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/emitter.rs

@@ -335,43 +335,56 @@ impl EmitterWriter {
         // which is...less weird, at least. In fact, in general, if
         // the rightmost span overlaps with any other span, we should
         // use the "hang below" version, so we can at least make it
-        // clear where the span *starts*.
+        // clear where the span *starts*. There's an exception for this
+        // logic, when the labels do not have a message:
+        //
+        //      fn foo(x: u32) {
+        //      --------------
+        //             |
+        //             x_span
+        //
+        // instead of:
+        //
+        //      fn foo(x: u32) {
+        //      --------------
+        //      |      |
+        //      |      x_span
+        //      <EMPTY LINE>
+        //
         let mut annotations_position = vec![];
         let mut line_len = 0;
         let mut p = 0;
         let mut ann_iter = annotations.iter().peekable();
         while let Some(annotation) = ann_iter.next() {
-            let is_line = if let AnnotationType::MultilineLine(_) = annotation.annotation_type {
-                true
-            } else {
-                false
-            };
             let peek = ann_iter.peek();
             if let Some(next) = peek {
-                let next_is_line = if let AnnotationType::MultilineLine(_) = next.annotation_type {
-                    true
-                } else {
-                    false
-                };
-
-                if overlaps(next, annotation) && !is_line && !next_is_line {
+                if overlaps(next, annotation) && !annotation.is_line() && !next.is_line()
+                    && annotation.has_label()
+                {
+                    // This annotation needs a new line in the output.
                     p += 1;
                 }
             }
             annotations_position.push((p, annotation));
             if let Some(next) = peek {
-                let next_is_line = if let AnnotationType::MultilineLine(_) = next.annotation_type {
-                    true
-                } else {
-                    false
-                };
                 let l = if let Some(ref label) = next.label {
                     label.len() + 2
                 } else {
                     0
                 };
-                if (overlaps(next, annotation) || next.end_col + l > annotation.start_col)
-                    && !is_line && !next_is_line
+                if (overlaps(next, annotation)  // Do not allow two labels to be in the same line
+                    || next.end_col + l > annotation.start_col)  // if they overlap including
+                                                // padding, to avoid situations like:
+                                                //
+                                                //      fn foo(x: u32) {
+                                                //      -------^------
+                                                //      |      |
+                                                //      fn_spanx_span
+                                                //
+                    && !annotation.is_line()    // Do not add a new line if this annotation or the
+                    && !next.is_line()          // next are vertical line placeholders.
+                    && annotation.has_label()   // Both labels must have some text, otherwise
+                    && next.has_label()         // they are not overlapping.
                 {
                     p += 1;
                 }
@@ -408,6 +421,17 @@ impl EmitterWriter {
             return;
         }
 
+        // Write the colunmn separator.
+        //
+        // After this we will have:
+        //
+        // 2 |   fn foo() {
+        //   |
+        //   |
+        //   |
+        // 3 |
+        // 4 |   }
+        //   |
         for pos in 0..line_len + 1 {
             draw_col_separator(buffer, line_offset + pos + 1, width_offset - 2);
             buffer.putc(line_offset + pos + 1,
@@ -468,7 +492,8 @@ impl EmitterWriter {
                 Style::UnderlineSecondary
             };
             let pos = pos + 1;
-            if pos > 1 {
+
+            if pos > 1 && annotation.has_label() {
                 for p in line_offset + 1..line_offset + pos + 1 {
                     buffer.putc(p,
                                 code_offset + annotation.start_col,
@@ -546,16 +571,8 @@ impl EmitterWriter {
         //   | |  something about `foo`
         //   | something about `fn foo()`
         annotations_position.sort_by(|a, b| {
-            fn len(a: &Annotation) -> usize {
-                // Account for usize underflows
-                if a.end_col > a.start_col {
-                    a.end_col - a.start_col
-                } else {
-                    a.start_col - a.end_col
-                }
-            }
             // Decreasing order
-            len(a.1).cmp(&len(b.1)).reverse()
+            a.1.len().cmp(&b.1.len()).reverse()
         });
 
         // Write the underlines.

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)]