use saturating substraction to calculate Line ranges

CHANGELOG.md

@@ -8,6 +8,7 @@
 ## Bugfixes
 
 - Python syntax highlighting no longer suffers from abysmal performance in specific scenarios. See #1688 (@keith-hall)
+- First line not shown in diff context. See #1891 (@divagant-martian)
 
 ## Performance
 

src/controller.rs

@@ -177,7 +177,8 @@ impl<'b> Controller<'b> {
                     if let Some(line_changes) = line_changes {
                         for &line in line_changes.keys() {
                             let line = line as usize;
-                            line_ranges.push(LineRange::new(line - context, line + context));
+                            line_ranges
+                                .push(LineRange::new(line.saturating_sub(context), line + context));
                         }
                     }
 

tests/snapshots/output/changes.snapshot.txt

@@ -1,3 +1,4 @@
++ /// A rectangle. First line is changed to prevent a regression of #1869
   struct Rectangle {
       width: u32,
       height: u32,

tests/snapshots/output/changes_grid.snapshot.txt

@@ -1,4 +1,5 @@
 ──┬─────────────────────────────────────────────────────────────────────────────
++ │ /// A rectangle. First line is changed to prevent a regression of #1869
   │ struct Rectangle {
   │     width: u32,
   │     height: u32,

tests/snapshots/output/changes_grid_header.snapshot.txt

@@ -1,6 +1,7 @@
 ──┬─────────────────────────────────────────────────────────────────────────────
   │ File: sample.rs
 ──┼─────────────────────────────────────────────────────────────────────────────
++ │ /// A rectangle. First line is changed to prevent a regression of #1869
   │ struct Rectangle {
   │     width: u32,
   │     height: u32,

tests/snapshots/output/changes_grid_header_numbers.snapshot.txt

@@ -1,26 +1,27 @@
 ───────┬────────────────────────────────────────────────────────────────────────
        │ File: sample.rs
 ───────┼────────────────────────────────────────────────────────────────────────
-   1   │ struct Rectangle {
-   2   │     width: u32,
-   3   │     height: u32,
-   4   │ }
-   5   │ 
-   6 _ │ fn main() {
-   7   │     let rect1 = Rectangle { width: 30, height: 50 };
-   8   │ 
-   9   │     println!(
-  10 ~ │         "The perimeter of the rectangle is {} pixels.",
-  11 ~ │         perimeter(&rect1)
-  12   │     );
-  13 + │     println!(r#"This line contains invalid utf8:  "�����"#;
-  14   │ }
-  15   │ 
-  16   │ fn area(rectangle: &Rectangle) -> u32 {
-  17   │     rectangle.width * rectangle.height
-  18   │ }
-  19 + │ 
-  20 + │ fn perimeter(rectangle: &Rectangle) -> u32 {
-  21 + │     (rectangle.width + rectangle.height) * 2
-  22 + │ }
+   1 + │ /// A rectangle. First line is changed to prevent a regression of #1869
+   2   │ struct Rectangle {
+   3   │     width: u32,
+   4   │     height: u32,
+   5   │ }
+   6   │ 
+   7 _ │ fn main() {
+   8   │     let rect1 = Rectangle { width: 30, height: 50 };
+   9   │ 
+  10   │     println!(
+  11 ~ │         "The perimeter of the rectangle is {} pixels.",
+  12 ~ │         perimeter(&rect1)
+  13   │     );
+  14 + │     println!(r#"This line contains invalid utf8:  "�����"#;
+  15   │ }
+  16   │ 
+  17   │ fn area(rectangle: &Rectangle) -> u32 {
+  18   │     rectangle.width * rectangle.height
+  19   │ }
+  20 + │ 
+  21 + │ fn perimeter(rectangle: &Rectangle) -> u32 {
+  22 + │     (rectangle.width + rectangle.height) * 2
+  23 + │ }
 ───────┴────────────────────────────────────────────────────────────────────────

tests/snapshots/output/changes_grid_header_numbers_rule.snapshot.txt

@@ -1,26 +1,27 @@
 ───────┬────────────────────────────────────────────────────────────────────────
        │ File: sample.rs
 ───────┼────────────────────────────────────────────────────────────────────────
-   1   │ struct Rectangle {
-   2   │     width: u32,
-   3   │     height: u32,
-   4   │ }
-   5   │ 
-   6 _ │ fn main() {
-   7   │     let rect1 = Rectangle { width: 30, height: 50 };
-   8   │ 
-   9   │     println!(
-  10 ~ │         "The perimeter of the rectangle is {} pixels.",
-  11 ~ │         perimeter(&rect1)
-  12   │     );
-  13 + │     println!(r#"This line contains invalid utf8:  "�����"#;
-  14   │ }
-  15   │ 
-  16   │ fn area(rectangle: &Rectangle) -> u32 {
-  17   │     rectangle.width * rectangle.height
-  18   │ }
-  19 + │ 
-  20 + │ fn perimeter(rectangle: &Rectangle) -> u32 {
-  21 + │     (rectangle.width + rectangle.height) * 2
-  22 + │ }
+   1 + │ /// A rectangle. First line is changed to prevent a regression of #1869
+   2   │ struct Rectangle {
+   3   │     width: u32,
+   4   │     height: u32,
+   5   │ }
+   6   │ 
+   7 _ │ fn main() {
+   8   │     let rect1 = Rectangle { width: 30, height: 50 };
+   9   │ 
+  10   │     println!(
+  11 ~ │         "The perimeter of the rectangle is {} pixels.",
+  12 ~ │         perimeter(&rect1)
+  13   │     );
+  14 + │     println!(r#"This line contains invalid utf8:  "�����"#;
+  15   │ }
+  16   │ 
+  17   │ fn area(rectangle: &Rectangle) -> u32 {
+  18   │     rectangle.width * rectangle.height
+  19   │ }
+  20 + │ 
+  21 + │ fn perimeter(rectangle: &Rectangle) -> u32 {
+  22 + │     (rectangle.width + rectangle.height) * 2
+  23 + │ }
 ───────┴────────────────────────────────────────────────────────────────────────

tests/snapshots/output/changes_grid_header_rule.snapshot.txt

@@ -1,6 +1,7 @@
 ──┬─────────────────────────────────────────────────────────────────────────────
   │ File: sample.rs
 ──┼─────────────────────────────────────────────────────────────────────────────
++ │ /// A rectangle. First line is changed to prevent a regression of #1869
   │ struct Rectangle {
   │     width: u32,
   │     height: u32,

tests/snapshots/output/changes_grid_numbers.snapshot.txt

@@ -1,24 +1,25 @@
 ───────┬────────────────────────────────────────────────────────────────────────
-   1   │ struct Rectangle {
-   2   │     width: u32,
-   3   │     height: u32,
-   4   │ }
-   5   │ 
-   6 _ │ fn main() {
-   7   │     let rect1 = Rectangle { width: 30, height: 50 };
-   8   │ 
-   9   │     println!(
-  10 ~ │         "The perimeter of the rectangle is {} pixels.",
-  11 ~ │         perimeter(&rect1)
-  12   │     );
-  13 + │     println!(r#"This line contains invalid utf8:  "�����"#;
-  14   │ }
-  15   │ 
-  16   │ fn area(rectangle: &Rectangle) -> u32 {
-  17   │     rectangle.width * rectangle.height
-  18   │ }
-  19 + │ 
-  20 + │ fn perimeter(rectangle: &Rectangle) -> u32 {
-  21 + │     (rectangle.width + rectangle.height) * 2
-  22 + │ }
+   1 + │ /// A rectangle. First line is changed to prevent a regression of #1869
+   2   │ struct Rectangle {
+   3   │     width: u32,
+   4   │     height: u32,
+   5   │ }
+   6   │ 
+   7 _ │ fn main() {
+   8   │     let rect1 = Rectangle { width: 30, height: 50 };
+   9   │ 
+  10   │     println!(
+  11 ~ │         "The perimeter of the rectangle is {} pixels.",
+  12 ~ │         perimeter(&rect1)
+  13   │     );
+  14 + │     println!(r#"This line contains invalid utf8:  "�����"#;
+  15   │ }
+  16   │ 
+  17   │ fn area(rectangle: &Rectangle) -> u32 {
+  18   │     rectangle.width * rectangle.height
+  19   │ }
+  20 + │ 
+  21 + │ fn perimeter(rectangle: &Rectangle) -> u32 {
+  22 + │     (rectangle.width + rectangle.height) * 2
+  23 + │ }
 ───────┴────────────────────────────────────────────────────────────────────────

tests/snapshots/output/changes_grid_numbers_rule.snapshot.txt

@@ -1,24 +1,25 @@
 ───────┬────────────────────────────────────────────────────────────────────────
-   1   │ struct Rectangle {
-   2   │     width: u32,
-   3   │     height: u32,
-   4   │ }
-   5   │ 
-   6 _ │ fn main() {
-   7   │     let rect1 = Rectangle { width: 30, height: 50 };
-   8   │ 
-   9   │     println!(
-  10 ~ │         "The perimeter of the rectangle is {} pixels.",
-  11 ~ │         perimeter(&rect1)
-  12   │     );
-  13 + │     println!(r#"This line contains invalid utf8:  "�����"#;
-  14   │ }
-  15   │ 
-  16   │ fn area(rectangle: &Rectangle) -> u32 {
-  17   │     rectangle.width * rectangle.height
-  18   │ }
-  19 + │ 
-  20 + │ fn perimeter(rectangle: &Rectangle) -> u32 {
-  21 + │     (rectangle.width + rectangle.height) * 2
-  22 + │ }
+   1 + │ /// A rectangle. First line is changed to prevent a regression of #1869
+   2   │ struct Rectangle {
+   3   │     width: u32,
+   4   │     height: u32,
+   5   │ }
+   6   │ 
+   7 _ │ fn main() {
+   8   │     let rect1 = Rectangle { width: 30, height: 50 };
+   9   │ 
+  10   │     println!(
+  11 ~ │         "The perimeter of the rectangle is {} pixels.",
+  12 ~ │         perimeter(&rect1)
+  13   │     );
+  14 + │     println!(r#"This line contains invalid utf8:  "�����"#;
+  15   │ }
+  16   │ 
+  17   │ fn area(rectangle: &Rectangle) -> u32 {
+  18   │     rectangle.width * rectangle.height
+  19   │ }
+  20 + │ 
+  21 + │ fn perimeter(rectangle: &Rectangle) -> u32 {
+  22 + │     (rectangle.width + rectangle.height) * 2
+  23 + │ }
 ───────┴────────────────────────────────────────────────────────────────────────

tests/snapshots/output/changes_grid_rule.snapshot.txt

@@ -1,4 +1,5 @@
 ──┬─────────────────────────────────────────────────────────────────────────────
++ │ /// A rectangle. First line is changed to prevent a regression of #1869
   │ struct Rectangle {
   │     width: u32,
   │     height: u32,

tests/snapshots/output/changes_header.snapshot.txt

@@ -1,4 +1,5 @@
   File: sample.rs
++ /// A rectangle. First line is changed to prevent a regression of #1869
   struct Rectangle {
       width: u32,
       height: u32,

tests/snapshots/output/changes_header_numbers.snapshot.txt

@@ -1,23 +1,24 @@
        File: sample.rs
-   1   struct Rectangle {
-   2       width: u32,
-   3       height: u32,
-   4   }
-   5   
-   6 _ fn main() {
-   7       let rect1 = Rectangle { width: 30, height: 50 };
-   8   
-   9       println!(
-  10 ~         "The perimeter of the rectangle is {} pixels.",
-  11 ~         perimeter(&rect1)
-  12       );
-  13 +     println!(r#"This line contains invalid utf8:  "�����"#;
-  14   }
-  15   
-  16   fn area(rectangle: &Rectangle) -> u32 {
-  17       rectangle.width * rectangle.height
-  18   }
-  19 + 
-  20 + fn perimeter(rectangle: &Rectangle) -> u32 {
-  21 +     (rectangle.width + rectangle.height) * 2
-  22 + }
+   1 + /// A rectangle. First line is changed to prevent a regression of #1869
+   2   struct Rectangle {
+   3       width: u32,
+   4       height: u32,
+   5   }
+   6   
+   7 _ fn main() {
+   8       let rect1 = Rectangle { width: 30, height: 50 };
+   9   
+  10       println!(
+  11 ~         "The perimeter of the rectangle is {} pixels.",
+  12 ~         perimeter(&rect1)
+  13       );
+  14 +     println!(r#"This line contains invalid utf8:  "�����"#;
+  15   }
+  16   
+  17   fn area(rectangle: &Rectangle) -> u32 {
+  18       rectangle.width * rectangle.height
+  19   }
+  20 + 
+  21 + fn perimeter(rectangle: &Rectangle) -> u32 {
+  22 +     (rectangle.width + rectangle.height) * 2
+  23 + }

tests/snapshots/output/changes_header_numbers_rule.snapshot.txt

@@ -1,23 +1,24 @@
        File: sample.rs
-   1   struct Rectangle {
-   2       width: u32,
-   3       height: u32,
-   4   }
-   5   
-   6 _ fn main() {
-   7       let rect1 = Rectangle { width: 30, height: 50 };
-   8   
-   9       println!(
-  10 ~         "The perimeter of the rectangle is {} pixels.",
-  11 ~         perimeter(&rect1)
-  12       );
-  13 +     println!(r#"This line contains invalid utf8:  "�����"#;
-  14   }
-  15   
-  16   fn area(rectangle: &Rectangle) -> u32 {
-  17       rectangle.width * rectangle.height
-  18   }
-  19 + 
-  20 + fn perimeter(rectangle: &Rectangle) -> u32 {
-  21 +     (rectangle.width + rectangle.height) * 2
-  22 + }
+   1 + /// A rectangle. First line is changed to prevent a regression of #1869
+   2   struct Rectangle {
+   3       width: u32,
+   4       height: u32,
+   5   }
+   6   
+   7 _ fn main() {
+   8       let rect1 = Rectangle { width: 30, height: 50 };
+   9   
+  10       println!(
+  11 ~         "The perimeter of the rectangle is {} pixels.",
+  12 ~         perimeter(&rect1)
+  13       );
+  14 +     println!(r#"This line contains invalid utf8:  "�����"#;
+  15   }
+  16   
+  17   fn area(rectangle: &Rectangle) -> u32 {
+  18       rectangle.width * rectangle.height
+  19   }
+  20 + 
+  21 + fn perimeter(rectangle: &Rectangle) -> u32 {
+  22 +     (rectangle.width + rectangle.height) * 2
+  23 + }

tests/snapshots/output/changes_header_rule.snapshot.txt

@@ -1,4 +1,5 @@
   File: sample.rs
++ /// A rectangle. First line is changed to prevent a regression of #1869
   struct Rectangle {
       width: u32,
       height: u32,

tests/snapshots/output/changes_numbers.snapshot.txt

@@ -1,22 +1,23 @@
-   1   struct Rectangle {
-   2       width: u32,
-   3       height: u32,
-   4   }
-   5   
-   6 _ fn main() {
-   7       let rect1 = Rectangle { width: 30, height: 50 };
-   8   
-   9       println!(
-  10 ~         "The perimeter of the rectangle is {} pixels.",
-  11 ~         perimeter(&rect1)
-  12       );
-  13 +     println!(r#"This line contains invalid utf8:  "�����"#;
-  14   }
-  15   
-  16   fn area(rectangle: &Rectangle) -> u32 {
-  17       rectangle.width * rectangle.height
-  18   }
-  19 + 
-  20 + fn perimeter(rectangle: &Rectangle) -> u32 {
-  21 +     (rectangle.width + rectangle.height) * 2
-  22 + }
+   1 + /// A rectangle. First line is changed to prevent a regression of #1869
+   2   struct Rectangle {
+   3       width: u32,
+   4       height: u32,
+   5   }
+   6   
+   7 _ fn main() {
+   8       let rect1 = Rectangle { width: 30, height: 50 };
+   9   
+  10       println!(
+  11 ~         "The perimeter of the rectangle is {} pixels.",
+  12 ~         perimeter(&rect1)
+  13       );
+  14 +     println!(r#"This line contains invalid utf8:  "�����"#;
+  15   }
+  16   
+  17   fn area(rectangle: &Rectangle) -> u32 {
+  18       rectangle.width * rectangle.height
+  19   }
+  20 + 
+  21 + fn perimeter(rectangle: &Rectangle) -> u32 {
+  22 +     (rectangle.width + rectangle.height) * 2
+  23 + }