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::keywords;
  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;
  24 use errors::Applicability;
  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 = [keywords::UnderscoreLifetime.name(),
 181                            keywords::StaticLifetime.name(),
 182                            keywords::Invalid.name()];
 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_non_exhaustive_attribute(&self, variant: &Variant) {
 196         let has_non_exhaustive = attr::contains_name(&variant.node.attrs, "non_exhaustive");
 197         if has_non_exhaustive {
 198             self.err_handler().span_err(variant.span,
 199                                         "#[non_exhaustive] is not yet supported on variants");
 200         }
 201     }
 202
 203     fn invalid_visibility(&self, vis: &Visibility, note: Option<&str>) {
 204         if let VisibilityKind::Inherited = vis.node {
 205             return
 206         }
 207
 208         let mut err = struct_span_err!(self.session,
 209                                         vis.span,
 210                                         E0449,
 211                                         "unnecessary visibility qualifier");
 212         if vis.node.is_pub() {
 213             err.span_label(vis.span, "`pub` not permitted here because it's implied");
 214         }
 215         if let Some(note) = note {
 216             err.note(note);
 217         }
 218         err.emit();
 219     }
 220
 221     fn check_decl_no_pat<ReportFn: Fn(Span, bool)>(&self, decl: &FnDecl, report_err: ReportFn) {
 222         for arg in &decl.inputs {
 223             match arg.pat.node {
 224                 PatKind::Ident(BindingMode::ByValue(Mutability::Immutable), _, None) |
 225                 PatKind::Wild => {}
 226                 PatKind::Ident(BindingMode::ByValue(Mutability::Mutable), _, None) =>
 227                     report_err(arg.pat.span, true),
 228                 _ => report_err(arg.pat.span, false),
 229             }
 230         }
 231     }
 232
 233     fn check_trait_fn_not_async(&self, span: Span, asyncness: IsAsync) {
 234         if asyncness.is_async() {
 235             struct_span_err!(self.session, span, E0706,
 236                              "trait fns cannot be declared `async`").emit()
 237         }
 238     }
 239
 240     fn check_trait_fn_not_const(&self, constness: Spanned<Constness>) {
 241         if constness.node == Constness::Const {
 242             struct_span_err!(self.session, constness.span, E0379,
 243                              "trait fns cannot be declared const")
 244                 .span_label(constness.span, "trait fns cannot be const")
 245                 .emit();
 246         }
 247     }
 248
 249     fn no_questions_in_bounds(&self, bounds: &GenericBounds, where_: &str, is_trait: bool) {
 250         for bound in bounds {
 251             if let GenericBound::Trait(ref poly, TraitBoundModifier::Maybe) = *bound {
 252                 let mut err = self.err_handler().struct_span_err(poly.span,
 253                     &format!("`?Trait` is not permitted in {}", where_));
 254                 if is_trait {
 255                     err.note(&format!("traits are `?{}` by default", poly.trait_ref.path));
 256                 }
 257                 err.emit();
 258             }
 259         }
 260     }
 261
 262     /// Matches `'-' lit | lit (cf. parser::Parser::parse_literal_maybe_minus)`,
 263     /// or paths for ranges.
 264     //
 265     // FIXME: do we want to allow `expr -> pattern` conversion to create path expressions?
 266     // That means making this work:
 267     //
 268     // ```rust,ignore (FIXME)
 269     // struct S;
 270     // macro_rules! m {
 271     //     ($a:expr) => {
 272     //         let $a = S;
 273     //     }
 274     // }
 275     // m!(S);
 276     // ```
 277     fn check_expr_within_pat(&self, expr: &Expr, allow_paths: bool) {
 278         match expr.node {
 279             ExprKind::Lit(..) => {}
 280             ExprKind::Path(..) if allow_paths => {}
 281             ExprKind::Unary(UnOp::Neg, ref inner)
 282                 if match inner.node { ExprKind::Lit(_) => true, _ => false } => {}
 283             _ => self.err_handler().span_err(expr.span, "arbitrary expressions aren't allowed \
 284                                                          in patterns")
 285         }
 286     }
 287
 288     fn check_late_bound_lifetime_defs(&self, params: &[GenericParam]) {
 289         // Check only lifetime parameters are present and that the lifetime
 290         // parameters that are present have no bounds.
 291         let non_lt_param_spans: Vec<_> = params.iter().filter_map(|param| match param.kind {
 292             GenericParamKind::Lifetime { .. } => {
 293                 if !param.bounds.is_empty() {
 294                     let spans: Vec<_> = param.bounds.iter().map(|b| b.span()).collect();
 295                     self.err_handler()
 296                         .span_err(spans, "lifetime bounds cannot be used in this context");
 297                 }
 298                 None
 299             }
 300             _ => Some(param.ident.span),
 301         }).collect();
 302         if !non_lt_param_spans.is_empty() {
 303             self.err_handler().span_err(non_lt_param_spans,
 304                 "only lifetime parameters can be used in this context");
 305         }
 306     }
 307
 308     /// With eRFC 2497, we need to check whether an expression is ambiguous and warn or error
 309     /// depending on the edition, this function handles that.
 310     fn while_if_let_ambiguity(&self, expr: &P<Expr>) {
 311         if let Some((span, op_kind)) = self.while_if_let_expr_ambiguity(&expr) {
 312             let mut err = self.err_handler().struct_span_err(
 313                 span, &format!("ambiguous use of `{}`", op_kind.to_string())
 314             );
 315
 316             err.note(
 317                 "this will be a error until the `let_chains` feature is stabilized"
 318             );
 319             err.note(
 320                 "see rust-lang/rust#53668 for more information"
 321             );
 322
 323             if let Ok(snippet) = self.session.source_map().span_to_snippet(span) {
 324                 err.span_suggestion(
 325                     span, "consider adding parentheses", format!("({})", snippet),
 326                     Applicability::MachineApplicable,
 327                 );
 328             }
 329
 330             err.emit();
 331         }
 332     }
 333
 334     /// With eRFC 2497 adding if-let chains, there is a requirement that the parsing of
 335     /// `&&` and `||` in a if-let statement be unambiguous. This function returns a span and
 336     /// a `BinOpKind` (either `&&` or `||` depending on what was ambiguous) if it is determined
 337     /// that the current expression parsed is ambiguous and will break in future.
 338     fn while_if_let_expr_ambiguity(&self, expr: &P<Expr>) -> Option<(Span, BinOpKind)> {
 339         debug!("while_if_let_expr_ambiguity: expr.node: {:?}", expr.node);
 340         match &expr.node {
 341             ExprKind::Binary(op, _, _) if op.node == BinOpKind::And || op.node == BinOpKind::Or => {
 342                 Some((expr.span, op.node))
 343             },
 344             ExprKind::Range(ref lhs, ref rhs, _) => {
 345                 let lhs_ambiguous = lhs.as_ref()
 346                     .and_then(|lhs| self.while_if_let_expr_ambiguity(lhs));
 347                 let rhs_ambiguous = rhs.as_ref()
 348                     .and_then(|rhs| self.while_if_let_expr_ambiguity(rhs));
 349
 350                 lhs_ambiguous.or(rhs_ambiguous)
 351             }
 352             _ => None,
 353         }
 354     }
 355 }
 356
 357 enum GenericPosition {
 358     Param,
 359     Arg,
 360 }
 361
 362 fn validate_generics_order<'a>(
 363     handler: &errors::Handler,
 364     generics: impl Iterator<Item = (ParamKindOrd, Span, Option<String>)>,
 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
 372     for (kind, span, ident) in generics {
 373         if let Some(ident) = ident {
 374             param_idents.push((kind, param_idents.len(), ident));
 375         }
 376         let max_param = &mut max_param;
 377         match max_param {
 378             Some(max_param) if *max_param > kind => {
 379                 let entry = out_of_order.entry(kind).or_insert((*max_param, vec![]));
 380                 entry.1.push(span);
 381             }
 382             Some(_) | None => *max_param = Some(kind),
 383         };
 384     }
 385
 386     let mut ordered_params = "<".to_string();
 387     if !out_of_order.is_empty() {
 388         param_idents.sort_by_key(|&(po, i, _)| (po, i));
 389         let mut first = true;
 390         for (_, _, ident) in param_idents {
 391             if !first {
 392                 ordered_params += ", ";
 393             }
 394             ordered_params += &ident;
 395             first = false;
 396         }
 397     }
 398     ordered_params += ">";
 399
 400     let pos_str = match pos {
 401         GenericPosition::Param => "parameter",
 402         GenericPosition::Arg => "argument",
 403     };
 404
 405     for (param_ord, (max_param, spans)) in out_of_order {
 406         let mut err = handler.struct_span_err(spans,
 407             &format!(
 408                 "{} {pos}s must be declared prior to {} {pos}s",
 409                 param_ord,
 410                 max_param,
 411                 pos = pos_str,
 412             ));
 413         if let GenericPosition::Param = pos {
 414             err.span_suggestion(
 415                 span,
 416                 &format!("reorder the {}s: lifetimes, then types, then consts", pos_str),
 417                 ordered_params.clone(),
 418                 Applicability::MachineApplicable,
 419             );
 420         }
 421         err.emit();
 422     }
 423 }
 424
 425 impl<'a> Visitor<'a> for AstValidator<'a> {
 426     fn visit_expr(&mut self, expr: &'a Expr) {
 427         match expr.node {
 428             ExprKind::IfLet(_, ref expr, _, _) | ExprKind::WhileLet(_, ref expr, _, _) =>
 429                 self.while_if_let_ambiguity(&expr),
 430             ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
 431                 span_err!(self.session, expr.span, E0472, "asm! is unsupported on this target");
 432             }
 433             ExprKind::ObsoleteInPlace(ref place, ref val) => {
 434                 let mut err = self.err_handler().struct_span_err(
 435                     expr.span,
 436                     "emplacement syntax is obsolete (for now, anyway)",
 437                 );
 438                 err.note(
 439                     "for more information, see \
 440                      <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>"
 441                 );
 442                 match val.node {
 443                     ExprKind::Lit(ref v) if v.node.is_numeric() => {
 444                         err.span_suggestion(
 445                             place.span.between(val.span),
 446                             "if you meant to write a comparison against a negative value, add a \
 447                              space in between `<` and `-`",
 448                             "< -".to_string(),
 449                             Applicability::MaybeIncorrect
 450                         );
 451                     }
 452                     _ => {}
 453                 }
 454                 err.emit();
 455             }
 456             _ => {}
 457         }
 458
 459         visit::walk_expr(self, expr)
 460     }
 461
 462     fn visit_ty(&mut self, ty: &'a Ty) {
 463         match ty.node {
 464             TyKind::BareFn(ref bfty) => {
 465                 self.check_decl_no_pat(&bfty.decl, |span, _| {
 466                     struct_span_err!(self.session, span, E0561,
 467                                      "patterns aren't allowed in function pointer types").emit();
 468                 });
 469                 self.check_late_bound_lifetime_defs(&bfty.generic_params);
 470             }
 471             TyKind::TraitObject(ref bounds, ..) => {
 472                 let mut any_lifetime_bounds = false;
 473                 for bound in bounds {
 474                     if let GenericBound::Outlives(ref lifetime) = *bound {
 475                         if any_lifetime_bounds {
 476                             span_err!(self.session, lifetime.ident.span, E0226,
 477                                       "only a single explicit lifetime bound is permitted");
 478                             break;
 479                         }
 480                         any_lifetime_bounds = true;
 481                     }
 482                 }
 483                 self.no_questions_in_bounds(bounds, "trait object types", false);
 484             }
 485             TyKind::ImplTrait(_, ref bounds) => {
 486                 if self.is_impl_trait_banned {
 487                     if self.warning_period_57979_impl_trait_in_proj {
 488                         self.session.buffer_lint(
 489                             NESTED_IMPL_TRAIT, ty.id, ty.span,
 490                             "`impl Trait` is not allowed in path parameters");
 491                     } else {
 492                         struct_span_err!(self.session, ty.span, E0667,
 493                             "`impl Trait` is not allowed in path parameters").emit();
 494                     }
 495                 }
 496
 497                 if let Some(outer_impl_trait) = self.outer_impl_trait {
 498                     if outer_impl_trait.should_warn_instead_of_error() {
 499                         self.session.buffer_lint_with_diagnostic(
 500                             NESTED_IMPL_TRAIT, ty.id, ty.span,
 501                             "nested `impl Trait` is not allowed",
 502                             BuiltinLintDiagnostics::NestedImplTrait {
 503                                 outer_impl_trait_span: outer_impl_trait.span,
 504                                 inner_impl_trait_span: ty.span,
 505                             });
 506                     } else {
 507                         struct_span_err!(self.session, ty.span, E0666,
 508                             "nested `impl Trait` is not allowed")
 509                             .span_label(outer_impl_trait.span, "outer `impl Trait`")
 510                             .span_label(ty.span, "nested `impl Trait` here")
 511                             .emit();
 512                     }
 513                 }
 514
 515                 if !bounds.iter()
 516                           .any(|b| if let GenericBound::Trait(..) = *b { true } else { false }) {
 517                     self.err_handler().span_err(ty.span, "at least one trait must be specified");
 518                 }
 519
 520                 self.with_impl_trait_in_proj_warning(true, |this| this.walk_ty(ty));
 521                 return;
 522             }
 523             _ => {}
 524         }
 525
 526         self.walk_ty(ty)
 527     }
 528
 529     fn visit_label(&mut self, label: &'a Label) {
 530         self.check_label(label.ident);
 531         visit::walk_label(self, label);
 532     }
 533
 534     fn visit_lifetime(&mut self, lifetime: &'a Lifetime) {
 535         self.check_lifetime(lifetime.ident);
 536         visit::walk_lifetime(self, lifetime);
 537     }
 538
 539     fn visit_item(&mut self, item: &'a Item) {
 540         if item.attrs.iter().any(|attr| is_proc_macro_attr(attr)  ) {
 541             self.has_proc_macro_decls = true;
 542         }
 543
 544         if attr::contains_name(&item.attrs, "global_allocator") {
 545             self.has_global_allocator = true;
 546         }
 547
 548         match item.node {
 549             ItemKind::Impl(unsafety, polarity, _, _, Some(..), ref ty, ref impl_items) => {
 550                 self.invalid_visibility(&item.vis, None);
 551                 if let TyKind::Err = ty.node {
 552                     self.err_handler()
 553                         .struct_span_err(item.span, "`impl Trait for .. {}` is an obsolete syntax")
 554                         .help("use `auto trait Trait {}` instead").emit();
 555                 }
 556                 if unsafety == Unsafety::Unsafe && polarity == ImplPolarity::Negative {
 557                     span_err!(self.session, item.span, E0198, "negative impls cannot be unsafe");
 558                 }
 559                 for impl_item in impl_items {
 560                     self.invalid_visibility(&impl_item.vis, None);
 561                     if let ImplItemKind::Method(ref sig, _) = impl_item.node {
 562                         self.check_trait_fn_not_const(sig.header.constness);
 563                         self.check_trait_fn_not_async(impl_item.span, sig.header.asyncness.node);
 564                     }
 565                 }
 566             }
 567             ItemKind::Impl(unsafety, polarity, defaultness, _, None, _, _) => {
 568                 self.invalid_visibility(&item.vis,
 569                                         Some("place qualifiers on individual impl items instead"));
 570                 if unsafety == Unsafety::Unsafe {
 571                     span_err!(self.session, item.span, E0197, "inherent impls cannot be unsafe");
 572                 }
 573                 if polarity == ImplPolarity::Negative {
 574                     self.err_handler().span_err(item.span, "inherent impls cannot be negative");
 575                 }
 576                 if defaultness == Defaultness::Default {
 577                     self.err_handler()
 578                         .struct_span_err(item.span, "inherent impls cannot be default")
 579                         .note("only trait implementations may be annotated with default").emit();
 580                 }
 581             }
 582             ItemKind::Fn(_, ref header, ref generics, _) => {
 583                 // We currently do not permit const generics in `const fn`, as
 584                 // this is tantamount to allowing compile-time dependent typing.
 585                 self.visit_fn_header(header);
 586                 if header.constness.node == Constness::Const {
 587                     // Look for const generics and error if we find any.
 588                     for param in &generics.params {
 589                         match param.kind {
 590                             GenericParamKind::Const { .. } => {
 591                                 self.err_handler()
 592                                     .struct_span_err(
 593                                         item.span,
 594                                         "const parameters are not permitted in `const fn`",
 595                                     )
 596                                     .emit();
 597                             }
 598                             _ => {}
 599                         }
 600                     }
 601                 }
 602             }
 603             ItemKind::ForeignMod(..) => {
 604                 self.invalid_visibility(
 605                     &item.vis,
 606                     Some("place qualifiers on individual foreign items instead"),
 607                 );
 608             }
 609             ItemKind::Enum(ref def, _) => {
 610                 for variant in &def.variants {
 611                     self.invalid_non_exhaustive_attribute(variant);
 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, "path");
 657                 if attr::contains_name(&item.attrs, "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 !vdata.is_struct() {
 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             _ => {}
 674         }
 675
 676         visit::walk_item(self, item)
 677     }
 678
 679     fn visit_foreign_item(&mut self, fi: &'a ForeignItem) {
 680         match fi.node {
 681             ForeignItemKind::Fn(ref decl, _) => {
 682                 self.check_decl_no_pat(decl, |span, _| {
 683                     struct_span_err!(self.session, span, E0130,
 684                                      "patterns aren't allowed in foreign function declarations")
 685                         .span_label(span, "pattern not allowed in foreign function").emit();
 686                 });
 687             }
 688             ForeignItemKind::Static(..) | ForeignItemKind::Ty | ForeignItemKind::Macro(..) => {}
 689         }
 690
 691         visit::walk_foreign_item(self, fi)
 692     }
 693
 694     // Mirrors visit::walk_generic_args, but tracks relevant state
 695     fn visit_generic_args(&mut self, _: Span, generic_args: &'a GenericArgs) {
 696         match *generic_args {
 697             GenericArgs::AngleBracketed(ref data) => {
 698                 walk_list!(self, visit_generic_arg, &data.args);
 699                 validate_generics_order(self.err_handler(), data.args.iter().map(|arg| {
 700                     (match arg {
 701                         GenericArg::Lifetime(..) => ParamKindOrd::Lifetime,
 702                         GenericArg::Type(..) => ParamKindOrd::Type,
 703                         GenericArg::Const(..) => ParamKindOrd::Const,
 704                     }, arg.span(), None)
 705                 }), GenericPosition::Arg, generic_args.span());
 706
 707                 // Type bindings such as `Item=impl Debug` in `Iterator<Item=Debug>`
 708                 // are allowed to contain nested `impl Trait`.
 709                 self.with_impl_trait(None, |this| {
 710                     walk_list!(this, visit_assoc_type_binding_from_generic_args, &data.bindings);
 711                 });
 712             }
 713             GenericArgs::Parenthesized(ref data) => {
 714                 walk_list!(self, visit_ty, &data.inputs);
 715                 if let Some(ref type_) = data.output {
 716                     // `-> Foo` syntax is essentially an associated type binding,
 717                     // so it is also allowed to contain nested `impl Trait`.
 718                     self.with_impl_trait(None, |this| this.visit_ty_from_generic_args(type_));
 719                 }
 720             }
 721         }
 722     }
 723
 724     fn visit_generics(&mut self, generics: &'a Generics) {
 725         let mut prev_ty_default = None;
 726         for param in &generics.params {
 727             if let GenericParamKind::Type { ref default, .. } = param.kind {
 728                 if default.is_some() {
 729                     prev_ty_default = Some(param.ident.span);
 730                 } else if let Some(span) = prev_ty_default {
 731                     self.err_handler()
 732                         .span_err(span, "type parameters with a default must be trailing");
 733                     break;
 734                 }
 735             }
 736         }
 737
 738         validate_generics_order(self.err_handler(), generics.params.iter().map(|param| {
 739             let span = param.ident.span;
 740             let ident = Some(param.ident.to_string());
 741             match &param.kind {
 742                 GenericParamKind::Lifetime { .. } => (ParamKindOrd::Lifetime, span, ident),
 743                 GenericParamKind::Type { .. } => (ParamKindOrd::Type, span, ident),
 744                 GenericParamKind::Const { ref ty } => {
 745                     let ty = pprust::ty_to_string(ty);
 746                     (ParamKindOrd::Const, span, Some(format!("const {}: {}", param.ident, ty)))
 747                 }
 748             }
 749         }), GenericPosition::Param, generics.span);
 750
 751         for predicate in &generics.where_clause.predicates {
 752             if let WherePredicate::EqPredicate(ref predicate) = *predicate {
 753                 self.err_handler()
 754                     .span_err(predicate.span, "equality constraints are not yet \
 755                                                supported in where clauses (see #20041)");
 756             }
 757         }
 758
 759         visit::walk_generics(self, generics)
 760     }
 761
 762     fn visit_generic_param(&mut self, param: &'a GenericParam) {
 763         if let GenericParamKind::Lifetime { .. } = param.kind {
 764             self.check_lifetime(param.ident);
 765         }
 766         visit::walk_generic_param(self, param);
 767     }
 768
 769     fn visit_pat(&mut self, pat: &'a Pat) {
 770         match pat.node {
 771             PatKind::Lit(ref expr) => {
 772                 self.check_expr_within_pat(expr, false);
 773             }
 774             PatKind::Range(ref start, ref end, _) => {
 775                 self.check_expr_within_pat(start, true);
 776                 self.check_expr_within_pat(end, true);
 777             }
 778             _ => {}
 779         }
 780
 781         visit::walk_pat(self, pat)
 782     }
 783
 784     fn visit_where_predicate(&mut self, p: &'a WherePredicate) {
 785         if let &WherePredicate::BoundPredicate(ref bound_predicate) = p {
 786             // A type binding, eg `for<'c> Foo: Send+Clone+'c`
 787             self.check_late_bound_lifetime_defs(&bound_predicate.bound_generic_params);
 788         }
 789         visit::walk_where_predicate(self, p);
 790     }
 791
 792     fn visit_poly_trait_ref(&mut self, t: &'a PolyTraitRef, m: &'a TraitBoundModifier) {
 793         self.check_late_bound_lifetime_defs(&t.bound_generic_params);
 794         visit::walk_poly_trait_ref(self, t, m);
 795     }
 796
 797     fn visit_mac(&mut self, mac: &Spanned<Mac_>) {
 798         // when a new macro kind is added but the author forgets to set it up for expansion
 799         // because that's the only part that won't cause a compiler error
 800         self.session.diagnostic()
 801             .span_bug(mac.span, "macro invocation missed in expansion; did you forget to override \
 802                                  the relevant `fold_*()` method in `PlaceholderExpander`?");
 803     }
 804
 805     fn visit_fn_header(&mut self, header: &'a FnHeader) {
 806         if header.asyncness.node.is_async() && self.session.rust_2015() {
 807             struct_span_err!(self.session, header.asyncness.span, E0670,
 808                              "`async fn` is not permitted in the 2015 edition").emit();
 809         }
 810     }
 811 }
 812
 813 pub fn check_crate(session: &Session, krate: &Crate) -> (bool, bool) {
 814     let mut validator = AstValidator {
 815         session,
 816         has_proc_macro_decls: false,
 817         has_global_allocator: false,
 818         outer_impl_trait: None,
 819         is_impl_trait_banned: false,
 820         warning_period_57979_didnt_record_next_impl_trait: false,
 821         warning_period_57979_impl_trait_in_proj: false,
 822     };
 823     visit::walk_crate(&mut validator, krate);
 824
 825     (validator.has_proc_macro_decls, validator.has_global_allocator)
 826 }