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 std::mem;
  10 use syntax::print::pprust;
  11 use rustc::lint;
  12 use rustc::lint::builtin::{BuiltinLintDiagnostics, NESTED_IMPL_TRAIT};
  13 use rustc::session::Session;
  14 use rustc_data_structures::fx::FxHashMap;
  15 use syntax::ast::*;
  16 use syntax::attr;
  17 use syntax::source_map::Spanned;
  18 use syntax::symbol::{kw, sym};
  19 use syntax::ptr::P;
  20 use syntax::visit::{self, Visitor};
  21 use syntax::{span_err, struct_span_err, walk_list};
  22 use syntax_ext::proc_macro_decls::is_proc_macro_attr;
  23 use syntax_pos::{Span, MultiSpan};
  24 use errors::{Applicability, FatalError};
  25 use log::debug;
  26
  27 #[derive(Copy, Clone, Debug)]
  28 struct OuterImplTrait {
  29     span: Span,
  30
  31     /// rust-lang/rust#57979: a bug in original implementation caused
  32     /// us to fail sometimes to record an outer `impl Trait`.
  33     /// Therefore, in order to reliably issue a warning (rather than
  34     /// an error) in the *precise* places where we are newly injecting
  35     /// the diagnostic, we have to distinguish between the places
  36     /// where the outer `impl Trait` has always been recorded, versus
  37     /// the places where it has only recently started being recorded.
  38     only_recorded_since_pull_request_57730: bool,
  39 }
  40
  41 impl OuterImplTrait {
  42     /// This controls whether we should downgrade the nested impl
  43     /// trait diagnostic to a warning rather than an error, based on
  44     /// whether the outer impl trait had been improperly skipped in
  45     /// earlier implementations of the analysis on the stable
  46     /// compiler.
  47     fn should_warn_instead_of_error(&self) -> bool {
  48         self.only_recorded_since_pull_request_57730
  49     }
  50 }
  51
  52 struct AstValidator<'a> {
  53     session: &'a Session,
  54     has_proc_macro_decls: bool,
  55     has_global_allocator: bool,
  56
  57     /// Used to ban nested `impl Trait`, e.g., `impl Into<impl Debug>`.
  58     /// Nested `impl Trait` _is_ allowed in associated type position,
  59     /// e.g `impl Iterator<Item=impl Debug>`
  60     outer_impl_trait: Option<OuterImplTrait>,
  61
  62     /// Used to ban `impl Trait` in path projections like `<impl Iterator>::Item`
  63     /// or `Foo::Bar<impl Trait>`
  64     is_impl_trait_banned: bool,
  65
  66     /// rust-lang/rust#57979: the ban of nested `impl Trait` was buggy
  67     /// until PRs #57730 and #57981 landed: it would jump directly to
  68     /// walk_ty rather than visit_ty (or skip recurring entirely for
  69     /// impl trait in projections), and thus miss some cases. We track
  70     /// whether we should downgrade to a warning for short-term via
  71     /// these booleans.
  72     warning_period_57979_didnt_record_next_impl_trait: bool,
  73     warning_period_57979_impl_trait_in_proj: bool,
  74 }
  75
  76 impl<'a> AstValidator<'a> {
  77     fn with_impl_trait_in_proj_warning<T>(&mut self, v: bool, f: impl FnOnce(&mut Self) -> T) -> T {
  78         let old = mem::replace(&mut self.warning_period_57979_impl_trait_in_proj, v);
  79         let ret = f(self);
  80         self.warning_period_57979_impl_trait_in_proj = old;
  81         ret
  82     }
  83
  84     fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
  85         let old = mem::replace(&mut self.is_impl_trait_banned, true);
  86         f(self);
  87         self.is_impl_trait_banned = old;
  88     }
  89
  90     fn with_impl_trait(&mut self, outer: Option<OuterImplTrait>, f: impl FnOnce(&mut Self)) {
  91         let old = mem::replace(&mut self.outer_impl_trait, outer);
  92         f(self);
  93         self.outer_impl_trait = old;
  94     }
  95
  96     fn visit_assoc_type_binding_from_generic_args(&mut self, type_binding: &'a TypeBinding) {
  97         // rust-lang/rust#57979: bug in old visit_generic_args called
  98         // walk_ty rather than visit_ty, skipping outer `impl Trait`
  99         // if it happened to occur at `type_binding.ty`
 100         if let TyKind::ImplTrait(..) = type_binding.ty.node {
 101             self.warning_period_57979_didnt_record_next_impl_trait = true;
 102         }
 103         self.visit_assoc_type_binding(type_binding);
 104     }
 105
 106     fn visit_ty_from_generic_args(&mut self, ty: &'a Ty) {
 107         // rust-lang/rust#57979: bug in old visit_generic_args called
 108         // walk_ty rather than visit_ty, skippping outer `impl Trait`
 109         // if it happened to occur at `ty`
 110         if let TyKind::ImplTrait(..) = ty.node {
 111             self.warning_period_57979_didnt_record_next_impl_trait = true;
 112         }
 113         self.visit_ty(ty);
 114     }
 115
 116     fn outer_impl_trait(&mut self, span: Span) -> OuterImplTrait {
 117         let only_recorded_since_pull_request_57730 =
 118             self.warning_period_57979_didnt_record_next_impl_trait;
 119
 120         // (this flag is designed to be set to true and then only
 121         // reach the construction point for the outer impl trait once,
 122         // so its safe and easiest to unconditionally reset it to
 123         // false)
 124         self.warning_period_57979_didnt_record_next_impl_trait = false;
 125
 126         OuterImplTrait {
 127             span, only_recorded_since_pull_request_57730,
 128         }
 129     }
 130
 131     // Mirrors visit::walk_ty, but tracks relevant state
 132     fn walk_ty(&mut self, t: &'a Ty) {
 133         match t.node {
 134             TyKind::ImplTrait(..) => {
 135                 let outer_impl_trait = self.outer_impl_trait(t.span);
 136                 self.with_impl_trait(Some(outer_impl_trait), |this| visit::walk_ty(this, t))
 137             }
 138             TyKind::Path(ref qself, ref path) => {
 139                 // We allow these:
 140                 //  - `Option<impl Trait>`
 141                 //  - `option::Option<impl Trait>`
 142                 //  - `option::Option<T>::Foo<impl Trait>
 143                 //
 144                 // But not these:
 145                 //  - `<impl Trait>::Foo`
 146                 //  - `option::Option<impl Trait>::Foo`.
 147                 //
 148                 // To implement this, we disallow `impl Trait` from `qself`
 149                 // (for cases like `<impl Trait>::Foo>`)
 150                 // but we allow `impl Trait` in `GenericArgs`
 151                 // iff there are no more PathSegments.
 152                 if let Some(ref qself) = *qself {
 153                     // `impl Trait` in `qself` is always illegal
 154                     self.with_banned_impl_trait(|this| this.visit_ty(&qself.ty));
 155                 }
 156
 157                 // Note that there should be a call to visit_path here,
 158                 // so if any logic is added to process `Path`s a call to it should be
 159                 // added both in visit_path and here. This code mirrors visit::walk_path.
 160                 for (i, segment) in path.segments.iter().enumerate() {
 161                     // Allow `impl Trait` iff we're on the final path segment
 162                     if i == path.segments.len() - 1 {
 163                         self.visit_path_segment(path.span, segment);
 164                     } else {
 165                         self.with_banned_impl_trait(|this| {
 166                             this.visit_path_segment(path.span, segment)
 167                         });
 168                     }
 169                 }
 170             }
 171             _ => visit::walk_ty(self, t),
 172         }
 173     }
 174
 175     fn err_handler(&self) -> &errors::Handler {
 176         &self.session.diagnostic()
 177     }
 178
 179     fn check_lifetime(&self, ident: Ident) {
 180         let valid_names = [kw::UnderscoreLifetime,
 181                            kw::StaticLifetime,
 182                            kw::Invalid];
 183         if !valid_names.contains(&ident.name) && ident.without_first_quote().is_reserved() {
 184             self.err_handler().span_err(ident.span, "lifetimes cannot use keyword names");
 185         }
 186     }
 187
 188     fn check_label(&self, ident: Ident) {
 189         if ident.without_first_quote().is_reserved() {
 190             self.err_handler()
 191                 .span_err(ident.span, &format!("invalid label name `{}`", ident.name));
 192         }
 193     }
 194
 195     fn invalid_visibility(&self, vis: &Visibility, note: Option<&str>) {
 196         if let VisibilityKind::Inherited = vis.node {
 197             return
 198         }
 199
 200         let mut err = struct_span_err!(self.session,
 201                                         vis.span,
 202                                         E0449,
 203                                         "unnecessary visibility qualifier");
 204         if vis.node.is_pub() {
 205             err.span_label(vis.span, "`pub` not permitted here because it's implied");
 206         }
 207         if let Some(note) = note {
 208             err.note(note);
 209         }
 210         err.emit();
 211     }
 212
 213     fn check_decl_no_pat<ReportFn: Fn(Span, bool)>(&self, decl: &FnDecl, report_err: ReportFn) {
 214         for arg in &decl.inputs {
 215             match arg.pat.node {
 216                 PatKind::Ident(BindingMode::ByValue(Mutability::Immutable), _, None) |
 217                 PatKind::Wild => {}
 218                 PatKind::Ident(BindingMode::ByValue(Mutability::Mutable), _, None) =>
 219                     report_err(arg.pat.span, true),
 220                 _ => report_err(arg.pat.span, false),
 221             }
 222         }
 223     }
 224
 225     fn check_trait_fn_not_async(&self, span: Span, asyncness: &IsAsync) {
 226         if asyncness.is_async() {
 227             struct_span_err!(self.session, span, E0706,
 228                              "trait fns cannot be declared `async`").emit()
 229         }
 230     }
 231
 232     fn check_trait_fn_not_const(&self, constness: Spanned<Constness>) {
 233         if constness.node == Constness::Const {
 234             struct_span_err!(self.session, constness.span, E0379,
 235                              "trait fns cannot be declared const")
 236                 .span_label(constness.span, "trait fns cannot be const")
 237                 .emit();
 238         }
 239     }
 240
 241     fn no_questions_in_bounds(&self, bounds: &GenericBounds, where_: &str, is_trait: bool) {
 242         for bound in bounds {
 243             if let GenericBound::Trait(ref poly, TraitBoundModifier::Maybe) = *bound {
 244                 let mut err = self.err_handler().struct_span_err(poly.span,
 245                     &format!("`?Trait` is not permitted in {}", where_));
 246                 if is_trait {
 247                     err.note(&format!("traits are `?{}` by default", poly.trait_ref.path));
 248                 }
 249                 err.emit();
 250             }
 251         }
 252     }
 253
 254     /// Matches `'-' lit | lit (cf. parser::Parser::parse_literal_maybe_minus)`,
 255     /// or paths for ranges.
 256     //
 257     // FIXME: do we want to allow `expr -> pattern` conversion to create path expressions?
 258     // That means making this work:
 259     //
 260     // ```rust,ignore (FIXME)
 261     // struct S;
 262     // macro_rules! m {
 263     //     ($a:expr) => {
 264     //         let $a = S;
 265     //     }
 266     // }
 267     // m!(S);
 268     // ```
 269     fn check_expr_within_pat(&self, expr: &Expr, allow_paths: bool) {
 270         match expr.node {
 271             ExprKind::Lit(..) => {}
 272             ExprKind::Path(..) if allow_paths => {}
 273             ExprKind::Unary(UnOp::Neg, ref inner)
 274                 if match inner.node { ExprKind::Lit(_) => true, _ => false } => {}
 275             _ => self.err_handler().span_err(expr.span, "arbitrary expressions aren't allowed \
 276                                                          in patterns")
 277         }
 278     }
 279
 280     fn check_late_bound_lifetime_defs(&self, params: &[GenericParam]) {
 281         // Check only lifetime parameters are present and that the lifetime
 282         // parameters that are present have no bounds.
 283         let non_lt_param_spans: Vec<_> = params.iter().filter_map(|param| match param.kind {
 284             GenericParamKind::Lifetime { .. } => {
 285                 if !param.bounds.is_empty() {
 286                     let spans: Vec<_> = param.bounds.iter().map(|b| b.span()).collect();
 287                     self.err_handler()
 288                         .span_err(spans, "lifetime bounds cannot be used in this context");
 289                 }
 290                 None
 291             }
 292             _ => Some(param.ident.span),
 293         }).collect();
 294         if !non_lt_param_spans.is_empty() {
 295             self.err_handler().span_err(non_lt_param_spans,
 296                 "only lifetime parameters can be used in this context");
 297         }
 298     }
 299
 300     /// With eRFC 2497, we need to check whether an expression is ambiguous and warn or error
 301     /// depending on the edition, this function handles that.
 302     fn while_if_let_ambiguity(&self, expr: &P<Expr>) {
 303         if let Some((span, op_kind)) = self.while_if_let_expr_ambiguity(&expr) {
 304             let mut err = self.err_handler().struct_span_err(
 305                 span, &format!("ambiguous use of `{}`", op_kind.to_string())
 306             );
 307
 308             err.note(
 309                 "this will be a error until the `let_chains` feature is stabilized"
 310             );
 311             err.note(
 312                 "see rust-lang/rust#53668 for more information"
 313             );
 314
 315             if let Ok(snippet) = self.session.source_map().span_to_snippet(span) {
 316                 err.span_suggestion(
 317                     span, "consider adding parentheses", format!("({})", snippet),
 318                     Applicability::MachineApplicable,
 319                 );
 320             }
 321
 322             err.emit();
 323         }
 324     }
 325
 326     /// With eRFC 2497 adding if-let chains, there is a requirement that the parsing of
 327     /// `&&` and `||` in a if-let statement be unambiguous. This function returns a span and
 328     /// a `BinOpKind` (either `&&` or `||` depending on what was ambiguous) if it is determined
 329     /// that the current expression parsed is ambiguous and will break in future.
 330     fn while_if_let_expr_ambiguity(&self, expr: &P<Expr>) -> Option<(Span, BinOpKind)> {
 331         debug!("while_if_let_expr_ambiguity: expr.node: {:?}", expr.node);
 332         match &expr.node {
 333             ExprKind::Binary(op, _, _) if op.node == BinOpKind::And || op.node == BinOpKind::Or => {
 334                 Some((expr.span, op.node))
 335             },
 336             ExprKind::Range(ref lhs, ref rhs, _) => {
 337                 let lhs_ambiguous = lhs.as_ref()
 338                     .and_then(|lhs| self.while_if_let_expr_ambiguity(lhs));
 339                 let rhs_ambiguous = rhs.as_ref()
 340                     .and_then(|rhs| self.while_if_let_expr_ambiguity(rhs));
 341
 342                 lhs_ambiguous.or(rhs_ambiguous)
 343             }
 344             _ => None,
 345         }
 346     }
 347 }
 348
 349 enum GenericPosition {
 350     Param,
 351     Arg,
 352 }
 353
 354 fn validate_generics_order<'a>(
 355     sess: &Session,
 356     handler: &errors::Handler,
 357     generics: impl Iterator<
 358         Item = (
 359             ParamKindOrd,
 360             Option<&'a [GenericBound]>,
 361             Span,
 362             Option<String>
 363         ),
 364     >,
 365     pos: GenericPosition,
 366     span: Span,
 367 ) {
 368     let mut max_param: Option<ParamKindOrd> = None;
 369     let mut out_of_order = FxHashMap::default();
 370     let mut param_idents = vec![];
 371     let mut found_type = false;
 372     let mut found_const = false;
 373
 374     for (kind, bounds, span, ident) in generics {
 375         if let Some(ident) = ident {
 376             param_idents.push((kind, bounds, param_idents.len(), ident));
 377         }
 378         let max_param = &mut max_param;
 379         match max_param {
 380             Some(max_param) if *max_param > kind => {
 381                 let entry = out_of_order.entry(kind).or_insert((*max_param, vec![]));
 382                 entry.1.push(span);
 383             }
 384             Some(_) | None => *max_param = Some(kind),
 385         };
 386         match kind {
 387             ParamKindOrd::Type => found_type = true,
 388             ParamKindOrd::Const => found_const = true,
 389             _ => {}
 390         }
 391     }
 392
 393     let mut ordered_params = "<".to_string();
 394     if !out_of_order.is_empty() {
 395         param_idents.sort_by_key(|&(po, _, i, _)| (po, i));
 396         let mut first = true;
 397         for (_, bounds, _, ident) in param_idents {
 398             if !first {
 399                 ordered_params += ", ";
 400             }
 401             ordered_params += &ident;
 402             if let Some(bounds) = bounds {
 403                 if !bounds.is_empty() {
 404                     ordered_params += ": ";
 405                     ordered_params += &pprust::bounds_to_string(&bounds);
 406                 }
 407             }
 408             first = false;
 409         }
 410     }
 411     ordered_params += ">";
 412
 413     let pos_str = match pos {
 414         GenericPosition::Param => "parameter",
 415         GenericPosition::Arg => "argument",
 416     };
 417
 418     for (param_ord, (max_param, spans)) in &out_of_order {
 419         let mut err = handler.struct_span_err(spans.clone(),
 420             &format!(
 421                 "{} {pos}s must be declared prior to {} {pos}s",
 422                 param_ord,
 423                 max_param,
 424                 pos = pos_str,
 425             ));
 426         if let GenericPosition::Param = pos {
 427             err.span_suggestion(
 428                 span,
 429                 &format!(
 430                     "reorder the {}s: lifetimes, then types{}",
 431                     pos_str,
 432                     if sess.features_untracked().const_generics { ", then consts" } else { "" },
 433                 ),
 434                 ordered_params.clone(),
 435                 Applicability::MachineApplicable,
 436             );
 437         }
 438         err.emit();
 439     }
 440
 441     // FIXME(const_generics): we shouldn't have to abort here at all, but we currently get ICEs
 442     // if we don't. Const parameters and type parameters can currently conflict if they
 443     // are out-of-order.
 444     if !out_of_order.is_empty() && found_type && found_const {
 445         FatalError.raise();
 446     }
 447 }
 448
 449 impl<'a> Visitor<'a> for AstValidator<'a> {
 450     fn visit_expr(&mut self, expr: &'a Expr) {
 451         match expr.node {
 452             ExprKind::IfLet(_, ref expr, _, _) | ExprKind::WhileLet(_, ref expr, _, _) =>
 453                 self.while_if_let_ambiguity(&expr),
 454             ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
 455                 span_err!(self.session, expr.span, E0472, "asm! is unsupported on this target");
 456             }
 457             _ => {}
 458         }
 459
 460         visit::walk_expr(self, expr)
 461     }
 462
 463     fn visit_ty(&mut self, ty: &'a Ty) {
 464         match ty.node {
 465             TyKind::BareFn(ref bfty) => {
 466                 self.check_decl_no_pat(&bfty.decl, |span, _| {
 467                     struct_span_err!(self.session, span, E0561,
 468                                      "patterns aren't allowed in function pointer types").emit();
 469                 });
 470                 self.check_late_bound_lifetime_defs(&bfty.generic_params);
 471             }
 472             TyKind::TraitObject(ref bounds, ..) => {
 473                 let mut any_lifetime_bounds = false;
 474                 for bound in bounds {
 475                     if let GenericBound::Outlives(ref lifetime) = *bound {
 476                         if any_lifetime_bounds {
 477                             span_err!(self.session, lifetime.ident.span, E0226,
 478                                       "only a single explicit lifetime bound is permitted");
 479                             break;
 480                         }
 481                         any_lifetime_bounds = true;
 482                     }
 483                 }
 484                 self.no_questions_in_bounds(bounds, "trait object types", false);
 485             }
 486             TyKind::ImplTrait(_, ref bounds) => {
 487                 if self.is_impl_trait_banned {
 488                     if self.warning_period_57979_impl_trait_in_proj {
 489                         self.session.buffer_lint(
 490                             NESTED_IMPL_TRAIT, ty.id, ty.span,
 491                             "`impl Trait` is not allowed in path parameters");
 492                     } else {
 493                         struct_span_err!(self.session, ty.span, E0667,
 494                             "`impl Trait` is not allowed in path parameters").emit();
 495                     }
 496                 }
 497
 498                 if let Some(outer_impl_trait) = self.outer_impl_trait {
 499                     if outer_impl_trait.should_warn_instead_of_error() {
 500                         self.session.buffer_lint_with_diagnostic(
 501                             NESTED_IMPL_TRAIT, ty.id, ty.span,
 502                             "nested `impl Trait` is not allowed",
 503                             BuiltinLintDiagnostics::NestedImplTrait {
 504                                 outer_impl_trait_span: outer_impl_trait.span,
 505                                 inner_impl_trait_span: ty.span,
 506                             });
 507                     } else {
 508                         struct_span_err!(self.session, ty.span, E0666,
 509                             "nested `impl Trait` is not allowed")
 510                             .span_label(outer_impl_trait.span, "outer `impl Trait`")
 511                             .span_label(ty.span, "nested `impl Trait` here")
 512                             .emit();
 513                     }
 514                 }
 515
 516                 if !bounds.iter()
 517                           .any(|b| if let GenericBound::Trait(..) = *b { true } else { false }) {
 518                     self.err_handler().span_err(ty.span, "at least one trait must be specified");
 519                 }
 520
 521                 self.with_impl_trait_in_proj_warning(true, |this| this.walk_ty(ty));
 522                 return;
 523             }
 524             _ => {}
 525         }
 526
 527         self.walk_ty(ty)
 528     }
 529
 530     fn visit_label(&mut self, label: &'a Label) {
 531         self.check_label(label.ident);
 532         visit::walk_label(self, label);
 533     }
 534
 535     fn visit_lifetime(&mut self, lifetime: &'a Lifetime) {
 536         self.check_lifetime(lifetime.ident);
 537         visit::walk_lifetime(self, lifetime);
 538     }
 539
 540     fn visit_item(&mut self, item: &'a Item) {
 541         if item.attrs.iter().any(|attr| is_proc_macro_attr(attr)  ) {
 542             self.has_proc_macro_decls = true;
 543         }
 544
 545         if attr::contains_name(&item.attrs, sym::global_allocator) {
 546             self.has_global_allocator = true;
 547         }
 548
 549         match item.node {
 550             ItemKind::Impl(unsafety, polarity, _, _, Some(..), ref ty, ref impl_items) => {
 551                 self.invalid_visibility(&item.vis, None);
 552                 if let TyKind::Err = ty.node {
 553                     self.err_handler()
 554                         .struct_span_err(item.span, "`impl Trait for .. {}` is an obsolete syntax")
 555                         .help("use `auto trait Trait {}` instead").emit();
 556                 }
 557                 if unsafety == Unsafety::Unsafe && polarity == ImplPolarity::Negative {
 558                     span_err!(self.session, item.span, E0198, "negative impls cannot be unsafe");
 559                 }
 560                 for impl_item in impl_items {
 561                     self.invalid_visibility(&impl_item.vis, None);
 562                     if let ImplItemKind::Method(ref sig, _) = impl_item.node {
 563                         self.check_trait_fn_not_const(sig.header.constness);
 564                         self.check_trait_fn_not_async(impl_item.span, &sig.header.asyncness.node);
 565                     }
 566                 }
 567             }
 568             ItemKind::Impl(unsafety, polarity, defaultness, _, None, _, _) => {
 569                 self.invalid_visibility(&item.vis,
 570                                         Some("place qualifiers on individual impl items instead"));
 571                 if unsafety == Unsafety::Unsafe {
 572                     span_err!(self.session, item.span, E0197, "inherent impls cannot be unsafe");
 573                 }
 574                 if polarity == ImplPolarity::Negative {
 575                     self.err_handler().span_err(item.span, "inherent impls cannot be negative");
 576                 }
 577                 if defaultness == Defaultness::Default {
 578                     self.err_handler()
 579                         .struct_span_err(item.span, "inherent impls cannot be default")
 580                         .note("only trait implementations may be annotated with default").emit();
 581                 }
 582             }
 583             ItemKind::Fn(_, ref header, ref generics, _) => {
 584                 // We currently do not permit const generics in `const fn`, as
 585                 // this is tantamount to allowing compile-time dependent typing.
 586                 self.visit_fn_header(header);
 587                 if header.constness.node == Constness::Const {
 588                     // Look for const generics and error if we find any.
 589                     for param in &generics.params {
 590                         match param.kind {
 591                             GenericParamKind::Const { .. } => {
 592                                 self.err_handler()
 593                                     .struct_span_err(
 594                                         item.span,
 595                                         "const parameters are not permitted in `const fn`",
 596                                     )
 597                                     .emit();
 598                             }
 599                             _ => {}
 600                         }
 601                     }
 602                 }
 603             }
 604             ItemKind::ForeignMod(..) => {
 605                 self.invalid_visibility(
 606                     &item.vis,
 607                     Some("place qualifiers on individual foreign items instead"),
 608                 );
 609             }
 610             ItemKind::Enum(ref def, _) => {
 611                 for variant in &def.variants {
 612                     for field in variant.node.data.fields() {
 613                         self.invalid_visibility(&field.vis, None);
 614                     }
 615                 }
 616             }
 617             ItemKind::Trait(is_auto, _, ref generics, ref bounds, ref trait_items) => {
 618                 if is_auto == IsAuto::Yes {
 619                     // Auto traits cannot have generics, super traits nor contain items.
 620                     if !generics.params.is_empty() {
 621                         struct_span_err!(self.session, item.span, E0567,
 622                                         "auto traits cannot have generic parameters").emit();
 623                     }
 624                     if !bounds.is_empty() {
 625                         struct_span_err!(self.session, item.span, E0568,
 626                                         "auto traits cannot have super traits").emit();
 627                     }
 628                     if !trait_items.is_empty() {
 629                         struct_span_err!(self.session, item.span, E0380,
 630                                 "auto traits cannot have methods or associated items").emit();
 631                     }
 632                 }
 633                 self.no_questions_in_bounds(bounds, "supertraits", true);
 634                 for trait_item in trait_items {
 635                     if let TraitItemKind::Method(ref sig, ref block) = trait_item.node {
 636                         self.check_trait_fn_not_async(trait_item.span, &sig.header.asyncness.node);
 637                         self.check_trait_fn_not_const(sig.header.constness);
 638                         if block.is_none() {
 639                             self.check_decl_no_pat(&sig.decl, |span, mut_ident| {
 640                                 if mut_ident {
 641                                     self.session.buffer_lint(
 642                                         lint::builtin::PATTERNS_IN_FNS_WITHOUT_BODY,
 643                                         trait_item.id, span,
 644                                         "patterns aren't allowed in methods without bodies");
 645                                 } else {
 646                                     struct_span_err!(self.session, span, E0642,
 647                                         "patterns aren't allowed in methods without bodies").emit();
 648                                 }
 649                             });
 650                         }
 651                     }
 652                 }
 653             }
 654             ItemKind::Mod(_) => {
 655                 // Ensure that `path` attributes on modules are recorded as used (cf. issue #35584).
 656                 attr::first_attr_value_str_by_name(&item.attrs, sym::path);
 657                 if attr::contains_name(&item.attrs, sym::warn_directory_ownership) {
 658                     let lint = lint::builtin::LEGACY_DIRECTORY_OWNERSHIP;
 659                     let msg = "cannot declare a new module at this location";
 660                     self.session.buffer_lint(lint, item.id, item.span, msg);
 661                 }
 662             }
 663             ItemKind::Union(ref vdata, _) => {
 664                 if let VariantData::Tuple(..) | VariantData::Unit(..) = vdata {
 665                     self.err_handler().span_err(item.span,
 666                                                 "tuple and unit unions are not permitted");
 667                 }
 668                 if vdata.fields().is_empty() {
 669                     self.err_handler().span_err(item.span,
 670                                                 "unions cannot have zero fields");
 671                 }
 672             }
 673             ItemKind::Existential(ref bounds, _) => {
 674                 if !bounds.iter()
 675                           .any(|b| if let GenericBound::Trait(..) = *b { true } else { false }) {
 676                     let msp = MultiSpan::from_spans(bounds.iter()
 677                         .map(|bound| bound.span()).collect());
 678                     self.err_handler().span_err(msp, "at least one trait must be specified");
 679                 }
 680             }
 681             _ => {}
 682         }
 683
 684         visit::walk_item(self, item)
 685     }
 686
 687     fn visit_foreign_item(&mut self, fi: &'a ForeignItem) {
 688         match fi.node {
 689             ForeignItemKind::Fn(ref decl, _) => {
 690                 self.check_decl_no_pat(decl, |span, _| {
 691                     struct_span_err!(self.session, span, E0130,
 692                                      "patterns aren't allowed in foreign function declarations")
 693                         .span_label(span, "pattern not allowed in foreign function").emit();
 694                 });
 695             }
 696             ForeignItemKind::Static(..) | ForeignItemKind::Ty | ForeignItemKind::Macro(..) => {}
 697         }
 698
 699         visit::walk_foreign_item(self, fi)
 700     }
 701
 702     // Mirrors visit::walk_generic_args, but tracks relevant state
 703     fn visit_generic_args(&mut self, _: Span, generic_args: &'a GenericArgs) {
 704         match *generic_args {
 705             GenericArgs::AngleBracketed(ref data) => {
 706                 walk_list!(self, visit_generic_arg, &data.args);
 707                 validate_generics_order(
 708                     self.session,
 709                     self.err_handler(),
 710                     data.args.iter().map(|arg| {
 711                         (match arg {
 712                             GenericArg::Lifetime(..) => ParamKindOrd::Lifetime,
 713                             GenericArg::Type(..) => ParamKindOrd::Type,
 714                             GenericArg::Const(..) => ParamKindOrd::Const,
 715                         }, None, arg.span(), None)
 716                     }),
 717                     GenericPosition::Arg,
 718                     generic_args.span(),
 719                 );
 720
 721                 // Type bindings such as `Item=impl Debug` in `Iterator<Item=Debug>`
 722                 // are allowed to contain nested `impl Trait`.
 723                 self.with_impl_trait(None, |this| {
 724                     walk_list!(this, visit_assoc_type_binding_from_generic_args, &data.bindings);
 725                 });
 726             }
 727             GenericArgs::Parenthesized(ref data) => {
 728                 walk_list!(self, visit_ty, &data.inputs);
 729                 if let Some(ref type_) = data.output {
 730                     // `-> Foo` syntax is essentially an associated type binding,
 731                     // so it is also allowed to contain nested `impl Trait`.
 732                     self.with_impl_trait(None, |this| this.visit_ty_from_generic_args(type_));
 733                 }
 734             }
 735         }
 736     }
 737
 738     fn visit_generics(&mut self, generics: &'a Generics) {
 739         let mut prev_ty_default = None;
 740         for param in &generics.params {
 741             if let GenericParamKind::Type { ref default, .. } = param.kind {
 742                 if default.is_some() {
 743                     prev_ty_default = Some(param.ident.span);
 744                 } else if let Some(span) = prev_ty_default {
 745                     self.err_handler()
 746                         .span_err(span, "type parameters with a default must be trailing");
 747                     break;
 748                 }
 749             }
 750         }
 751
 752         validate_generics_order(
 753             self.session,
 754             self.err_handler(),
 755             generics.params.iter().map(|param| {
 756                 let ident = Some(param.ident.to_string());
 757                 let (kind, ident) = match &param.kind {
 758                     GenericParamKind::Lifetime { .. } => (ParamKindOrd::Lifetime, ident),
 759                     GenericParamKind::Type { .. } => (ParamKindOrd::Type, ident),
 760                     GenericParamKind::Const { ref ty } => {
 761                         let ty = pprust::ty_to_string(ty);
 762                         (ParamKindOrd::Const, Some(format!("const {}: {}", param.ident, ty)))
 763                     }
 764                 };
 765                 (kind, Some(&*param.bounds), param.ident.span, ident)
 766             }),
 767             GenericPosition::Param,
 768             generics.span,
 769         );
 770
 771         for predicate in &generics.where_clause.predicates {
 772             if let WherePredicate::EqPredicate(ref predicate) = *predicate {
 773                 self.err_handler()
 774                     .span_err(predicate.span, "equality constraints are not yet \
 775                                                supported in where clauses (see #20041)");
 776             }
 777         }
 778
 779         visit::walk_generics(self, generics)
 780     }
 781
 782     fn visit_generic_param(&mut self, param: &'a GenericParam) {
 783         if let GenericParamKind::Lifetime { .. } = param.kind {
 784             self.check_lifetime(param.ident);
 785         }
 786         visit::walk_generic_param(self, param);
 787     }
 788
 789     fn visit_pat(&mut self, pat: &'a Pat) {
 790         match pat.node {
 791             PatKind::Lit(ref expr) => {
 792                 self.check_expr_within_pat(expr, false);
 793             }
 794             PatKind::Range(ref start, ref end, _) => {
 795                 self.check_expr_within_pat(start, true);
 796                 self.check_expr_within_pat(end, true);
 797             }
 798             _ => {}
 799         }
 800
 801         visit::walk_pat(self, pat)
 802     }
 803
 804     fn visit_where_predicate(&mut self, p: &'a WherePredicate) {
 805         if let &WherePredicate::BoundPredicate(ref bound_predicate) = p {
 806             // A type binding, eg `for<'c> Foo: Send+Clone+'c`
 807             self.check_late_bound_lifetime_defs(&bound_predicate.bound_generic_params);
 808         }
 809         visit::walk_where_predicate(self, p);
 810     }
 811
 812     fn visit_poly_trait_ref(&mut self, t: &'a PolyTraitRef, m: &'a TraitBoundModifier) {
 813         self.check_late_bound_lifetime_defs(&t.bound_generic_params);
 814         visit::walk_poly_trait_ref(self, t, m);
 815     }
 816
 817     fn visit_mac(&mut self, mac: &Spanned<Mac_>) {
 818         // when a new macro kind is added but the author forgets to set it up for expansion
 819         // because that's the only part that won't cause a compiler error
 820         self.session.diagnostic()
 821             .span_bug(mac.span, "macro invocation missed in expansion; did you forget to override \
 822                                  the relevant `fold_*()` method in `PlaceholderExpander`?");
 823     }
 824
 825     fn visit_fn_header(&mut self, header: &'a FnHeader) {
 826         if header.asyncness.node.is_async() && self.session.rust_2015() {
 827             struct_span_err!(self.session, header.asyncness.span, E0670,
 828                              "`async fn` is not permitted in the 2015 edition").emit();
 829         }
 830     }
 831 }
 832
 833 pub fn check_crate(session: &Session, krate: &Crate) -> (bool, bool) {
 834     let mut validator = AstValidator {
 835         session,
 836         has_proc_macro_decls: false,
 837         has_global_allocator: false,
 838         outer_impl_trait: None,
 839         is_impl_trait_banned: false,
 840         warning_period_57979_didnt_record_next_impl_trait: false,
 841         warning_period_57979_impl_trait_in_proj: false,
 842     };
 843     visit::walk_crate(&mut validator, krate);
 844
 845     (validator.has_proc_macro_decls, validator.has_global_allocator)
 846 }