Auto merge of #40887 - estebank:ty-placeholder, r=petrochenkov

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
```

Fix #34264.

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

@@ -0,0 +1,20 @@
+// 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

@@ -0,0 +1,49 @@
+error: expected one of `:` or `@`, found `<`
+  --> $DIR/issue-34264.rs:11:14
+   |
+11 | fn foo(Option<i32>, String) {}
+   |              ^ expected one of `:` or `@` here
+
+error: expected one of `:` or `@`, found `)`
+  --> $DIR/issue-34264.rs:11:27
+   |
+11 | fn foo(Option<i32>, String) {}
+   |                           ^ expected one of `:` or `@` here
+
+error: expected one of `:` or `@`, found `,`
+  --> $DIR/issue-34264.rs:12:9
+   |
+12 | fn bar(x, y: usize) {}
+   |         ^ expected one of `:` or `@` here
+
+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
+