Set unified context iff needed

Cargo.toml

@@ -69,6 +69,7 @@ features = []
 
 [dev-dependencies]
 insta = { version = "1.*", features = ["colors", "filters"] }
+rstest = "0.21.0"
 
 [profile.test]
 opt-level = 2

src/subcommands/diff.rs

@@ -3,6 +3,7 @@ use std::path::{Path, PathBuf};
 use std::process;
 
 use bytelines::ByteLinesReader;
+use itertools::Itertools;
 
 use crate::config::{self, delta_unreachable};
 use crate::delta;
@@ -23,15 +24,27 @@ pub fn diff(
     let via_process_substitution =
         |f: &Path| f.starts_with("/proc/self/fd/") || f.starts_with("/dev/fd/");
 
-    let diff_cmd = if via_process_substitution(minus_file) || via_process_substitution(plus_file) {
-        format!("diff -U3 {} --", config.diff_args)
-    } else {
-        format!("git diff --no-index --color {} --", config.diff_args)
+    let diff_args = match shell_words::split(&config.diff_args) {
+        Ok(words) => words,
+        Err(err) => {
+            eprintln!("Failed to parse diff args: {}: {err}", config.diff_args);
+            return config.error_exit_code;
+        }
     };
 
-    let mut diff_args = diff_cmd.split_whitespace();
-    let diff_bin = diff_args.next().unwrap();
+    let can_use_git = !via_process_substitution(minus_file) && !via_process_substitution(plus_file);
+    let mut diff_cmd = if can_use_git {
+        vec!["git", "diff", "--no-index", "--color"]
+    } else if diff_args_set_unified_context(&diff_args) {
+        vec!["diff"]
+    } else {
+        vec!["diff", "-U3"]
+    };
+    diff_cmd.extend(diff_args.iter().map(String::as_str));
+    diff_cmd.push("--");
 
+    let mut diff_cmd = diff_cmd.iter();
+    let diff_bin = diff_cmd.next().unwrap();
     let diff_path = match grep_cli::resolve_binary(PathBuf::from(diff_bin)) {
         Ok(path) => path,
         Err(err) => {
@@ -40,8 +53,18 @@ pub fn diff(
         }
     };
 
+    let diff_cmd_string = format!(
+        "{:?} {} {:?} {:?}",
+        &diff_path,
+        diff_cmd.join(" "),
+        minus_file,
+        plus_file
+    );
+
+    eprintln!("{}", diff_cmd_string);
+
     let diff_process = process::Command::new(diff_path)
-        .args(diff_args)
+        .args(diff_cmd)
         .args([minus_file, plus_file])
         .stdout(process::Stdio::piped())
         .spawn();
@@ -80,21 +103,58 @@ pub fn diff(
             config.error_exit_code
         });
     if code >= 2 {
-        eprintln!("'{diff_bin}' process failed with exit status {code}. Command was {diff_cmd}");
+        eprintln!(
+            "'{diff_bin}' process failed with exit status {code}. Command was {diff_cmd_string}"
+        );
         config.error_exit_code
     } else {
         code
     }
 }
 
+fn diff_args_set_unified_context<I, S>(args: I) -> bool
+where
+    I: IntoIterator<Item = S>,
+    S: AsRef<str>,
+{
+    for arg in args {
+        let arg = arg.as_ref();
+        if arg == "-u" {
+            return true;
+        } else if (arg.starts_with("-U") || arg.starts_with("-u"))
+            && arg.split_at(2).1.parse::<u32>().is_ok()
+        {
+            return true;
+        }
+    }
+    false
+}
+
 #[cfg(test)]
 mod main_tests {
     use std::io::{Cursor, Read, Seek};
     use std::path::PathBuf;
 
-    use super::diff;
+    use super::{diff, diff_args_set_unified_context};
     use crate::tests::integration_test_utils;
 
+    use rstest::rstest;
+
+    #[rstest]
+    #[case(&["-u"], true)]
+    #[case(&["-u7"], true)]
+    #[case(&["-u77"], true)]
+    #[case(&["-ux"], false)]
+    #[case(&["-U"], false)]
+    #[case(&["-U7"], true)]
+    #[case(&["-U77"], true)]
+    fn test_unified_diff_arg_is_detected_in_diff_args(
+        #[case] diff_args: &[&str],
+        #[case] expected: bool,
+    ) {
+        assert_eq!(diff_args_set_unified_context(diff_args), expected)
+    }
+
     #[test]
     #[ignore] // https://github.com/dandavison/delta/pull/546
     fn test_diff_same_empty_file() {
@@ -120,7 +180,7 @@ mod main_tests {
     }
 
     fn _do_diff_test(file_a: &str, file_b: &str, expect_diff: bool) {
-        let config = integration_test_utils::make_config_from_args(&[]);
+        let config = integration_test_utils::make_config_from_args(&["-@-U99"]);
         let mut writer = Cursor::new(vec![]);
         let exit_code = diff(
             &PathBuf::from(file_a),