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 rustc::lint;
  10 use rustc::session::Session;
  11 use syntax::ast::*;
  12 use syntax::attr;
  13 use syntax::source_map::Spanned;
  14 use syntax::symbol::keywords;
  15 use syntax::ptr::P;
  16 use syntax::visit::{self, Visitor};
  17 use syntax_pos::Span;
  18 use errors;
  19 use errors::Applicability;
  20
  21 struct AstValidator<'a> {
  22     session: &'a Session,
  23 }
  24
  25 impl<'a> AstValidator<'a> {
  26     fn err_handler(&self) -> &errors::Handler {
  27         &self.session.diagnostic()
  28     }
  29
  30     fn check_lifetime(&self, ident: Ident) {
  31         let valid_names = [keywords::UnderscoreLifetime.name(),
  32                            keywords::StaticLifetime.name(),
  33                            keywords::Invalid.name()];
  34         if !valid_names.contains(&ident.name) && ident.without_first_quote().is_reserved() {
  35             self.err_handler().span_err(ident.span, "lifetimes cannot use keyword names");
  36         }
  37     }
  38
  39     fn check_label(&self, ident: Ident) {
  40         if ident.without_first_quote().is_reserved() {
  41             self.err_handler()
  42                 .span_err(ident.span, &format!("invalid label name `{}`", ident.name));
  43         }
  44     }
  45
  46     fn invalid_non_exhaustive_attribute(&self, variant: &Variant) {
  47         let has_non_exhaustive = attr::contains_name(&variant.node.attrs, "non_exhaustive");
  48         if has_non_exhaustive {
  49             self.err_handler().span_err(variant.span,
  50                                         "#[non_exhaustive] is not yet supported on variants");
  51         }
  52     }
  53
  54     fn invalid_visibility(&self, vis: &Visibility, note: Option<&str>) {
  55         if let VisibilityKind::Inherited = vis.node {
  56             return
  57         }
  58
  59         let mut err = struct_span_err!(self.session,
  60                                         vis.span,
  61                                         E0449,
  62                                         "unnecessary visibility qualifier");
  63         if vis.node.is_pub() {
  64             err.span_label(vis.span, "`pub` not permitted here because it's implied");
  65         }
  66         if let Some(note) = note {
  67             err.note(note);
  68         }
  69         err.emit();
  70     }
  71
  72     fn check_decl_no_pat<ReportFn: Fn(Span, bool)>(&self, decl: &FnDecl, report_err: ReportFn) {
  73         for arg in &decl.inputs {
  74             match arg.pat.node {
  75                 PatKind::Ident(BindingMode::ByValue(Mutability::Immutable), _, None) |
  76                 PatKind::Wild => {}
  77                 PatKind::Ident(BindingMode::ByValue(Mutability::Mutable), _, None) =>
  78                     report_err(arg.pat.span, true),
  79                 _ => report_err(arg.pat.span, false),
  80             }
  81         }
  82     }
  83
  84     fn check_trait_fn_not_async(&self, span: Span, asyncness: IsAsync) {
  85         if asyncness.is_async() {
  86             struct_span_err!(self.session, span, E0706,
  87                              "trait fns cannot be declared `async`").emit()
  88         }
  89     }
  90
  91     fn check_trait_fn_not_const(&self, constness: Spanned<Constness>) {
  92         if constness.node == Constness::Const {
  93             struct_span_err!(self.session, constness.span, E0379,
  94                              "trait fns cannot be declared const")
  95                 .span_label(constness.span, "trait fns cannot be const")
  96                 .emit();
  97         }
  98     }
  99
 100     fn no_questions_in_bounds(&self, bounds: &GenericBounds, where_: &str, is_trait: bool) {
 101         for bound in bounds {
 102             if let GenericBound::Trait(ref poly, TraitBoundModifier::Maybe) = *bound {
 103                 let mut err = self.err_handler().struct_span_err(poly.span,
 104                     &format!("`?Trait` is not permitted in {}", where_));
 105                 if is_trait {
 106                     err.note(&format!("traits are `?{}` by default", poly.trait_ref.path));
 107                 }
 108                 err.emit();
 109             }
 110         }
 111     }
 112
 113     /// matches '-' lit | lit (cf. parser::Parser::parse_literal_maybe_minus),
 114     /// or path for ranges.
 115     ///
 116     /// FIXME: do we want to allow expr -> pattern conversion to create path expressions?
 117     /// That means making this work:
 118     ///
 119     /// ```rust,ignore (FIXME)
 120     ///     struct S;
 121     ///     macro_rules! m {
 122     ///         ($a:expr) => {
 123     ///             let $a = S;
 124     ///         }
 125     ///     }
 126     ///     m!(S);
 127     /// ```
 128     fn check_expr_within_pat(&self, expr: &Expr, allow_paths: bool) {
 129         match expr.node {
 130             ExprKind::Lit(..) => {}
 131             ExprKind::Path(..) if allow_paths => {}
 132             ExprKind::Unary(UnOp::Neg, ref inner)
 133                 if match inner.node { ExprKind::Lit(_) => true, _ => false } => {}
 134             _ => self.err_handler().span_err(expr.span, "arbitrary expressions aren't allowed \
 135                                                          in patterns")
 136         }
 137     }
 138
 139     fn check_late_bound_lifetime_defs(&self, params: &[GenericParam]) {
 140         // Check only lifetime parameters are present and that the lifetime
 141         // parameters that are present have no bounds.
 142         let non_lt_param_spans: Vec<_> = params.iter().filter_map(|param| match param.kind {
 143             GenericParamKind::Lifetime { .. } => {
 144                 if !param.bounds.is_empty() {
 145                     let spans: Vec<_> = param.bounds.iter().map(|b| b.span()).collect();
 146                     self.err_handler()
 147                         .span_err(spans, "lifetime bounds cannot be used in this context");
 148                 }
 149                 None
 150             }
 151             _ => Some(param.ident.span),
 152         }).collect();
 153         if !non_lt_param_spans.is_empty() {
 154             self.err_handler().span_err(non_lt_param_spans,
 155                 "only lifetime parameters can be used in this context");
 156         }
 157     }
 158
 159     /// With eRFC 2497, we need to check whether an expression is ambiguous and warn or error
 160     /// depending on the edition, this function handles that.
 161     fn while_if_let_ambiguity(&self, expr: &P<Expr>) {
 162         if let Some((span, op_kind)) = self.while_if_let_expr_ambiguity(&expr) {
 163             let mut err = self.err_handler().struct_span_err(
 164                 span, &format!("ambiguous use of `{}`", op_kind.to_string())
 165             );
 166
 167             err.note(
 168                 "this will be a error until the `let_chains` feature is stabilized"
 169             );
 170             err.note(
 171                 "see rust-lang/rust#53668 for more information"
 172             );
 173
 174             if let Ok(snippet) = self.session.source_map().span_to_snippet(span) {
 175                 err.span_suggestion_with_applicability(
 176                     span, "consider adding parentheses", format!("({})", snippet),
 177                     Applicability::MachineApplicable,
 178                 );
 179             }
 180
 181             err.emit();
 182         }
 183     }
 184
 185     /// With eRFC 2497 adding if-let chains, there is a requirement that the parsing of
 186     /// `&&` and `||` in a if-let statement be unambiguous. This function returns a span and
 187     /// a `BinOpKind` (either `&&` or `||` depending on what was ambiguous) if it is determined
 188     /// that the current expression parsed is ambiguous and will break in future.
 189     fn while_if_let_expr_ambiguity(&self, expr: &P<Expr>) -> Option<(Span, BinOpKind)> {
 190         debug!("while_if_let_expr_ambiguity: expr.node: {:?}", expr.node);
 191         match &expr.node {
 192             ExprKind::Binary(op, _, _) if op.node == BinOpKind::And || op.node == BinOpKind::Or => {
 193                 Some((expr.span, op.node))
 194             },
 195             ExprKind::Range(ref lhs, ref rhs, _) => {
 196                 let lhs_ambiguous = lhs.as_ref()
 197                     .and_then(|lhs| self.while_if_let_expr_ambiguity(lhs));
 198                 let rhs_ambiguous = rhs.as_ref()
 199                     .and_then(|rhs| self.while_if_let_expr_ambiguity(rhs));
 200
 201                 lhs_ambiguous.or(rhs_ambiguous)
 202             }
 203             _ => None,
 204         }
 205     }
 206
 207 }
 208
 209 impl<'a> Visitor<'a> for AstValidator<'a> {
 210     fn visit_expr(&mut self, expr: &'a Expr) {
 211         match expr.node {
 212             ExprKind::IfLet(_, ref expr, _, _) | ExprKind::WhileLet(_, ref expr, _, _) =>
 213                 self.while_if_let_ambiguity(&expr),
 214             ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
 215                 span_err!(self.session, expr.span, E0472, "asm! is unsupported on this target");
 216             }
 217             ExprKind::ObsoleteInPlace(ref place, ref val) => {
 218                 let mut err = self.err_handler().struct_span_err(
 219                     expr.span,
 220                     "emplacement syntax is obsolete (for now, anyway)",
 221                 );
 222                 err.note(
 223                     "for more information, see \
 224                      <https://github.com/rust-lang/rust/issues/27779#issuecomment-378416911>"
 225                 );
 226                 match val.node {
 227                     ExprKind::Lit(ref v) if v.node.is_numeric() => {
 228                         err.span_suggestion_with_applicability(
 229                             place.span.between(val.span),
 230                             "if you meant to write a comparison against a negative value, add a \
 231                              space in between `<` and `-`",
 232                             "< -".to_string(),
 233                             Applicability::MaybeIncorrect
 234                         );
 235                     }
 236                     _ => {}
 237                 }
 238                 err.emit();
 239             }
 240             _ => {}
 241         }
 242
 243         visit::walk_expr(self, expr)
 244     }
 245
 246     fn visit_ty(&mut self, ty: &'a Ty) {
 247         match ty.node {
 248             TyKind::BareFn(ref bfty) => {
 249                 self.check_decl_no_pat(&bfty.decl, |span, _| {
 250                     struct_span_err!(self.session, span, E0561,
 251                                      "patterns aren't allowed in function pointer types").emit();
 252                 });
 253                 self.check_late_bound_lifetime_defs(&bfty.generic_params);
 254             }
 255             TyKind::TraitObject(ref bounds, ..) => {
 256                 let mut any_lifetime_bounds = false;
 257                 for bound in bounds {
 258                     if let GenericBound::Outlives(ref lifetime) = *bound {
 259                         if any_lifetime_bounds {
 260                             span_err!(self.session, lifetime.ident.span, E0226,
 261                                       "only a single explicit lifetime bound is permitted");
 262                             break;
 263                         }
 264                         any_lifetime_bounds = true;
 265                     }
 266                 }
 267                 self.no_questions_in_bounds(bounds, "trait object types", false);
 268             }
 269             TyKind::ImplTrait(_, ref bounds) => {
 270                 if !bounds.iter()
 271                           .any(|b| if let GenericBound::Trait(..) = *b { true } else { false }) {
 272                     self.err_handler().span_err(ty.span, "at least one trait must be specified");
 273                 }
 274             }
 275             _ => {}
 276         }
 277
 278         visit::walk_ty(self, ty)
 279     }
 280
 281     fn visit_label(&mut self, label: &'a Label) {
 282         self.check_label(label.ident);
 283         visit::walk_label(self, label);
 284     }
 285
 286     fn visit_lifetime(&mut self, lifetime: &'a Lifetime) {
 287         self.check_lifetime(lifetime.ident);
 288         visit::walk_lifetime(self, lifetime);
 289     }
 290
 291     fn visit_item(&mut self, item: &'a Item) {
 292         match item.node {
 293             ItemKind::Impl(unsafety, polarity, _, _, Some(..), ref ty, ref impl_items) => {
 294                 self.invalid_visibility(&item.vis, None);
 295                 if let TyKind::Err = ty.node {
 296                     self.err_handler()
 297                         .struct_span_err(item.span, "`impl Trait for .. {}` is an obsolete syntax")
 298                         .help("use `auto trait Trait {}` instead").emit();
 299                 }
 300                 if unsafety == Unsafety::Unsafe && polarity == ImplPolarity::Negative {
 301                     span_err!(self.session, item.span, E0198, "negative impls cannot be unsafe");
 302                 }
 303                 for impl_item in impl_items {
 304                     self.invalid_visibility(&impl_item.vis, None);
 305                     if let ImplItemKind::Method(ref sig, _) = impl_item.node {
 306                         self.check_trait_fn_not_const(sig.header.constness);
 307                         self.check_trait_fn_not_async(impl_item.span, sig.header.asyncness);
 308                     }
 309                 }
 310             }
 311             ItemKind::Impl(unsafety, polarity, defaultness, _, None, _, _) => {
 312                 self.invalid_visibility(&item.vis,
 313                                         Some("place qualifiers on individual impl items instead"));
 314                 if unsafety == Unsafety::Unsafe {
 315                     span_err!(self.session, item.span, E0197, "inherent impls cannot be unsafe");
 316                 }
 317                 if polarity == ImplPolarity::Negative {
 318                     self.err_handler().span_err(item.span, "inherent impls cannot be negative");
 319                 }
 320                 if defaultness == Defaultness::Default {
 321                     self.err_handler()
 322                         .struct_span_err(item.span, "inherent impls cannot be default")
 323                         .note("only trait implementations may be annotated with default").emit();
 324                 }
 325             }
 326             ItemKind::ForeignMod(..) => {
 327                 self.invalid_visibility(
 328                     &item.vis,
 329                     Some("place qualifiers on individual foreign items instead"),
 330                 );
 331             }
 332             ItemKind::Enum(ref def, _) => {
 333                 for variant in &def.variants {
 334                     self.invalid_non_exhaustive_attribute(variant);
 335                     for field in variant.node.data.fields() {
 336                         self.invalid_visibility(&field.vis, None);
 337                     }
 338                 }
 339             }
 340             ItemKind::Trait(is_auto, _, ref generics, ref bounds, ref trait_items) => {
 341                 if is_auto == IsAuto::Yes {
 342                     // Auto traits cannot have generics, super traits nor contain items.
 343                     if !generics.params.is_empty() {
 344                         struct_span_err!(self.session, item.span, E0567,
 345                                         "auto traits cannot have generic parameters").emit();
 346                     }
 347                     if !bounds.is_empty() {
 348                         struct_span_err!(self.session, item.span, E0568,
 349                                         "auto traits cannot have super traits").emit();
 350                     }
 351                     if !trait_items.is_empty() {
 352                         struct_span_err!(self.session, item.span, E0380,
 353                                 "auto traits cannot have methods or associated items").emit();
 354                     }
 355                 }
 356                 self.no_questions_in_bounds(bounds, "supertraits", true);
 357                 for trait_item in trait_items {
 358                     if let TraitItemKind::Method(ref sig, ref block) = trait_item.node {
 359                         self.check_trait_fn_not_async(trait_item.span, sig.header.asyncness);
 360                         self.check_trait_fn_not_const(sig.header.constness);
 361                         if block.is_none() {
 362                             self.check_decl_no_pat(&sig.decl, |span, mut_ident| {
 363                                 if mut_ident {
 364                                     self.session.buffer_lint(
 365                                         lint::builtin::PATTERNS_IN_FNS_WITHOUT_BODY,
 366                                         trait_item.id, span,
 367                                         "patterns aren't allowed in methods without bodies");
 368                                 } else {
 369                                     struct_span_err!(self.session, span, E0642,
 370                                         "patterns aren't allowed in methods without bodies").emit();
 371                                 }
 372                             });
 373                         }
 374                     }
 375                 }
 376             }
 377             ItemKind::Mod(_) => {
 378                 // Ensure that `path` attributes on modules are recorded as used (cf. issue #35584).
 379                 attr::first_attr_value_str_by_name(&item.attrs, "path");
 380                 if attr::contains_name(&item.attrs, "warn_directory_ownership") {
 381                     let lint = lint::builtin::LEGACY_DIRECTORY_OWNERSHIP;
 382                     let msg = "cannot declare a new module at this location";
 383                     self.session.buffer_lint(lint, item.id, item.span, msg);
 384                 }
 385             }
 386             ItemKind::Union(ref vdata, _) => {
 387                 if !vdata.is_struct() {
 388                     self.err_handler().span_err(item.span,
 389                                                 "tuple and unit unions are not permitted");
 390                 }
 391                 if vdata.fields().is_empty() {
 392                     self.err_handler().span_err(item.span,
 393                                                 "unions cannot have zero fields");
 394                 }
 395             }
 396             _ => {}
 397         }
 398
 399         visit::walk_item(self, item)
 400     }
 401
 402     fn visit_foreign_item(&mut self, fi: &'a ForeignItem) {
 403         match fi.node {
 404             ForeignItemKind::Fn(ref decl, _) => {
 405                 self.check_decl_no_pat(decl, |span, _| {
 406                     struct_span_err!(self.session, span, E0130,
 407                                      "patterns aren't allowed in foreign function declarations")
 408                         .span_label(span, "pattern not allowed in foreign function").emit();
 409                 });
 410             }
 411             ForeignItemKind::Static(..) | ForeignItemKind::Ty | ForeignItemKind::Macro(..) => {}
 412         }
 413
 414         visit::walk_foreign_item(self, fi)
 415     }
 416
 417     fn visit_generics(&mut self, generics: &'a Generics) {
 418         let mut seen_non_lifetime_param = false;
 419         let mut seen_default = None;
 420         for param in &generics.params {
 421             match (&param.kind, seen_non_lifetime_param) {
 422                 (GenericParamKind::Lifetime { .. }, true) => {
 423                     self.err_handler()
 424                         .span_err(param.ident.span, "lifetime parameters must be leading");
 425                 },
 426                 (GenericParamKind::Lifetime { .. }, false) => {}
 427                 (GenericParamKind::Type { ref default, .. }, _) => {
 428                     seen_non_lifetime_param = true;
 429                     if default.is_some() {
 430                         seen_default = Some(param.ident.span);
 431                     } else if let Some(span) = seen_default {
 432                         self.err_handler()
 433                             .span_err(span, "type parameters with a default must be trailing");
 434                         break;
 435                     }
 436                 }
 437             }
 438         }
 439         for predicate in &generics.where_clause.predicates {
 440             if let WherePredicate::EqPredicate(ref predicate) = *predicate {
 441                 self.err_handler().span_err(predicate.span, "equality constraints are not yet \
 442                                                              supported in where clauses (#20041)");
 443             }
 444         }
 445         visit::walk_generics(self, generics)
 446     }
 447
 448     fn visit_generic_param(&mut self, param: &'a GenericParam) {
 449         if let GenericParamKind::Lifetime { .. } = param.kind {
 450             self.check_lifetime(param.ident);
 451         }
 452         visit::walk_generic_param(self, param);
 453     }
 454
 455     fn visit_pat(&mut self, pat: &'a Pat) {
 456         match pat.node {
 457             PatKind::Lit(ref expr) => {
 458                 self.check_expr_within_pat(expr, false);
 459             }
 460             PatKind::Range(ref start, ref end, _) => {
 461                 self.check_expr_within_pat(start, true);
 462                 self.check_expr_within_pat(end, true);
 463             }
 464             _ => {}
 465         }
 466
 467         visit::walk_pat(self, pat)
 468     }
 469
 470     fn visit_where_predicate(&mut self, p: &'a WherePredicate) {
 471         if let &WherePredicate::BoundPredicate(ref bound_predicate) = p {
 472             // A type binding, eg `for<'c> Foo: Send+Clone+'c`
 473             self.check_late_bound_lifetime_defs(&bound_predicate.bound_generic_params);
 474         }
 475         visit::walk_where_predicate(self, p);
 476     }
 477
 478     fn visit_poly_trait_ref(&mut self, t: &'a PolyTraitRef, m: &'a TraitBoundModifier) {
 479         self.check_late_bound_lifetime_defs(&t.bound_generic_params);
 480         visit::walk_poly_trait_ref(self, t, m);
 481     }
 482
 483     fn visit_mac(&mut self, mac: &Spanned<Mac_>) {
 484         // when a new macro kind is added but the author forgets to set it up for expansion
 485         // because that's the only part that won't cause a compiler error
 486         self.session.diagnostic()
 487             .span_bug(mac.span, "macro invocation missed in expansion; did you forget to override \
 488                                  the relevant `fold_*()` method in `PlaceholderExpander`?");
 489     }
 490 }
 491
 492 // Bans nested `impl Trait`, e.g., `impl Into<impl Debug>`.
 493 // Nested `impl Trait` _is_ allowed in associated type position,
 494 // e.g `impl Iterator<Item=impl Debug>`
 495 struct NestedImplTraitVisitor<'a> {
 496     session: &'a Session,
 497     outer_impl_trait: Option<Span>,
 498 }
 499
 500 impl<'a> NestedImplTraitVisitor<'a> {
 501     fn with_impl_trait<F>(&mut self, outer_impl_trait: Option<Span>, f: F)
 502         where F: FnOnce(&mut NestedImplTraitVisitor<'a>)
 503     {
 504         let old_outer_impl_trait = self.outer_impl_trait;
 505         self.outer_impl_trait = outer_impl_trait;
 506         f(self);
 507         self.outer_impl_trait = old_outer_impl_trait;
 508     }
 509 }
 510
 511
 512 impl<'a> Visitor<'a> for NestedImplTraitVisitor<'a> {
 513     fn visit_ty(&mut self, t: &'a Ty) {
 514         if let TyKind::ImplTrait(..) = t.node {
 515             if let Some(outer_impl_trait) = self.outer_impl_trait {
 516                 struct_span_err!(self.session, t.span, E0666,
 517                                  "nested `impl Trait` is not allowed")
 518                     .span_label(outer_impl_trait, "outer `impl Trait`")
 519                     .span_label(t.span, "nested `impl Trait` here")
 520                     .emit();
 521
 522             }
 523             self.with_impl_trait(Some(t.span), |this| visit::walk_ty(this, t));
 524         } else {
 525             visit::walk_ty(self, t);
 526         }
 527     }
 528     fn visit_generic_args(&mut self, _: Span, generic_args: &'a GenericArgs) {
 529         match *generic_args {
 530             GenericArgs::AngleBracketed(ref data) => {
 531                 for arg in &data.args {
 532                     self.visit_generic_arg(arg)
 533                 }
 534                 for type_binding in &data.bindings {
 535                     // Type bindings such as `Item=impl Debug` in `Iterator<Item=Debug>`
 536                     // are allowed to contain nested `impl Trait`.
 537                     self.with_impl_trait(None, |this| visit::walk_ty(this, &type_binding.ty));
 538                 }
 539             }
 540             GenericArgs::Parenthesized(ref data) => {
 541                 for type_ in &data.inputs {
 542                     self.visit_ty(type_);
 543                 }
 544                 if let Some(ref type_) = data.output {
 545                     // `-> Foo` syntax is essentially an associated type binding,
 546                     // so it is also allowed to contain nested `impl Trait`.
 547                     self.with_impl_trait(None, |this| visit::walk_ty(this, type_));
 548                 }
 549             }
 550         }
 551     }
 552
 553     fn visit_mac(&mut self, _mac: &Spanned<Mac_>) {
 554         // covered in AstValidator
 555     }
 556 }
 557
 558 // Bans `impl Trait` in path projections like `<impl Iterator>::Item` or `Foo::Bar<impl Trait>`.
 559 struct ImplTraitProjectionVisitor<'a> {
 560     session: &'a Session,
 561     is_banned: bool,
 562 }
 563
 564 impl<'a> ImplTraitProjectionVisitor<'a> {
 565     fn with_ban<F>(&mut self, f: F)
 566         where F: FnOnce(&mut ImplTraitProjectionVisitor<'a>)
 567     {
 568         let old_is_banned = self.is_banned;
 569         self.is_banned = true;
 570         f(self);
 571         self.is_banned = old_is_banned;
 572     }
 573 }
 574
 575 impl<'a> Visitor<'a> for ImplTraitProjectionVisitor<'a> {
 576     fn visit_ty(&mut self, t: &'a Ty) {
 577         match t.node {
 578             TyKind::ImplTrait(..) => {
 579                 if self.is_banned {
 580                     struct_span_err!(self.session, t.span, E0667,
 581                         "`impl Trait` is not allowed in path parameters").emit();
 582                 }
 583             }
 584             TyKind::Path(ref qself, ref path) => {
 585                 // We allow these:
 586                 //  - `Option<impl Trait>`
 587                 //  - `option::Option<impl Trait>`
 588                 //  - `option::Option<T>::Foo<impl Trait>
 589                 //
 590                 // But not these:
 591                 //  - `<impl Trait>::Foo`
 592                 //  - `option::Option<impl Trait>::Foo`.
 593                 //
 594                 // To implement this, we disallow `impl Trait` from `qself`
 595                 // (for cases like `<impl Trait>::Foo>`)
 596                 // but we allow `impl Trait` in `GenericArgs`
 597                 // iff there are no more PathSegments.
 598                 if let Some(ref qself) = *qself {
 599                     // `impl Trait` in `qself` is always illegal
 600                     self.with_ban(|this| this.visit_ty(&qself.ty));
 601                 }
 602
 603                 for (i, segment) in path.segments.iter().enumerate() {
 604                     // Allow `impl Trait` iff we're on the final path segment
 605                     if i == path.segments.len() - 1 {
 606                         visit::walk_path_segment(self, path.span, segment);
 607                     } else {
 608                         self.with_ban(|this|
 609                             visit::walk_path_segment(this, path.span, segment));
 610                     }
 611                 }
 612             }
 613             _ => visit::walk_ty(self, t),
 614         }
 615     }
 616
 617     fn visit_mac(&mut self, _mac: &Spanned<Mac_>) {
 618         // covered in AstValidator
 619     }
 620 }
 621
 622 pub fn check_crate(session: &Session, krate: &Crate) {
 623     visit::walk_crate(
 624         &mut NestedImplTraitVisitor {
 625             session,
 626             outer_impl_trait: None,
 627         }, krate);
 628
 629     visit::walk_crate(
 630         &mut ImplTraitProjectionVisitor {
 631             session,
 632             is_banned: false,
 633         }, krate);
 634
 635     visit::walk_crate(&mut AstValidator { session }, krate)
 636 }