Detect = -> : typo in let bindings

When encountering a let binding type error, attempt to parse as
initializer instead. If successful, it is likely just a typo:

```rust
fn main() {
    let x: Vec::with_capacity(10);
}
```

```
error: expected type, found `10`
 --> file.rs:3:31
  |
3 |     let x: Vec::with_capacity(10, 20);
  |         --                    ^^
  |         ||
  |         |help: did you mean assign here?: `=`
  |         while parsing the type for `x`
```

src/test/compile-fail/self-vs-path-ambiguity.rs

@@ -17,7 +17,6 @@ impl S {
     fn g(&self::S: &S) {}
     fn h(&mut self::S: &mut S) {}
     fn i(&'a self::S: &S) {} //~ ERROR unexpected lifetime `'a` in pattern
-                             //~^ ERROR expected one of `)` or `mut`, found `'a`
 }
 
 fn main() {}

src/test/parse-fail/issue-33413.rs

@@ -12,5 +12,4 @@
 
 impl S {
     fn f(*, a: u8) -> u8 {} //~ ERROR expected pattern, found `*`
-    //~^ ERROR expected one of `)`, `-`, `box`, `false`, `mut`, `ref`, or `true`, found `*`
 }

src/test/ui/issue-44406.stderr

@@ -13,14 +13,5 @@ error: expected type, found keyword `true`
 18 |     foo!(true);
    |          ^^^^ expecting a type here because of type ascription
 
-error: expected one of `!`, `&&`, `&`, `(`, `*`, `.`, `;`, `<`, `?`, `[`, `_`, `dyn`, `extern`, `fn`, `for`, `impl`, `unsafe`, `}`, an operator, or lifetime, found `true`
-  --> $DIR/issue-44406.rs:18:10
-   |
-13 |         bar(baz: $rest)
-   |                 - expected one of 20 possible tokens here
-...
-18 |     foo!(true);
-   |          ^^^^ unexpected token
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 

src/test/ui/resolve/token-error-correct.stderr

@@ -28,29 +28,11 @@ error: expected expression, found `;`
 14 |     foo(bar(;
    |             ^
 
-error: expected one of `)`, `,`, `.`, `<`, `?`, `break`, `continue`, `false`, `for`, `if`, `loop`, `match`, `move`, `return`, `true`, `unsafe`, `while`, `yield`, or an operator, found `;`
-  --> $DIR/token-error-correct.rs:14:13
-   |
-14 |     foo(bar(;
-   |             ^ expected one of 19 possible tokens here
-
 error: expected expression, found `)`
   --> $DIR/token-error-correct.rs:23:1
    |
 23 | }
    | ^
 
-error[E0425]: cannot find function `foo` in this scope
-  --> $DIR/token-error-correct.rs:14:5
-   |
-14 |     foo(bar(;
-   |     ^^^ not found in this scope
-
-error[E0425]: cannot find function `bar` in this scope
-  --> $DIR/token-error-correct.rs:14:9
-   |
-14 |     foo(bar(;
-   |         ^^^ not found in this scope
-
-error: aborting due to 7 previous errors
+error: aborting due to 4 previous errors
 

src/test/ui/suggestions/type-ascription-instead-of-initializer.rs

@@ -0,0 +1,13 @@
+// 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 main() {
+    let x: Vec::with_capacity(10, 20);
+}

src/test/ui/suggestions/type-ascription-instead-of-initializer.stderr

@@ -0,0 +1,17 @@
+error: expected type, found `10`
+  --> $DIR/type-ascription-instead-of-initializer.rs:12:31
+   |
+12 |     let x: Vec::with_capacity(10, 20);
+   |         --                    ^^
+   |         ||
+   |         |help: use `=` if you meant to assign
+   |         while parsing the type for `x`
+
+error[E0061]: this function takes 1 parameter but 2 parameters were supplied
+  --> $DIR/type-ascription-instead-of-initializer.rs:12:31
+   |
+12 |     let x: Vec::with_capacity(10, 20);
+   |                               ^^^^^^ expected 1 parameter
+
+error: aborting due to 2 previous errors
+