Show some context around a diff

Author: oli-obk

src/diff.rs

@@ -3,24 +3,58 @@ use diff::{chars, lines, Result, Result::*};
 
 #[derive(Default)]
 struct DiffState<'a> {
-    skipped_lines: usize,
-    /// When we skip a line, remember it, in case
-    /// we end up only skipping one line. In that case we just
-    /// print the line instead of `... skipped one line ...`
-    last_skipped_line: Option<&'a str>,
+    /// Whether we've already printed something, so we should print starting context, too.
+    print_start_context: bool,
+    /// When we skip lines, remember the last `CONTEXT` ones to
+    /// display after the "skipped N lines" message
+    skipped_lines: Vec<&'a str>,
     /// When we see a removed line, we don't print it, we
     /// keep it around to compare it with the next added line.
     prev_left: Option<&'a str>,
 }
 
+/// How many lines of context are displayed around the actual diffs
+const CONTEXT: usize = 2;
+
 impl<'a> DiffState<'a> {
-    fn print_skip(&mut self) {
-        match self.skipped_lines {
+    /// Print `... n lines skipped ...` followed by the last `CONTEXT` lines.
+    fn print_end_skip(&self, skipped: usize) {
+        self.print_skipped_msg(skipped);
+        for line in self.skipped_lines.iter().rev().take(CONTEXT).rev() {
+            eprintln!(" {line}");
+        }
+    }
+    fn print_skipped_msg(&self, skipped: usize) {
+        match skipped {
             0 => {}
-            1 => eprintln!(" {}", self.last_skipped_line.unwrap()),
-            _ => eprintln!("... {} lines skipped ...", self.skipped_lines),
+            1 => eprintln!(" {}", self.skipped_lines[CONTEXT]),
+            _ => eprintln!("... {skipped} lines skipped ..."),
+        }
+    }
+    fn print_start_skip(&self) {
+        for line in self.skipped_lines.iter().take(CONTEXT) {
+            eprintln!(" {line}");
         }
-        self.skipped_lines = 0;
+    }
+    fn print_skip(&mut self) {
+        let half = self.skipped_lines.len() / 2;
+        if !self.print_start_context {
+            self.print_start_context = true;
+            self.print_end_skip(self.skipped_lines.len().saturating_sub(CONTEXT));
+        } else if half < CONTEXT {
+            for line in self.skipped_lines.drain(..) {
+                eprintln!(" {line}");
+            }
+        } else {
+            self.print_start_skip();
+            let skipped = self.skipped_lines.len() - CONTEXT * 2;
+            self.print_end_skip(skipped);
+        }
+        self.skipped_lines.clear();
+    }
+
+    fn skip(&mut self, line: &'a str) {
+        self.skipped_lines.push(line);
     }
 
     fn print_prev(&mut self) {
@@ -46,8 +80,7 @@ impl<'a> DiffState<'a> {
             }
             Both(l, _) => {
                 self.print_prev();
-                self.last_skipped_line = Some(l);
-                self.skipped_lines += 1
+                self.skip(l);
             }
             Right(r) => {
                 if let Some(l) = self.prev_left.take() {
@@ -101,8 +134,9 @@ impl<'a> DiffState<'a> {
         }
     }
 
-    fn finish(mut self) {
-        self.print_skip();
+    fn finish(self) {
+        self.print_start_skip();
+        self.print_skipped_msg(self.skipped_lines.len().saturating_sub(CONTEXT));
         eprintln!()
     }
 }

tests/integrations/basic-fail/Cargo.stderr

@@ -40,17 +40,23 @@ tests/actual_tests/foomp.rs FAILED:
 command: "rustc" "--error-format=json" "--edition=2021" "--extern" "basic_fail=$DIR/$DIR/../../../target/debug/libbasic_fail-$HASH.rmeta" "-L" "$DIR/$DIR/../../../target/debug" "-L" "$DIR/$DIR/../../../target/debug" "tests/actual_tests/foomp.rs"
 
 actual output differed from expected tests/actual_tests/foomp.stderr
-... 5 lines skipped ...
+... 3 lines skipped ...
+ 4 |     add("42", 3);
+   |     --- ^^^^ expected `usize`, found `&str`
 +  |     |
 +  |     arguments to this function are incorrect
-... 2 lines skipped ...
+   |
+ note: function defined here
 ~ --> $DIR/$DIR/src/lib.rs:1:8
-... 2 lines skipped ...
+   |
+ 1 | pub fn add(left: usize, right: usize) -> usize {
 ~  |        ^^^ some expected text that isn't in the actual message
 
 -error: aborting doo to previous error
 +error: aborting due to previous error
-... 3 lines skipped ...
+ 
+ For more information about this error, try `rustc --explain E0308`.
+ 
 
 
 full stderr: