Create a more compact diff format

Cargo.toml

@@ -15,7 +15,7 @@ rustc_version = "0.4"
 colored = "2"
 # Features chosen to match those required by env_logger, to avoid rebuilds
 regex = { version = "1.5.5", default-features = false, features = ["perf", "std"] }
-pretty_assertions = "1.2.1"
+diff = "0.1.13"
 crossbeam = "0.8.1"
 lazy_static = "1.4.0"
 serde = { version = "1.0", features = ["derive"] }

src/diff.rs

@@ -0,0 +1,161 @@
+use colored::*;
+use diff::{chars, lines, Result, Result::*};
+
+#[derive(Default)]
+struct DiffState<'a> {
+    /// 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> {
+    /// 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 {
+            // When the amount of skipped lines is exactly `CONTEXT * 2`, we already
+            // print all the context and don't actually skip anything.
+            0 => {}
+            // Instead of writing a line saying we skipped one line, print that one line
+            1 => eprintln!(" {}", self.skipped_lines[CONTEXT]),
+            _ => eprintln!("... {skipped} lines skipped ..."),
+        }
+    }
+
+    /// Print an initial `CONTEXT` amount of lines.
+    fn print_start_skip(&self) {
+        for line in self.skipped_lines.iter().take(CONTEXT) {
+            eprintln!(" {line}");
+        }
+    }
+
+    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 {
+            // Print all the skipped lines if the amount of context desired is less than the amount of lines
+            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) {
+        if let Some(l) = self.prev_left.take() {
+            self.print_left(l);
+        }
+    }
+
+    fn print_left(&self, l: &str) {
+        eprintln!("{}{}", "-".red(), l.red());
+    }
+
+    fn print_right(&self, r: &str) {
+        eprintln!("{}{}", "+".green(), r.green());
+    }
+
+    fn row(&mut self, row: Result<&'a str>) {
+        match row {
+            Left(l) => {
+                self.print_skip();
+                self.print_prev();
+                self.prev_left = Some(l);
+            }
+            Both(l, _) => {
+                self.print_prev();
+                self.skip(l);
+            }
+            Right(r) => {
+                // When there's an added line after a removed line, we'll want to special case some print cases.
+                // FIXME(oli-obk): also do special printing modes when there are multiple lines that only have minor changes.
+                if let Some(l) = self.prev_left.take() {
+                    let diff = chars(l, r);
+                    let mut seen_l = false;
+                    let mut seen_r = false;
+                    for char in &diff {
+                        match char {
+                            Left(l) if !l.is_whitespace() => seen_l = true,
+                            Right(r) if !r.is_whitespace() => seen_r = true,
+                            _ => {}
+                        }
+                    }
+                    if seen_l && seen_r {
+                        // The line both adds and removes chars, print both lines, but highlight their differences instead of
+                        // drawing the entire line in red/green.
+                        eprint!("{}", "-".red());
+                        for char in &diff {
+                            match char {
+                                Left(l) => eprint!("{}", l.to_string().red()),
+                                Right(_) => {}
+                                Both(l, _) => eprint!("{}", l),
+                            }
+                        }
+                        eprintln!();
+                        eprint!("{}", "+".green());
+                        for char in &diff {
+                            match char {
+                                Left(_) => {}
+                                Right(r) => eprint!("{}", r.to_string().green()),
+                                Both(l, _) => eprint!("{}", l),
+                            }
+                        }
+                        eprintln!();
+                    } else {
+                        // The line only adds or only removes chars, print a single line highlighting their differences.
+                        eprint!("{}", "~".yellow());
+                        for char in diff {
+                            match char {
+                                Left(l) => eprint!("{}", l.to_string().red()),
+                                Both(l, _) => eprint!("{}", l),
+                                Right(r) => eprint!("{}", r.to_string().green()),
+                            }
+                        }
+                        eprintln!();
+                    }
+                } else {
+                    self.print_skip();
+                    self.print_right(r);
+                }
+            }
+        }
+    }
+
+    fn finish(self) {
+        self.print_start_skip();
+        self.print_skipped_msg(self.skipped_lines.len().saturating_sub(CONTEXT));
+        eprintln!()
+    }
+}
+
+pub fn print_diff(expected: &str, actual: &str) {
+    let mut state = DiffState::default();
+    for row in lines(expected, actual) {
+        state.row(row);
+    }
+    state.finish();
+}

src/lib.rs

@@ -25,6 +25,7 @@ use crate::dependencies::build_dependencies;
 use crate::parser::{Comments, Condition};
 
 mod dependencies;
+mod diff;
 mod parser;
 mod rustc_stderr;
 #[cfg(test)]
@@ -321,12 +322,10 @@ pub fn run_tests_generic(config: Config, file_filter: impl Fn(&Path) -> bool + S
                         actual,
                         expected,
                     } => {
-                        eprintln!("actual output differed from expected {}", path.display());
-                        eprintln!(
-                            "{}",
-                            pretty_assertions::StrComparison::new(expected, actual)
-                        );
-                        eprintln!()
+                        eprintln!("{}", "actual output differed from expected".underline());
+                        eprintln!("{}", format!("--- {}", path.display()).red());
+                        eprintln!("{}", "+++ <stderr output>".green());
+                        diff::print_diff(expected, actual);
                     }
                     Error::ErrorsWithoutPattern { path: None, msgs } => {
                         eprintln!(