From 34ce564627cac9ca677b73f7c1291b0dec9e094a Mon Sep 17 00:00:00 2001
From: Martin Pool <mbp@sourcefrog.net>
Date: Mon, 4 Dec 2023 08:16:27 -0800
Subject: [PATCH 1/2] Warning not error if no mutants are found

Fixes #175
---
 NEWS.md           |  4 ++++
 src/lab.rs        | 10 ++++++++--
 src/visit.rs      |  1 -
 tests/cli/main.rs |  8 +++++---
 4 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index d230938d..c1133201 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,9 @@
 # cargo-mutants changelog
 
+## Unreleased
+
+- Changed: If no mutants are generated then `cargo mutants` now exits successfully, showing a warning. (Previously it would exit with an error.) This works better with `--in-diff` in CI, where it's normal that some changes may not have any mutants.
+
 ## 23.11.2
 
 - Changed: If `--file` or `--exclude` are set on the command line, then they replace the corresponding config file options. Similarly, if `--re` is given then the `examine_re` config key is ignored, and if `--exclude-re` is given then `exclude_regex` is ignored. (Previously the values were combined.) This makes it easier to use the command line to test files or mutants that are normally not tested.
diff --git a/src/lab.rs b/src/lab.rs
index e1eada8c..949023a3 100644
--- a/src/lab.rs
+++ b/src/lab.rs
@@ -7,7 +7,7 @@ use std::sync::Mutex;
 use std::thread;
 use std::time::{Duration, Instant};
 
-use anyhow::{ensure, Result};
+use anyhow::Result;
 use itertools::Itertools;
 use tracing::warn;
 #[allow(unused)]
@@ -22,6 +22,9 @@ use crate::*;
 
 /// Run all possible mutation experiments.
 ///
+/// This is called after all filtering is complete, so all the mutants here will be tested
+/// or checked.
+///
 /// Before testing the mutants, the lab checks that the source tree passes its tests with no
 /// mutations applied.
 pub fn test_mutants(
@@ -43,7 +46,10 @@ pub fn test_mutants(
     }
     output_dir.write_mutants_list(&mutants)?;
     console.discovered_mutants(&mutants);
-    ensure!(!mutants.is_empty(), "No mutants found");
+    if mutants.is_empty() {
+        warn!("No mutants found under the active filters");
+        return Ok(LabOutcome::default());
+    }
     let all_packages = mutants.iter().map(|m| m.package()).unique().collect_vec();
     debug!(?all_packages);
 
diff --git a/src/visit.rs b/src/visit.rs
index 680cf31e..005128f3 100644
--- a/src/visit.rs
+++ b/src/visit.rs
@@ -41,7 +41,6 @@ pub fn walk_tree(
     options: &Options,
     console: &Console,
 ) -> Result<Discovered> {
-    // TODO: Lift up parsing the error expressions...
     let error_exprs = options
         .error_values
         .iter()
diff --git a/tests/cli/main.rs b/tests/cli/main.rs
index a33f2ef5..3b4b5f5f 100644
--- a/tests/cli/main.rs
+++ b/tests/cli/main.rs
@@ -657,15 +657,17 @@ fn integration_test_source_is_not_mutated() {
     check_text_list_output(tmp_src_dir.path(), "integration_test_source_is_not_mutated");
 }
 #[test]
-fn error_when_no_mutants_found() {
+fn warning_when_no_mutants_found() {
     let tmp_src_dir = copy_of_testdata("everything_skipped");
     run()
         .args(["mutants", "--check", "--no-times", "--no-shuffle"])
         .current_dir(tmp_src_dir.path())
         .assert()
-        .stderr(predicate::str::contains("Error: No mutants found"))
+        .stderr(predicate::str::contains(
+            "No mutants found under the active filters",
+        ))
         .stdout(predicate::str::contains("Found 0 mutants to test"))
-        .failure();
+        .success(); // It's arguable, but better if CI doesn't fail in this case.
 }
 
 #[test]

From 235be7ee1586adbac9608aa26ccd09feabe8fd19 Mon Sep 17 00:00:00 2001
From: Martin Pool <mbp@sourcefrog.net>
Date: Mon, 4 Dec 2023 08:18:22 -0800
Subject: [PATCH 2/2] Rename job to "pr-mutants"

We might use the name "incremental" for iterating on failing mutants
---
 .github/workflows/tests.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 955e0440..d4e64991 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -47,7 +47,7 @@ jobs:
       - name: Test
         run: cargo test --workspace
 
-  incremental-mutants:
+  pr-mutants:
     runs-on: ubuntu-latest
     if: github.event_name == 'pull_request'
     steps: