Warning not error if no mutants are found (#176)

Fixes #175

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
 
 A big internal refactor to allow mutations smaller than a whole function. Only one pattern is added in this release, mutation of `==` operators, but many more are possible.

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);
 

src/visit.rs

@@ -46,7 +46,6 @@ pub fn walk_tree(
     options: &Options,
     console: &Console,
 ) -> Result<Discovered> {
-    // TODO: Lift up parsing the error expressions...
     let error_exprs = options
         .error_values
         .iter()

tests/cli/main.rs

@@ -666,15 +666,17 @@ 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]