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
```
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
```
and from
```
error: foo
--> test.rs:3:6
|
3 | a { b { c } d }
| ^^^^-------^^^^
| | |
| |
|
```
to
```
error: foo
--> test.rs:3:6
|
3 | a { b { c } d }
| ^^^^-------^^^^
```
r? @nikomatsakis
cc @jonathandturner, @GuillaumeGomez, @nrc
d2d8ae6575