Special case when a code line only has multiline span starts

```
3 │       X0 Y0 Z0
  │ ┏━━━━━┛  │  │
  │ ┃┌───────┘  │
  │ ┃│┌─────────┘
4 │ ┃││   X1 Y1 Z1
5 │ ┃││   X2 Y2 Z2
  │ ┃│└────╿──│──┘ `Z` label
  │ ┃└─────│──┤
  │ ┗━━━━━━┥  `Y` is a good letter too
  ╰╴       `X` is a good letter
```

Author: estebank

compiler/rustc_errors/src/emitter.rs

@@ -915,7 +915,7 @@ impl HumanEmitter {
         //      <EMPTY LINE>
         //
         let mut annotations_position = vec![];
-        let mut line_len = 0;
+        let mut line_len: usize = 0;
         let mut p = 0;
         for (i, annotation) in annotations.iter().enumerate() {
             for (j, next) in annotations.iter().enumerate() {
@@ -986,6 +986,31 @@ impl HumanEmitter {
             return vec![];
         }
 
+        if annotations_position
+            .iter()
+            .all(|(_, ann)| matches!(ann.annotation_type, AnnotationType::MultilineStart(_)))
+            && let Some(max_pos) = annotations_position.iter().map(|(pos, _)| *pos).max()
+        {
+            // Special case the following, so that we minimize overlapping multiline spans.
+            //
+            // 3 │       X0 Y0 Z0
+            //   │ ┏━━━━━┛  │  │     < We are writing these lines
+            //   │ ┃┌───────┘  │     < by reverting the "depth" of
+            //   │ ┃│┌─────────┘     < their multilne spans.
+            // 4 │ ┃││   X1 Y1 Z1
+            // 5 │ ┃││   X2 Y2 Z2
+            //   │ ┃│└────╿──│──┘ `Z` label
+            //   │ ┃└─────│──┤
+            //   │ ┗━━━━━━┥  `Y` is a good letter too
+            //   ╰╴       `X` is a good letter
+            for (pos, _) in &mut annotations_position {
+                *pos = max_pos - *pos;
+            }
+            // We know then that we don't need an additional line for the span label, saving us
+            // one line of vertical space.
+            line_len = line_len.saturating_sub(1);
+        }
+
         // Write the column separator.
         //
         // After this we will have:

compiler/rustc_parse/src/parser/tests.rs

@@ -369,9 +369,8 @@ error: foo
  --> test.rs:3:3
   |
 3 |      X0 Y0
-  |   ___^__-
-  |  |___|
-  | ||
+  |  ____^  -
+  | | ______|
 4 | ||   X1 Y1
 5 | ||   X2 Y2
   | ||____^__- `Y` is a good letter too
@@ -384,9 +383,8 @@ error: foo
   ╭▸ test.rs:3:3
   │
 3 │      X0 Y0
-  │  ┌───╿──┘
-  │ ┏│━━━┙
-  │ ┃│
+  │ ┏━━━━┛  │
+  │ ┃┌──────┘
 4 │ ┃│   X1 Y1
 5 │ ┃│   X2 Y2
   │ ┃└────╿──┘ `Y` is a good letter too
@@ -424,9 +422,8 @@ error: foo
  --> test.rs:3:3
   |
 3 |      X0 Y0
-  |   ___^__-
-  |  |___|
-  | ||
+  |  ____^  -
+  | | ______|
 4 | ||   Y1 X1
   | ||____-__^ `X` is a good letter
   |  |____|
@@ -438,9 +435,8 @@ error: foo
   ╭▸ test.rs:3:3
   │
 3 │      X0 Y0
-  │  ┌───╿──┘
-  │ ┏│━━━┙
-  │ ┃│
+  │ ┏━━━━┛  │
+  │ ┃┌──────┘
 4 │ ┃│   Y1 X1
   │ ┗│━━━━│━━┛ `X` is a good letter
   │  └────┤
@@ -1360,10 +1356,9 @@ error: foo
  --> test.rs:3:3
   |
 3 |       X0 Y0 Z0
-  |    ___^__-__-
-  |   |___|__|
-  |  ||___|
-  | |||
+  |  _____^  -  -
+  | | _______|  |
+  | || _________|
 4 | |||   X1 Y1 Z1
 5 | |||   X2 Y2 Z2
   | |||____^__-__- `Z` label
@@ -1377,10 +1372,9 @@ error: foo
   ╭▸ test.rs:3:3
   │
 3 │       X0 Y0 Z0
-  │   ┌───╿──│──┘
-  │  ┌│───│──┘
-  │ ┏││━━━┙
-  │ ┃││
+  │ ┏━━━━━┛  │  │
+  │ ┃┌───────┘  │
+  │ ┃│┌─────────┘
 4 │ ┃││   X1 Y1 Z1
 5 │ ┃││   X2 Y2 Z2
   │ ┃│└────╿──│──┘ `Z` label
@@ -1389,17 +1383,6 @@ error: foo
   ╰╴       `X` is a good letter
 
 "#,
-        // FIXME: This would ideally instead look like this
-        // 3 │       X0 Y0 Z0
-        //   │ ┏━━━━━┛  │  │
-        //   │ ┃┌───────┘  │
-        //   │ ┃│┌─────────┘
-        // 4 │ ┃││   X1 Y1 Z1
-        // 5 │ ┃││   X2 Y2 Z2
-        //   │ ┃│└────╿──│──┘ `Z` label
-        //   │ ┃└─────│──┤
-        //   │ ┗━━━━━━┥  `Y` is a good letter too
-        //   ╰╴       `X` is a good letter
     );
 }
 

tests/ui/async-await/async-is-unwindsafe.stderr

@@ -2,9 +2,8 @@ error[E0277]: the type `&mut Context<'_>` may not be safely transferred across a
   --> $DIR/async-is-unwindsafe.rs:12:5
    |
 LL |        is_unwindsafe(async {
-   |   _____^_____________-
-   |  |_____|
-   | ||
+   |  ______^             -
+   | | ___________________|
 LL | ||
 LL | ||         use std::ptr::null;
 LL | ||         use std::task::{Context, RawWaker, RawWakerVTable, Waker};

tests/ui/lint/suggestions.stderr

@@ -41,9 +41,8 @@ warning: variable does not need to be mutable
   --> $DIR/suggestions.rs:54:13
    |
 LL |            let mut
-   |  ______________^
-   | | _____________|
-   | ||
+   |   _____________^
+   |  |_____________|
 LL | ||             b = 1;
    | ||____________-^
    | |_____________|