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
```

1 files changed, 388 insertions, 0 deletions

diff --git a/src/libsyntax/test_snippet.rs b/src/libsyntax/test_snippet.rs
index 98e574867b4..c6d6e6237f2 100644
--- a/src/libsyntax/test_snippet.rs
+++ b/src/libsyntax/test_snippet.rs
@@ -494,6 +494,7 @@ error: foo
 
 "#);
 }
+
 #[test]
 fn overlaping_start_and_end() {
     test_harness(r#"
@@ -544,3 +545,390 @@ error: foo
 
 "#);
 }
+
+#[test]
+fn multiple_labels_primary_without_message() {
+    test_harness(r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+    vec![
+        SpanLabel {
+            start: Position {
+                string: "b",
+                count: 1,
+            },
+            end: Position {
+                string: "}",
+                count: 1,
+            },
+            label: "",
+        },
+        SpanLabel {
+            start: Position {
+                string: "a",
+                count: 1,
+            },
+            end: Position {
+                string: "d",
+                count: 1,
+            },
+            label: "`a` is a good letter",
+        },
+        SpanLabel {
+            start: Position {
+                string: "c",
+                count: 1,
+            },
+            end: Position {
+                string: "c",
+                count: 1,
+            },
+            label: "",
+        },
+    ],
+    r#"
+error: foo
+ --> test.rs:3:7
+  |
+3 |   a { b { c } d }
+  |   ----^^^^-^^-- `a` is a good letter
+
+"#);
+}
+
+#[test]
+fn multiple_labels_secondary_without_message() {
+    test_harness(r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+    vec![
+        SpanLabel {
+            start: Position {
+                string: "a",
+                count: 1,
+            },
+            end: Position {
+                string: "d",
+                count: 1,
+            },
+            label: "`a` is a good letter",
+        },
+        SpanLabel {
+            start: Position {
+                string: "b",
+                count: 1,
+            },
+            end: Position {
+                string: "}",
+                count: 1,
+            },
+            label: "",
+        },
+    ],
+    r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^-------^^ `a` is a good letter
+
+"#);
+}
+
+#[test]
+fn multiple_labels_primary_without_message_2() {
+    test_harness(r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+    vec![
+        SpanLabel {
+            start: Position {
+                string: "b",
+                count: 1,
+            },
+            end: Position {
+                string: "}",
+                count: 1,
+            },
+            label: "`b` is a good letter",
+        },
+        SpanLabel {
+            start: Position {
+                string: "a",
+                count: 1,
+            },
+            end: Position {
+                string: "d",
+                count: 1,
+            },
+            label: "",
+        },
+        SpanLabel {
+            start: Position {
+                string: "c",
+                count: 1,
+            },
+            end: Position {
+                string: "c",
+                count: 1,
+            },
+            label: "",
+        },
+    ],
+    r#"
+error: foo
+ --> test.rs:3:7
+  |
+3 |   a { b { c } d }
+  |   ----^^^^-^^--
+  |       |
+  |       `b` is a good letter
+
+"#);
+}
+
+#[test]
+fn multiple_labels_secondary_without_message_2() {
+    test_harness(r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+    vec![
+        SpanLabel {
+            start: Position {
+                string: "a",
+                count: 1,
+            },
+            end: Position {
+                string: "d",
+                count: 1,
+            },
+            label: "",
+        },
+        SpanLabel {
+            start: Position {
+                string: "b",
+                count: 1,
+            },
+            end: Position {
+                string: "}",
+                count: 1,
+            },
+            label: "`b` is a good letter",
+        },
+    ],
+    r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^-------^^
+  |       |
+  |       `b` is a good letter
+
+"#);
+}
+
+#[test]
+fn multiple_labels_without_message() {
+    test_harness(r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+    vec![
+        SpanLabel {
+            start: Position {
+                string: "a",
+                count: 1,
+            },
+            end: Position {
+                string: "d",
+                count: 1,
+            },
+            label: "",
+        },
+        SpanLabel {
+            start: Position {
+                string: "b",
+                count: 1,
+            },
+            end: Position {
+                string: "}",
+                count: 1,
+            },
+            label: "",
+        },
+    ],
+    r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^-------^^
+
+"#);
+}
+
+#[test]
+fn multiple_labels_without_message_2() {
+    test_harness(r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+    vec![
+        SpanLabel {
+            start: Position {
+                string: "b",
+                count: 1,
+            },
+            end: Position {
+                string: "}",
+                count: 1,
+            },
+            label: "",
+        },
+        SpanLabel {
+            start: Position {
+                string: "a",
+                count: 1,
+            },
+            end: Position {
+                string: "d",
+                count: 1,
+            },
+            label: "",
+        },
+        SpanLabel {
+            start: Position {
+                string: "c",
+                count: 1,
+            },
+            end: Position {
+                string: "c",
+                count: 1,
+            },
+            label: "",
+        },
+    ],
+    r#"
+error: foo
+ --> test.rs:3:7
+  |
+3 |   a { b { c } d }
+  |   ----^^^^-^^--
+
+"#);
+}
+
+#[test]
+fn multiple_labels_with_message() {
+    test_harness(r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+    vec![
+        SpanLabel {
+            start: Position {
+                string: "a",
+                count: 1,
+            },
+            end: Position {
+                string: "d",
+                count: 1,
+            },
+            label: "`a` is a good letter",
+        },
+        SpanLabel {
+            start: Position {
+                string: "b",
+                count: 1,
+            },
+            end: Position {
+                string: "}",
+                count: 1,
+            },
+            label: "`b` is a good letter",
+        },
+    ],
+    r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^-------^^
+  |   |   |
+  |   |   `b` is a good letter
+  |   `a` is a good letter
+
+"#);
+}
+
+#[test]
+fn single_label_with_message() {
+    test_harness(r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+    vec![
+        SpanLabel {
+            start: Position {
+                string: "a",
+                count: 1,
+            },
+            end: Position {
+                string: "d",
+                count: 1,
+            },
+            label: "`a` is a good letter",
+        },
+    ],
+    r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^^^^^^^^^^ `a` is a good letter
+
+"#);
+}
+
+#[test]
+fn single_label_without_message() {
+    test_harness(r#"
+fn foo() {
+  a { b { c } d }
+}
+"#,
+    vec![
+        SpanLabel {
+            start: Position {
+                string: "a",
+                count: 1,
+            },
+            end: Position {
+                string: "d",
+                count: 1,
+            },
+            label: "",
+        },
+    ],
+    r#"
+error: foo
+ --> test.rs:3:3
+  |
+3 |   a { b { c } d }
+  |   ^^^^^^^^^^^^^
+
+"#);
+}