Introduce `TyErr` independent from `TyInfer`

Add a `TyErr` type to represent unknown types in places where
parse errors have happened, while still able to build the AST.

Initially only used to represent incorrectly written fn arguments and
avoid "expected X parameters, found Y" errors when called with the
appropriate amount of parameters. We cannot use `TyInfer` for this as
`_` is not allowed as a valid argument type.

Example output:

```rust
error: expected one of `:` or `@`, found `,`
  --> file.rs:12:9
   |
12 | fn bar(x, y: usize) {}
   |         ^

error[E0061]: this function takes 2 parameters but 3 parameters were supplied
  --> file.rs:19:9
   |
12 | fn bar(x, y) {}
   | --------------- defined here
...
19 |     bar(1, 2, 3);
   |         ^^^^^^^ expected 2 parameters
```

src/librustc/hir/intravisit.rs

@@ -578,7 +578,7 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty) {
         TyTypeof(expression) => {
             visitor.visit_nested_body(expression)
         }
-        TyInfer => {}
+        TyInfer | TyErr => {}
     }
 }
 

src/librustc/hir/lowering.rs

@@ -555,6 +555,7 @@ fn lower_ty_binding(&mut self, b: &TypeBinding) -> hir::TypeBinding {
     fn lower_ty(&mut self, t: &Ty) -> P<hir::Ty> {
         let kind = match t.node {
             TyKind::Infer => hir::TyInfer,
+            TyKind::Err => hir::TyErr,
             TyKind::Slice(ref ty) => hir::TySlice(self.lower_ty(ty)),
             TyKind::Ptr(ref mt) => hir::TyPtr(self.lower_mt(mt)),
             TyKind::Rptr(ref region, ref mt) => {

src/librustc/hir/mod.rs

@@ -1351,6 +1351,8 @@ pub enum Ty_ {
     /// TyInfer means the type should be inferred instead of it having been
     /// specified. This can appear anywhere in a type.
     TyInfer,
+    /// Placeholder for a type that has failed to be defined.
+    TyErr,
 }
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]

src/librustc/hir/print.rs

@@ -450,6 +450,9 @@ pub fn print_type(&mut self, ty: &hir::Ty) -> io::Result<()> {
             hir::TyInfer => {
                 word(&mut self.s, "_")?;
             }
+            hir::TyErr => {
+                word(&mut self.s, "?")?;
+            }
         }
         self.end()
     }

src/librustc_incremental/calculate_svh/svh_visitor.rs

@@ -432,7 +432,8 @@ enum SawTyComponent {
     SawTyObjectSum,
     SawTyImplTrait,
     SawTyTypeof,
-    SawTyInfer
+    SawTyInfer,
+    SawTyErr,
 }
 
 fn saw_ty(node: &Ty_) -> SawTyComponent {
@@ -448,7 +449,8 @@ fn saw_ty(node: &Ty_) -> SawTyComponent {
       TyTraitObject(..) => SawTyObjectSum,
       TyImplTrait(..) => SawTyImplTrait,
       TyTypeof(..) => SawTyTypeof,
-      TyInfer => SawTyInfer
+      TyInfer => SawTyInfer,
+      TyErr => SawTyErr,
     }
 }
 

src/librustc_typeck/astconv.rs

@@ -1229,6 +1229,9 @@ pub fn ast_ty_to_ty(&self, ast_ty: &hir::Ty) -> Ty<'tcx> {
                 // handled specially and will not descend into this routine.
                 self.ty_infer(ast_ty.span)
             }
+            hir::TyErr => {
+                tcx.types.err
+            }
         };
 
         cache.borrow_mut().insert(ast_ty.id, result_ty);

src/librustdoc/clean/mod.rs

@@ -1805,7 +1805,7 @@ fn clean(&self, cx: &DocContext) -> Type {
             }
             TyBareFn(ref barefn) => BareFunction(box barefn.clean(cx)),
             TyImplTrait(ref bounds) => ImplTrait(bounds.clean(cx)),
-            TyInfer => Infer,
+            TyInfer | TyErr => Infer,
             TyTypeof(..) => panic!("Unimplemented type {:?}", self.node),
         }
     }

src/libsyntax/ast.rs

@@ -1361,6 +1361,8 @@ pub enum TyKind {
     ImplicitSelf,
     // A macro in the type position.
     Mac(Mac),
+    /// Placeholder for a kind that has failed to be defined.
+    Err,
 }
 
 /// Inline assembly dialect.

src/libsyntax/fold.rs

@@ -358,7 +358,7 @@ pub fn noop_fold_ty<T: Folder>(t: P<Ty>, fld: &mut T) -> P<Ty> {
     t.map(|Ty {id, node, span}| Ty {
         id: fld.new_id(id),
         node: match node {
-            TyKind::Infer | TyKind::ImplicitSelf => node,
+            TyKind::Infer | TyKind::ImplicitSelf | TyKind::Err => node,
             TyKind::Slice(ty) => TyKind::Slice(fld.fold_ty(ty)),
             TyKind::Ptr(mt) => TyKind::Ptr(fld.fold_mt(mt)),
             TyKind::Rptr(region, mt) => {

src/libsyntax/parse/parser.rs

@@ -407,6 +407,25 @@ fn from(expr: P<Expr>) -> Self {
     }
 }
 
+/// Create a placeholder argument.
+fn dummy_arg(span: Span) -> Arg {
+    let spanned = Spanned {
+        span: span,
+        node: keywords::Invalid.ident()
+    };
+    let pat = P(Pat {
+        id: ast::DUMMY_NODE_ID,
+        node: PatKind::Ident(BindingMode::ByValue(Mutability::Immutable), spanned, None),
+        span: span
+    });
+    let ty = Ty {
+        node: TyKind::Err,
+        span: span,
+        id: ast::DUMMY_NODE_ID
+    };
+    Arg { ty: P(ty), pat: pat, id: ast::DUMMY_NODE_ID }
+}
+
 impl<'a> Parser<'a> {
     pub fn new(sess: &'a ParseSess,
                tokens: TokenStream,
@@ -4343,8 +4362,12 @@ fn parse_fn_args(&mut self, named_args: bool, allow_variadic: bool)
                             Ok(arg) => Ok(Some(arg)),
                             Err(mut e) => {
                                 e.emit();
+                                let lo = p.prev_span;
+                                // Skip every token until next possible arg or end.
                                 p.eat_to_tokens(&[&token::Comma, &token::CloseDelim(token::Paren)]);
-                                Ok(None)
+                                // Create a placeholder argument for proper arg count (#34264).
+                                let span = lo.to(p.prev_span);
+                                Ok(Some(dummy_arg(span)))
                             }
                         }
                     }

src/libsyntax/print/pprust.rs

@@ -1095,6 +1095,9 @@ pub fn print_type(&mut self, ty: &ast::Ty) -> io::Result<()> {
             ast::TyKind::Infer => {
                 word(&mut self.s, "_")?;
             }
+            ast::TyKind::Err => {
+                word(&mut self.s, "?")?;
+            }
             ast::TyKind::ImplicitSelf => {
                 word(&mut self.s, "Self")?;
             }

src/libsyntax/visit.rs

@@ -350,7 +350,7 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) {
         TyKind::Typeof(ref expression) => {
             visitor.visit_expr(expression)
         }
-        TyKind::Infer | TyKind::ImplicitSelf => {}
+        TyKind::Infer | TyKind::ImplicitSelf | TyKind::Err => {}
         TyKind::Mac(ref mac) => {
             visitor.visit_mac(mac)
         }

src/test/ui/span/issue-34264.rs

+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn foo(Option<i32>, String) {}
+fn bar(x, y: usize) {}
+
+fn main() {
+    foo(Some(42), 2);
+    foo(Some(42), 2, "");
+    bar("", "");
+    bar(1, 2);
+    bar(1, 2, 3);
+}

src/test/ui/span/issue-34264.stderr

+error: expected one of `:` or `@`, found `<`
+  --> $DIR/issue-34264.rs:11:14
+   |
+11 | fn foo(Option<i32>, String) {}
+   |              ^
+
+error: expected one of `:` or `@`, found `)`
+  --> $DIR/issue-34264.rs:11:27
+   |
+11 | fn foo(Option<i32>, String) {}
+   |                           ^
+
+error: expected one of `:` or `@`, found `,`
+  --> $DIR/issue-34264.rs:12:9
+   |
+12 | fn bar(x, y: usize) {}
+   |         ^
+
+error[E0061]: this function takes 2 parameters but 3 parameters were supplied
+  --> $DIR/issue-34264.rs:16:9
+   |
+11 | fn foo(Option<i32>, String) {}
+   | ------------------------------ defined here
+...
+16 |     foo(Some(42), 2, "");
+   |         ^^^^^^^^^^^^^^^ expected 2 parameters
+
+error[E0308]: mismatched types
+  --> $DIR/issue-34264.rs:17:13
+   |
+17 |     bar("", "");
+   |             ^^ expected usize, found reference
+   |
+   = note: expected type `usize`
+              found type `&'static str`
+   = help: here are some functions which might fulfill your needs:
+           - .len()
+
+error[E0061]: this function takes 2 parameters but 3 parameters were supplied
+  --> $DIR/issue-34264.rs:19:9
+   |
+12 | fn bar(x, y: usize) {}
+   | ---------------------- defined here
+...
+19 |     bar(1, 2, 3);
+   |         ^^^^^^^ expected 2 parameters
+
+error: aborting due to 3 previous errors
+