ctx) = self.bound_context {
 904                         let msg = format!("`?const` is not permitted in {}", ctx.description());
 905                         self.err_handler().span_err(bound.span(), &msg);
 906                     }
 907                 }
 908
 909                 Some(Constness::Const) => bug!("Parser should reject bare `const` on bounds"),
 910                 None => {}
 911             }
 912         }
 913
 914         visit::walk_param_bound(self, bound)
 915     }
 916
 917     fn visit_pat(&mut self, pat: &'a Pat) {
 918         match pat.kind {
 919             PatKind::Lit(ref expr) => {
 920                 self.check_expr_within_pat(expr, false);
 921             }
 922             PatKind::Range(ref start, ref end, _) => {
 923                 self.check_expr_within_pat(start, true);
 924                 self.check_expr_within_pat(end, true);
 925             }
 926             _ => {}
 927         }
 928
 929         visit::walk_pat(self, pat)
 930     }
 931
 932     fn visit_where_predicate(&mut self, p: &'a WherePredicate) {
 933         if let &WherePredicate::BoundPredicate(ref bound_predicate) = p {
 934             // A type binding, eg `for<'c> Foo: Send+Clone+'c`
 935             self.check_late_bound_lifetime_defs(&bound_predicate.bound_generic_params);
 936         }
 937         visit::walk_where_predicate(self, p);
 938     }
 939
 940     fn visit_poly_trait_ref(&mut self, t: &'a PolyTraitRef, m: &'a TraitBoundModifier) {
 941         self.check_late_bound_lifetime_defs(&t.bound_generic_params);
 942         visit::walk_poly_trait_ref(self, t, m);
 943     }
 944
 945     fn visit_variant_data(&mut self, s: &'a VariantData) {
 946         self.with_banned_assoc_ty_bound(|this| visit::walk_struct_def(this, s))
 947     }
 948
 949     fn visit_enum_def(
 950         &mut self,
 951         enum_definition: &'a EnumDef,
 952         generics: &'a Generics,
 953         item_id: NodeId,
 954         _: Span,
 955     ) {
 956         self.with_banned_assoc_ty_bound(|this| {
 957             visit::walk_enum_def(this, enum_definition, generics, item_id)
 958         })
 959     }
 960
 961     fn visit_impl_item(&mut self, ii: &'a AssocItem) {
 962         match &ii.kind {
 963             AssocItemKind::Const(_, body) => {
 964                 self.check_impl_item_provided(ii.span, body, "constant", " = <expr>;");
 965             }
 966             AssocItemKind::Fn(sig, body) => {
 967                 self.check_impl_item_provided(ii.span, body, "function", " { <body> }");
 968                 self.check_fn_decl(&sig.decl);
 969             }
 970             AssocItemKind::TyAlias(bounds, body) => {
 971                 self.check_impl_item_provided(ii.span, body, "type", " = <type>;");
 972                 self.check_impl_assoc_type_no_bounds(bounds);
 973             }
 974             _ => {}
 975         }
 976         visit::walk_impl_item(self, ii);
 977     }
 978
 979     fn visit_trait_item(&mut self, ti: &'a AssocItem) {
 980         self.invalid_visibility(&ti.vis, None);
 981         self.check_defaultness(ti.span, ti.defaultness);
 982
 983         if let AssocItemKind::Fn(sig, block) = &ti.kind {
 984             self.check_fn_decl(&sig.decl);
 985             self.check_trait_fn_not_async(ti.span, sig.header.asyncness.node);
 986             self.check_trait_fn_not_const(sig.header.constness);
 987             if block.is_none() {
 988                 Self::check_decl_no_pat(&sig.decl, |span, mut_ident| {
 989                     if mut_ident {
 990                         self.lint_buffer.buffer_lint(
 991                             lint::builtin::PATTERNS_IN_FNS_WITHOUT_BODY,
 992                             ti.id,
 993                             span,
 994                             "patterns aren't allowed in methods without bodies",
 995                         );
 996                     } else {
 997                         struct_span_err!(
 998                             self.session,
 999                             span,
1000                             E0642,
1001                             "patterns aren't allowed in methods without bodies"
1002                         )
1003                         .emit();
1004                     }
1005                 });
1006             }
1007         }
1008
1009         visit::walk_trait_item(self, ti);
1010     }
1011
1012     fn visit_assoc_item(&mut self, item: &'a AssocItem) {
1013         if let AssocItemKind::Fn(sig, _) = &item.kind {
1014             self.check_c_varadic_type(&sig.decl);
1015         }
1016         visit::walk_assoc_item(self, item);
1017     }
1018 }
1019
1020 pub fn check_crate(session: &Session, krate: &Crate, lints: &mut lint::LintBuffer) -> bool {
1021     let mut validator = AstValidator {
1022         session,
1023         has_proc_macro_decls: false,
1024         outer_impl_trait: None,
1025         bound_context: None,
1026         is_impl_trait_banned: false,
1027         is_assoc_ty_bound_banned: false,
1028         lint_buffer: lints,
1029     };
1030     visit::walk_crate(&mut validator, krate);
1031
1032     validator.has_proc_macro_decls
1033 }