From 11ce6bcad16bb8400783c28e52d5fe47e88da8a3 Mon Sep 17 00:00:00 2001
From: Martin Pool <mbp@sourcefrog.net>
Date: Sat, 13 Jul 2024 18:20:21 -0700
Subject: [PATCH] Don't error on an empty --in-diff file

Fixes #331
---
 NEWS.md          |  2 ++
 src/in_diff.rs   |  6 +++++-
 tests/in_diff.rs | 17 +++++++++++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/NEWS.md b/NEWS.md
index 479df323..8a6ac3c6 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -6,6 +6,8 @@
 
 - Changed: No build timeouts by default. Previously, cargo-mutants set a default build timeout based on the baseline build, but experience showed that this would sometimes make builds flaky, because build times can be quite variable. If mutants cause builds to hang, then you can still set a timeout using `--build-timeout` or `--build-timeout-multiplier`.
 
+- Fixed: Don't error if the `--in-diff` file is empty.
+
 ## 24.5.0
 
 - Fixed: Follow `path` attributes on `mod` statements.
diff --git a/src/in_diff.rs b/src/in_diff.rs
index b7be4e98..755d32db 100644
--- a/src/in_diff.rs
+++ b/src/in_diff.rs
@@ -10,7 +10,7 @@ use camino::Utf8Path;
 use indoc::formatdoc;
 use itertools::Itertools;
 use patch::{Line, Patch};
-use tracing::{trace, warn};
+use tracing::{info, trace, warn};
 
 use crate::mutate::Mutant;
 use crate::source::SourceFile;
@@ -19,6 +19,10 @@ use crate::Result;
 /// Return only mutants to functions whose source was touched by this diff.
 pub fn diff_filter(mutants: Vec<Mutant>, diff_text: &str) -> Result<Vec<Mutant>> {
     // Flatten the error to a string because otherwise it references the diff, and can't be returned.
+    if diff_text.trim().is_empty() {
+        info!("diff file is empty; no mutants will match");
+        return Ok(Vec::new());
+    }
     let patches =
         Patch::from_multiple(diff_text).map_err(|err| anyhow!("Failed to parse diff: {err}"))?;
     check_diff_new_text_matches(&patches, &mutants)?;
diff --git a/tests/in_diff.rs b/tests/in_diff.rs
index 6f0af9cd..81a0eb51 100644
--- a/tests/in_diff.rs
+++ b/tests/in_diff.rs
@@ -4,6 +4,7 @@ use std::fs::read_to_string;
 use std::io::Write;
 
 use indoc::indoc;
+use predicates::prelude::predicate;
 use similar::TextDiff;
 use tempfile::NamedTempFile;
 
@@ -68,6 +69,22 @@ fn list_mutants_changed_in_diff1() {
     );
 }
 
+#[test]
+fn empty_diff_is_not_an_error_and_matches_nothing() {
+    let diff_file = NamedTempFile::new().unwrap();
+    let tmp = copy_of_testdata("diff1");
+    run()
+        .args(["mutants", "--no-shuffle", "-d"])
+        .arg(tmp.path())
+        .arg("--in-diff")
+        .arg(diff_file.path())
+        .arg("--list")
+        .assert()
+        .success()
+        .stdout("")
+        .stderr(predicate::str::contains("diff file is empty"));
+}
+
 /// If the text in the diff doesn't look like the tree then error out.
 #[test]
 fn mismatched_diff_causes_error() {