When displaying multispans, ignore empty lines adjacent to `...`

```
error[E0308]: `match` arms have incompatible types
   --> tests/ui/codemap_tests/huge_multispan_highlight.rs:98:18
    |
6   |       let _ = match true {
    |               ---------- `match` arms have incompatible types
7   |           true => (
    |  _________________-
8   | |             // last line shown in multispan header
...   |
96  | |
97  | |         ),
    | |_________- this is found to be of type `()`
98  |           false => "
    |  __________________^
...   |
119 | |
120 | |         ",
    | |_________^ expected `()`, found `&str`

error[E0308]: `match` arms have incompatible types
   --> tests/ui/codemap_tests/huge_multispan_highlight.rs:215:18
    |
122 |       let _ = match true {
    |               ---------- `match` arms have incompatible types
123 |           true => (
    |  _________________-
124 | |
125 | |         1 // last line shown in multispan header
...   |
213 | |
214 | |         ),
    | |_________- this is found to be of type `{integer}`
215 |           false => "
    |  __________________^
216 | |
217 | |
218 | |         1 last line shown in multispan
...   |
237 | |
238 | |         ",
    | |_________^ expected integer, found `&str`
```

1 files changed, 48 insertions, 1 deletions

diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index 4f033e3fefa..fceccb7e9b6 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/compiler/rustc_errors/src/emitter.rs
@@ -1638,6 +1638,27 @@ impl HumanEmitter {
                                     *style,
                                 );
                             }
+                            if let Some(line) = annotated_file.lines.get(line_idx) {
+                                for ann in &line.annotations {
+                                    if let AnnotationType::MultilineStart(pos) = ann.annotation_type
+                                    {
+                                        // In the case where we have elided the entire start of the
+                                        // multispan because those lines were empty, we still need
+                                        // to draw the `|`s across the `...`.
+                                        draw_multiline_line(
+                                            &mut buffer,
+                                            last_buffer_line_num,
+                                            width_offset,
+                                            pos,
+                                            if ann.is_primary {
+                                                Style::UnderlinePrimary
+                                            } else {
+                                                Style::UnderlineSecondary
+                                            },
+                                        );
+                                    }
+                                }
+                            }
                         } else if line_idx_delta == 2 {
                             let unannotated_line = annotated_file
                                 .file
@@ -1665,6 +1686,24 @@ impl HumanEmitter {
                                     *style,
                                 );
                             }
+                            if let Some(line) = annotated_file.lines.get(line_idx) {
+                                for ann in &line.annotations {
+                                    if let AnnotationType::MultilineStart(pos) = ann.annotation_type
+                                    {
+                                        draw_multiline_line(
+                                            &mut buffer,
+                                            last_buffer_line_num,
+                                            width_offset,
+                                            pos,
+                                            if ann.is_primary {
+                                                Style::UnderlinePrimary
+                                            } else {
+                                                Style::UnderlineSecondary
+                                            },
+                                        );
+                                    }
+                                }
+                            }
                         }
                     }
 
@@ -2417,7 +2456,15 @@ impl FileWithAnnotatedLines {
                 // the beginning doesn't have an underline, but the current logic seems to be
                 // working correctly.
                 let middle = min(ann.line_start + 4, ann.line_end);
-                for line in ann.line_start + 1..middle {
+                // We'll show up to 4 lines past the beginning of the multispan start.
+                // We will *not* include the tail of lines that are only whitespace.
+                let until = (ann.line_start..middle)
+                    .rev()
+                    .filter_map(|line| file.get_line(line - 1).map(|s| (line + 1, s)))
+                    .find(|(_, s)| !s.trim().is_empty())
+                    .map(|(line, _)| line)
+                    .unwrap_or(ann.line_start);
+                for line in ann.line_start + 1..until {
                     // Every `|` that joins the beginning of the span (`___^`) to the end (`|__^`).
                     add_annotation_to_file(&mut output, file.clone(), line, ann.as_line());
                 }