feat: Match Rust's overlapping multiline starts

Author: Muscraft

src/renderer/display_list.rs

@@ -269,7 +269,7 @@ impl<'a> DisplaySet<'a> {
         }
     }
 
-    // Adapted from https://github.com/rust-lang/rust/blob/894f7a4ba6554d3797404bbf550d9919df060b97/compiler/rustc_errors/src/emitter.rs#L706-L1155
+    // Adapted from https://github.com/rust-lang/rust/blob/d371d17496f2ce3a56da76aa083f4ef157572c20/compiler/rustc_errors/src/emitter.rs#L706-L1211
     #[inline]
     fn format_line(
         &self,
@@ -366,7 +366,7 @@ impl<'a> DisplaySet<'a> {
                     annotations.sort_by_key(|a| Reverse(a.range.0));
 
                     let mut annotations_positions = 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() {
@@ -442,6 +442,36 @@ impl<'a> DisplaySet<'a> {
                         line_len += 1;
                     }
 
+                    if annotations_positions.iter().all(|(_, ann)| {
+                        matches!(
+                            ann.annotation_part,
+                            DisplayAnnotationPart::MultilineStart(_)
+                        )
+                    }) {
+                        if let Some(max_pos) =
+                            annotations_positions.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_positions {
+                                *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);
+                        }
+                    }
+
                     // This is a special case where we have a multiline
                     // annotation that is at the start of the line disregarding
                     // any leading whitespace, and no other multiline

tests/rustc_tests.rs

@@ -91,9 +91,8 @@ error: foo
  --> test.rs:3:3
   |
 3 |      X0 Y0
-  |   ___^__-
-  |  |___|
-  | ||
+  |  ____^  -
+  | | ______|
 4 | ||   X1 Y1
 5 | ||   X2 Y2
   | ||____^__- `Y` is a good letter too
@@ -130,9 +129,8 @@ error: foo
  --> test.rs:3:3
   |
 3 |      X0 Y0
-  |   ___^__-
-  |  |___|
-  | ||
+  |  ____^  -
+  | | ______|
 4 | ||   Y1 X1
   | ||____-__^ `X` is a good letter
   |  |____|
@@ -210,10 +208,9 @@ error: foo
  --> test.rs:3:3
   |
 3 |       X0 Y0 Z0
-  |    ___^__-__-
-  |   |___|__|
-  |  ||___|
-  | |||
+  |  _____^  -  -
+  | | _______|  |
+  | || _________|
 4 | |||   X1 Y1 Z1
 5 | |||   X2 Y2 Z2
   | |||____^__-__- `Z` label