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 rustc::lint;
  11 use rustc::session::Session;
  12 use syntax::ast::*;
  13 use syntax::attr;
  14 use syntax::source_map::Spanned;
  15 use syntax::symbol::keywords;
  16 use syntax::ptr::P;
  17 use syntax::visit::{self, Visitor};
  18 use syntax_pos::Span;
  19 use errors;
  20 use errors::Applicability;
  21
  22 struct AstValidator<'a> {
  23     session: &'a Session,
  24
  25     // Used to ban nested `impl Trait`, e.g., `impl Into<impl Debug>`.
  26     // Nested `impl Trait` _is_ allowed in associated type position,
  27     // e.g `impl Iterator<Item=impl Debug>`
  28     outer_impl_trait: Option<Span>,
  29
  30     // Used to ban `impl Trait` in path projections like `<impl Iterator>::Item`
  31     // or `Foo::Bar<impl Trait>`
  32     is_impl_trait_banned: bool,
  33 }
  34
  35 impl<'a> AstValidator<'a> {
  36     fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
  37         let old = mem::replace(&mut self.is_impl_trait_banned, true);
  38         f(self);
  39         self.is_impl_trait_banned = old;
  40     }
  41
  42     fn with_impl_trait(&mut self, outer_impl_trait: Option<Span>, f: impl FnOnce(&mut Self)) {
  43         let old = mem::replace(&mut self.outer_impl_trait, outer_impl_trait);
  44         f(self);
  45         self.outer_impl_trait = old;
  46     }
  47
  48     // Mirrors visit::walk_ty, but tracks relevant state
  49     fn walk_ty(&mut self, t: &'a Ty) {
  50         match t.node {
  51             TyKind::ImplTrait(..) => {
  52                 self.with_impl_trait(Some(t.span), |this| visit::walk_ty(this, t))
  53             }
  54             TyKind::Path(ref qself, ref path) => {
  55                 // We allow these:
  56                 //  - `Option<impl Trait>`
  57                 //  - `option::Option<impl Trait>`
  58                 //  - `option::Option<T>::Foo<impl Trait>
  59                 //
  60                 // But not these:
  61                 //  - `<impl Trait>::Foo`
  62                 //  - `option::Option<impl Trait>::Foo`.
  63                 //
  64                 // To implement this, we disallow `impl Trait` from `qself`
  65                 // (for cases like `<impl Trait>::Foo>`)
  66                 // but we allow `impl Trait` in `GenericArgs`
  67                 // iff there are no more PathSegments.
  68                 if let Some(ref qself) = *qself {
  69                     // `impl Trait` in `qself` is always illegal
  70                     self.with_banned_impl_trait(|this| this.visit_ty(&qself.ty));
  71                 }
  72
  73                 // Note that there should be a call to visit_path here,
  74                 // so if any logic is added to process `Path`s a call to it should be
  75                 // added both in visit_path and here. This code mirrors visit::walk_path.
  76                 for (i, segment) in path.segments.iter().enumerate() {
  77                     // Allow `impl Trait` iff we're on the final path segment
  78                     if i == path.segments.len() - 1 {
  79                         self.visit_path_segment(path.span, segment);
  80                     } else {
  81                         self.with_banned_impl_trait(|this| {
  82                             this.visit_path_segment(path.span, segment)
  83                         });
  84                     }
  85                 }
  86             }
  87             _ => visit::walk_ty(self, t),
  88         }
  89     }
  90
  91     fn err_handler(&self) -> &errors::Handler {
  92         &self.session.diagnostic()
  93     }
  94
  95     fn check_lifetime(&self, ident: Ident) {
  96         let valid_names = [keywords::UnderscoreLifetime.name(),
  97                            keywords::StaticLifetime.name(),
  98                            keywords::Invalid.name()];
  99         if !valid_names.contains(&ident.name) && ident.without_first_quote().is_reserved() {
 100             self.err_handler().span_err(ident.span, "lifetimes cannot use keyword names");
 101         }
 102     }
 103
 104     fn check_label(&self, ident: Ident) {
 105         if ident.without_first_quote().is_reserved() {
 106             self.err_handler()
 107                 .span_err(ident.span, &format!("invalid label name `{}`", ident.name));
 108         }
 109     }
 110
 111     fn invalid_non_exhaustive_attribute(&self, variant: &Variant) {
 112         let has_non_exhaustive = attr::contains_name(&variant.node.attrs, "non_exhaustive");
 113         if has_non_exhaustive {
 114             self.err_handler().span_err(variant.span,
 115                                         "#[non_exhaustive] is not yet supported on variants");
 116         }
 117     }
 118
 119     fn invalid_visibility(&self, vis: &Visibility, note: Option<&str>) {
 120         if let VisibilityKind::Inherited = vis.node {
 121             return
 122         }
 123
 124         let mut err = struct_span_err!(self.session,
 125                                         vis.span,
 126                                         E0449,
 127                                         "unnecessary visibility qualifier");
 128         if vis.node.is_pub() {
 129             err.span_label(vis.span, "`pub` not permitted here because it's implied");
 130         }
 131         if let Some(note) = note {
 132             err.note(note);
 133         }
 134         err.emit();
 135     }
 136
 137     fn check_decl_no_pat<ReportFn: Fn(Span, bool)>(&self, decl: &FnDecl, report_err: ReportFn) {
 138         for arg in &decl.inputs {
 139             match arg.pat.node {
 140                 PatKind::Ident(BindingMode::ByValue(Mutability::Immutable), _, None) |
 141                 PatKind::Wild => {}
 142                 PatKind::Ident(BindingMode::ByValue(Mutability::Mutable), _, None) =>
 143                     report_err(arg.pat.span, true),
 144                 _ => report_err(arg.pat.span, false),
 145             }
 146         }
 147     }
 148
 149     fn check_trait_fn_not_async(&self, span: Span, asyncness: IsAsync) {
 150         if asyncness.is_async() {
 151             struct_span_err!(self.session, span, E0706,
 152                              "trait fns cannot be declared `async`").emit()
 153         }
 154     }
 155
 156     fn check_trait_fn_not_const(&self, constness: Spanned<Constness>) {
 157         if constness.node == Constness::Const {
 158             struct_span_err!(self.session, constness.span, E0379,
 159                              "trait fns cannot be declared const")
 160                 .span_label(constness.span, "trait fns cannot be const")
 161                 .emit();
 162         }
 163     }
 164
 165     fn no_questions_in_bounds(&self, bounds: &GenericBounds, where_: &str, is_trait: bool) {
 166         for bound in bounds {
 167             if let GenericBound::Trait(ref poly, TraitBoundModifier::Maybe) = *bound {
 168                 let mut err = self.err_handler().struct_span_err(poly.span,
 169                     &format!("`?Trait` is not permitted in {}", where_));
 170                 if is_trait {
 171                     err.note(&format!("traits are `?{}` by default", poly.trait_ref.path));
 172                 }
 173                 err.emit();
 174             }
 175         }
 176     }
 177
 178     /// matches '-' lit | lit (cf. parser::Parser::parse_literal_maybe_minus),
 179     /// or path for ranges.
 180     ///
 181     /// FIXME: do we want to allow expr -> pattern conversion to create path expressions?
 182     /// That means making this work:
 183     ///
 184     /// ```rust,ignore (FIXME)
 185     ///     struct S;
 186     ///     macro_rules! m {
 187     ///         ($a:expr) => {
 188     ///             let $a = S;
 189     ///         }
 190     ///     }
 191     ///     m!(S);
 192     /// ```
 193     fn check_expr_within_pat(&self, expr: &Expr, allow_paths: bool) {
 194         match expr.node {
 195             ExprKind::Lit(..) => {}
 196             ExprKind::Path(..) if allow_paths => {}
 197             ExprKind::Unary(UnOp::Neg, ref inner)
 198                 if match inner.node { ExprKind::Lit(_) => true, _ => false } => {}
 199             _ => self.err_handler().span_err(expr.span, "arbitrary expressions aren't allowed \
 200                                                          in patterns")
 201         }
 202     }
 203
 204     fn check_late_bound_lifetime_defs(&self, params: &[GenericParam]) {
 205         // Check only lifetime parameters are present and that the lifetime
 206         // parameters that are present have no bounds.
 207         let non_lt_param_spans: Vec<_> = params.iter().filter_map(|param| match param.kind {
 208             GenericParamKind::Lifetime { .. } => {
 209                 if !param.bounds.is_empty() {
 210                     let spans: Vec<_> = param.bounds.iter().map(|b| b.span()).collect();
 211                     self.err_handler()
 212                         .span_err(spans, "lifetime bounds cannot be used in this context");
 213                 }
 214                 None
 215             }
 216             _ => Some(param.ident.span),
 217         }).collect();
 218         if !non_lt_param_spans.is_empty() {
 219             self.err_handler().span_err(non_lt_param_spans,
 220                 "only lifetime parameters can be used in this context");
 221         }
 222     }
 223
 224     /// With eRFC 2497, we need to check whether an expression is ambiguous and warn or error
 225     /// depending on the edition, this function handles that.
 226     fn while_if_let_ambiguity(&self, expr: &P<Expr>) {
 227         if let Some((span, op_kind)) = self.while_if_let_expr_ambiguity(&expr) {
 228             let mut err = self.err_handler().struct_span_err(
 229                 span, &format!("ambiguous use of `{}`", op_kind.to_string())
 230             );
 231
 232             err.note(
 233                 "this will be a error until the `let_chains` feature is stabilized"
 234             );
 235             err.note(
 236                 "see rust-lang/rust#53668 for more information"
 237             );
 238
 239             if let Ok(snippet) = self.session.source_map().span_to_snippet(span) {
 240                 err.span_suggestion_with_applicability(
 241                     span, "consider adding parentheses", format!("({})", snippet),
 242                     Applicability::MachineApplicable,
 243                 );
 244             }
 245
 246             err.emit();
 247         }
 248     }
 249
 250     /// With eRFC 2497 adding if-let chains, there is a requirement that the parsing of
 251     /// `&&` and `||` in a if-let statement be unambiguous. This function returns a span and
 252     /// a `BinOpKind` (either `&&` or `||` depending on what was ambiguous) if it is determined
 253     /// that the current expression parsed is ambiguous and will break in future.
 254     fn while_if_let_expr_ambiguity(&self, expr: &P<Expr>) -> Option<(Span, BinOpKind)> {
 255         debug!("while_if_let_expr_ambiguity: expr.node: {:?}", expr.node);
 256         match &expr.node {
 257             ExprKind::Binary(op, _, _) if op.node == BinOpKind::And || op.node == BinOpKind::Or => {
 258                 Some((expr.span, op.node))
 259             },
 260             ExprKind::Range(ref lhs, ref rhs, _) => {
 261                 let lhs_ambiguous = lhs.as_ref()
 262                     .and_then(|lhs| self.while_if_let_expr_ambiguity(lhs));
 263                 let rhs_ambiguous = rhs.as_ref()
 264                     .and_then(|rhs| self.while_if_let_expr_ambiguity(rhs));
 265
 266                 lhs_ambiguous.or(rhs_ambiguous)
 267             }
 268             _ => None,
 269         }
 270     }
 271
 272 }
 273
 274 impl<'a> Visitor<'a> for AstValidator<'a> {
 275     fn visit_expr(&mut self, expr: &'a Expr) {
 276         match expr.node {
 277             ExprKind::IfLet(_, ref expr, _, _) | ExprKind::WhileLet(_, ref expr, _, _) =>
 278                 self.while_if_let_ambiguity(&expr),
 279             ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
 280                 span_err!(self.session, expr.span, E0472, "asm! is unsupported on this target");
 281             }
 282             ExprKind::ObsoleteInPlace(ref place, ref val) => {
 283                 let mut err = self.err_handler().struct_span_err(
 284                     expr.span,
 285                     "emplacement syntax is obsolete (for now, anyway)",
 286                 );
 287                 err.note(
 288                     "for more information, see \
 289                      <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>"
 290                 );
 291                 match val.node {
 292                     ExprKind::Lit(ref v) if v.node.is_numeric() => {
 293                         err.span_suggestion_with_applicability(
 294                             place.span.between(val.span),
 295                             "if you meant to write a comparison against a negative value, add a \
 296                              space in between `<` and `-`",
 297                             "< -".to_string(),
 298                             Applicability::MaybeIncorrect
 299                         );
 300                     }
 301                     _ => {}
 302                 }
 303                 err.emit();
 304             }
 305             _ => {}
 306         }
 307
 308         visit::walk_expr(self, expr)
 309     }
 310
 311     fn visit_ty(&mut self, ty: &'a Ty) {
 312         match ty.node {
 313             TyKind::BareFn(ref bfty) => {
 314                 self.check_decl_no_pat(&bfty.decl, |span, _| {
 315                     struct_span_err!(self.session, span, E0561,
 316                                      "patterns aren't allowed in function pointer types").emit();
 317                 });
 318                 self.check_late_bound_lifetime_defs(&bfty.generic_params);
 319             }
 320             TyKind::TraitObject(ref bounds, ..) => {
 321                 let mut any_lifetime_bounds = false;
 322                 for bound in bounds {
 323                     if let GenericBound::Outlives(ref lifetime) = *bound {
 324                         if any_lifetime_bounds {
 325                             span_err!(self.session, lifetime.ident.span, E0226,
 326                                       "only a single explicit lifetime bound is permitted");
 327                             break;
 328                         }
 329                         any_lifetime_bounds = true;
 330                     }
 331                 }
 332                 self.no_questions_in_bounds(bounds, "trait object types", false);
 333             }
 334             TyKind::ImplTrait(_, ref bounds) => {
 335                 if self.is_impl_trait_banned {
 336                     struct_span_err!(self.session, ty.span, E0667,
 337                         "`impl Trait` is not allowed in path parameters").emit();
 338                 }
 339
 340                 if let Some(outer_impl_trait) = self.outer_impl_trait {
 341                     struct_span_err!(self.session, ty.span, E0666,
 342                                     "nested `impl Trait` is not allowed")
 343                         .span_label(outer_impl_trait, "outer `impl Trait`")
 344                         .span_label(ty.span, "nested `impl Trait` here")
 345                         .emit();
 346
 347                 }
 348                 if !bounds.iter()
 349                           .any(|b| if let GenericBound::Trait(..) = *b { true } else { false }) {
 350                     self.err_handler().span_err(ty.span, "at least one trait must be specified");
 351                 }
 352             }
 353             _ => {}
 354         }
 355
 356         self.walk_ty(ty)
 357     }
 358
 359     fn visit_label(&mut self, label: &'a Label) {
 360         self.check_label(label.ident);
 361         visit::walk_label(self, label);
 362     }
 363
 364     fn visit_lifetime(&mut self, lifetime: &'a Lifetime) {
 365         self.check_lifetime(lifetime.ident);
 366         visit::walk_lifetime(self, lifetime);
 367     }
 368
 369     fn visit_item(&mut self, item: &'a Item) {
 370         match item.node {
 371             ItemKind::Impl(unsafety, polarity, _, _, Some(..), ref ty, ref impl_items) => {
 372                 self.invalid_visibility(&item.vis, None);
 373                 if let TyKind::Err = ty.node {
 374                     self.err_handler()
 375                         .struct_span_err(item.span, "`impl Trait for .. {}` is an obsolete syntax")
 376                         .help("use `auto trait Trait {}` instead").emit();
 377                 }
 378                 if unsafety == Unsafety::Unsafe && polarity == ImplPolarity::Negative {
 379                     span_err!(self.session, item.span, E0198, "negative impls cannot be unsafe");
 380                 }
 381                 for impl_item in impl_items {
 382                     self.invalid_visibility(&impl_item.vis, None);
 383                     if let ImplItemKind::Method(ref sig, _) = impl_item.node {
 384                         self.check_trait_fn_not_const(sig.header.constness);
 385                         self.check_trait_fn_not_async(impl_item.span, sig.header.asyncness);
 386                     }
 387                 }
 388             }
 389             ItemKind::Impl(unsafety, polarity, defaultness, _, None, _, _) => {
 390                 self.invalid_visibility(&item.vis,
 391                                         Some("place qualifiers on individual impl items instead"));
 392                 if unsafety == Unsafety::Unsafe {
 393                     span_err!(self.session, item.span, E0197, "inherent impls cannot be unsafe");
 394                 }
 395                 if polarity == ImplPolarity::Negative {
 396                     self.err_handler().span_err(item.span, "inherent impls cannot be negative");
 397                 }
 398                 if defaultness == Defaultness::Default {
 399                     self.err_handler()
 400                         .struct_span_err(item.span, "inherent impls cannot be default")
 401                         .note("only trait implementations may be annotated with default").emit();
 402                 }
 403             }
 404             ItemKind::ForeignMod(..) => {
 405                 self.invalid_visibility(
 406                     &item.vis,
 407                     Some("place qualifiers on individual foreign items instead"),
 408                 );
 409             }
 410             ItemKind::Enum(ref def, _) => {
 411                 for variant in &def.variants {
 412                     self.invalid_non_exhaustive_attribute(variant);
 413                     for field in variant.node.data.fields() {
 414                         self.invalid_visibility(&field.vis, None);
 415                     }
 416                 }
 417             }
 418             ItemKind::Trait(is_auto, _, ref generics, ref bounds, ref trait_items) => {
 419                 if is_auto == IsAuto::Yes {
 420                     // Auto traits cannot have generics, super traits nor contain items.
 421                     if !generics.params.is_empty() {
 422                         struct_span_err!(self.session, item.span, E0567,
 423                                         "auto traits cannot have generic parameters").emit();
 424                     }
 425                     if !bounds.is_empty() {
 426                         struct_span_err!(self.session, item.span, E0568,
 427                                         "auto traits cannot have super traits").emit();
 428                     }
 429                     if !trait_items.is_empty() {
 430                         struct_span_err!(self.session, item.span, E0380,
 431                                 "auto traits cannot have methods or associated items").emit();
 432                     }
 433                 }
 434                 self.no_questions_in_bounds(bounds, "supertraits", true);
 435                 for trait_item in trait_items {
 436                     if let TraitItemKind::Method(ref sig, ref block) = trait_item.node {
 437                         self.check_trait_fn_not_async(trait_item.span, sig.header.asyncness);
 438                         self.check_trait_fn_not_const(sig.header.constness);
 439                         if block.is_none() {
 440                             self.check_decl_no_pat(&sig.decl, |span, mut_ident| {
 441                                 if mut_ident {
 442                                     self.session.buffer_lint(
 443                                         lint::builtin::PATTERNS_IN_FNS_WITHOUT_BODY,
 444                                         trait_item.id, span,
 445                                         "patterns aren't allowed in methods without bodies");
 446                                 } else {
 447                                     struct_span_err!(self.session, span, E0642,
 448                                         "patterns aren't allowed in methods without bodies").emit();
 449                                 }
 450                             });
 451                         }
 452                     }
 453                 }
 454             }
 455             ItemKind::Mod(_) => {
 456                 // Ensure that `path` attributes on modules are recorded as used (cf. issue #35584).
 457                 attr::first_attr_value_str_by_name(&item.attrs, "path");
 458                 if attr::contains_name(&item.attrs, "warn_directory_ownership") {
 459                     let lint = lint::builtin::LEGACY_DIRECTORY_OWNERSHIP;
 460                     let msg = "cannot declare a new module at this location";
 461                     self.session.buffer_lint(lint, item.id, item.span, msg);
 462                 }
 463             }
 464             ItemKind::Union(ref vdata, _) => {
 465                 if !vdata.is_struct() {
 466                     self.err_handler().span_err(item.span,
 467                                                 "tuple and unit unions are not permitted");
 468                 }
 469                 if vdata.fields().is_empty() {
 470                     self.err_handler().span_err(item.span,
 471                                                 "unions cannot have zero fields");
 472                 }
 473             }
 474             _ => {}
 475         }
 476
 477         visit::walk_item(self, item)
 478     }
 479
 480     fn visit_foreign_item(&mut self, fi: &'a ForeignItem) {
 481         match fi.node {
 482             ForeignItemKind::Fn(ref decl, _) => {
 483                 self.check_decl_no_pat(decl, |span, _| {
 484                     struct_span_err!(self.session, span, E0130,
 485                                      "patterns aren't allowed in foreign function declarations")
 486                         .span_label(span, "pattern not allowed in foreign function").emit();
 487                 });
 488             }
 489             ForeignItemKind::Static(..) | ForeignItemKind::Ty | ForeignItemKind::Macro(..) => {}
 490         }
 491
 492         visit::walk_foreign_item(self, fi)
 493     }
 494
 495     // Mirrors visit::walk_generic_args, but tracks relevant state
 496     fn visit_generic_args(&mut self, _: Span, generic_args: &'a GenericArgs) {
 497         match *generic_args {
 498             GenericArgs::AngleBracketed(ref data) => {
 499                 walk_list!(self, visit_generic_arg, &data.args);
 500                 // Type bindings such as `Item=impl Debug` in `Iterator<Item=Debug>`
 501                 // are allowed to contain nested `impl Trait`.
 502                 self.with_impl_trait(None, |this| {
 503                     walk_list!(this, visit_assoc_type_binding, &data.bindings);
 504                 });
 505             }
 506             GenericArgs::Parenthesized(ref data) => {
 507                 walk_list!(self, visit_ty, &data.inputs);
 508                 if let Some(ref type_) = data.output {
 509                     // `-> Foo` syntax is essentially an associated type binding,
 510                     // so it is also allowed to contain nested `impl Trait`.
 511                     self.with_impl_trait(None, |this| visit::walk_ty(this, type_));
 512                 }
 513             }
 514         }
 515     }
 516
 517     fn visit_generics(&mut self, generics: &'a Generics) {
 518         let mut seen_non_lifetime_param = false;
 519         let mut seen_default = None;
 520         for param in &generics.params {
 521             match (&param.kind, seen_non_lifetime_param) {
 522                 (GenericParamKind::Lifetime { .. }, true) => {
 523                     self.err_handler()
 524                         .span_err(param.ident.span, "lifetime parameters must be leading");
 525                 },
 526                 (GenericParamKind::Lifetime { .. }, false) => {}
 527                 (GenericParamKind::Type { ref default, .. }, _) => {
 528                     seen_non_lifetime_param = true;
 529                     if default.is_some() {
 530                         seen_default = Some(param.ident.span);
 531                     } else if let Some(span) = seen_default {
 532                         self.err_handler()
 533                             .span_err(span, "type parameters with a default must be trailing");
 534                         break;
 535                     }
 536                 }
 537             }
 538         }
 539         for predicate in &generics.where_clause.predicates {
 540             if let WherePredicate::EqPredicate(ref predicate) = *predicate {
 541                 self.err_handler()
 542                     .span_err(predicate.span, "equality constraints are not yet \
 543                                                supported in where clauses (see #20041)");
 544             }
 545         }
 546         visit::walk_generics(self, generics)
 547     }
 548
 549     fn visit_generic_param(&mut self, param: &'a GenericParam) {
 550         if let GenericParamKind::Lifetime { .. } = param.kind {
 551             self.check_lifetime(param.ident);
 552         }
 553         visit::walk_generic_param(self, param);
 554     }
 555
 556     fn visit_pat(&mut self, pat: &'a Pat) {
 557         match pat.node {
 558             PatKind::Lit(ref expr) => {
 559                 self.check_expr_within_pat(expr, false);
 560             }
 561             PatKind::Range(ref start, ref end, _) => {
 562                 self.check_expr_within_pat(start, true);
 563                 self.check_expr_within_pat(end, true);
 564             }
 565             _ => {}
 566         }
 567
 568         visit::walk_pat(self, pat)
 569     }
 570
 571     fn visit_where_predicate(&mut self, p: &'a WherePredicate) {
 572         if let &WherePredicate::BoundPredicate(ref bound_predicate) = p {
 573             // A type binding, eg `for<'c> Foo: Send+Clone+'c`
 574             self.check_late_bound_lifetime_defs(&bound_predicate.bound_generic_params);
 575         }
 576         visit::walk_where_predicate(self, p);
 577     }
 578
 579     fn visit_poly_trait_ref(&mut self, t: &'a PolyTraitRef, m: &'a TraitBoundModifier) {
 580         self.check_late_bound_lifetime_defs(&t.bound_generic_params);
 581         visit::walk_poly_trait_ref(self, t, m);
 582     }
 583
 584     fn visit_mac(&mut self, mac: &Spanned<Mac_>) {
 585         // when a new macro kind is added but the author forgets to set it up for expansion
 586         // because that's the only part that won't cause a compiler error
 587         self.session.diagnostic()
 588             .span_bug(mac.span, "macro invocation missed in expansion; did you forget to override \
 589                                  the relevant `fold_*()` method in `PlaceholderExpander`?");
 590     }
 591 }
 592
 593 pub fn check_crate(session: &Session, krate: &Crate) {
 594     visit::walk_crate(&mut AstValidator {
 595         session,
 596         outer_impl_trait: None,
 597         is_impl_trait_banned: false,
 598     }, krate)
 599 }