Fix multiple labels when some don't have message

The diagnostic emitter now accounts for labels with no text message,
presenting the underline on its own, without drawing the line for the
non existing message below it. Go from

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ----^^^^^^^----
  |   |   |
  |   |   `b` is a good letter
  |
```

to

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ----^^^^^^^----
  |       |
  |       `b` is a good letter
```

and from

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ^^^^-------^^^^
  |   |   |
  |   |
  |   `a` is a good letter
```

to

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ^^^^-------^^^^ `a` is a good letter
```

src/librustc_errors/snippet.rs

@@ -151,6 +151,15 @@ impl Annotation {
         }
     }
 
+    /// Wether this annotation is a vertical line placeholder.
+    pub fn is_line(&self) -> bool {
+        if let AnnotationType::MultilineLine(_) = self.annotation_type {
+            true
+        } else {
+            false
+        }
+    }
+
     pub fn is_multiline(&self) -> bool {
         match self.annotation_type {
             AnnotationType::Multiline(_) |
@@ -161,6 +170,32 @@ impl Annotation {
         }
     }
 
+    pub fn len(&self) -> usize {
+        // Account for usize underflows
+        if self.end_col > self.start_col {
+            self.end_col - self.start_col
+        } else {
+            self.start_col - self.end_col
+        }
+    }
+
+    pub fn has_label(&self) -> bool {
+        if let Some(ref label) = self.label {
+            // Consider labels with no text as effectively not being there
+            // to avoid weird output with unnecessary vertical lines, like:
+            //
+            //     X | fn foo(x: u32) {
+            //       | -------^------
+            //       | |      |
+            //       | |
+            //       |
+            //
+            // Note that this would be the complete output users would see.
+            label.len() > 0
+        } else {
+            false
+        }
+    }
 }
 
 #[derive(Debug)]