Detect missing if blocks

When unnecessarily using a fat arrow after an if condition, suggest the
removal of it.

When finding an if statement with no block, point at the `if` keyword to
provide more context.

src/test/ui/did_you_mean/issue-40006.stderr

@@ -19,7 +19,7 @@ error: expected `[`, found `#`
   --> $DIR/issue-40006.rs:20:17
    |
 LL |     fn xxx() { ### } //~ ERROR missing
-   |                 ^
+   |                 ^ expected `[`
 
 error: missing `fn`, `type`, or `const` for trait-item declaration
   --> $DIR/issue-40006.rs:20:21

src/test/ui/if-without-block.rs

@@ -0,0 +1,18 @@
+// Copyright 2018 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 n = 1;
+    if 5 == {
+    //~^ NOTE this `if` statement has a condition, but no block
+        println!("five");
+    }
+}
+//~^ ERROR expected `{`, found `}`

src/test/ui/if-without-block.stderr

@@ -0,0 +1,11 @@
+error: expected `{`, found `}`
+  --> $DIR/if-without-block.rs:17:1
+   |
+13 |     if 5 == {
+   |     -- this `if` statement has a condition, but no block
+...
+17 | }
+   | ^
+
+error: aborting due to previous error
+

src/test/ui/missing-block-hint.stderr

@@ -2,11 +2,13 @@ error: expected `{`, found `=>`
   --> $DIR/missing-block-hint.rs:13:18
    |
 LL |         if (foo) => {} //~ ERROR expected `{`, found `=>`
-   |                  ^^
+   |                  ^^ help: only necessary in match arms, not before if blocks
 
 error: expected `{`, found `bar`
   --> $DIR/missing-block-hint.rs:17:13
    |
+LL |         if (foo)
+   |         -- this `if` statement has a condition, but no block
 LL |             bar; //~ ERROR expected `{`, found `bar`
    |             ^^^-
    |             |