Warn if no mutants are viable

Author: sourcefrog

NEWS.md

@@ -17,6 +17,8 @@
 
 - Changed: Renamed fields in `outcomes.json` from `cargo_result` to `process_status` and from `command` to `argv`.
 
+- Warn if no mutants were generated or if all mutants were unviable.
+
 ## 1.2.1
 
 Released 2023-01-05

src/lab.rs

@@ -8,6 +8,7 @@ use std::thread;
 use std::time::{Duration, Instant};
 
 use anyhow::{anyhow, Context, Result};
+use tracing::warn;
 #[allow(unused)]
 use tracing::{debug, debug_span, error, info, trace};
 
@@ -144,7 +145,15 @@ pub fn test_unmutated_then_all_mutants(
         .into_inner()
         .expect("final unlock mutants queue");
     console.lab_finished(&output_dir.lab_outcome, start_time, &options);
-    Ok(output_dir.take_lab_outcome())
+    let lab_outcome = output_dir.take_lab_outcome();
+    if lab_outcome.total_mutants == 0 {
+        // This should be unreachable as we also bail out before copying
+        // the tree if no mutants are generated.
+        warn!("No mutants were generated");
+    } else if lab_outcome.unviable == lab_outcome.total_mutants {
+        warn!("No mutants were viable; perhaps there is a problem with building in a scratch directory");
+    }
+    Ok(lab_outcome)
 }
 
 /// Test various phases of one scenario in a build dir.

src/outcome.rs

@@ -56,14 +56,14 @@ impl fmt::Display for Phase {
 #[derive(Debug, Default, Serialize)]
 pub struct LabOutcome {
     /// All the scenario outcomes, including baseline builds.
-    outcomes: Vec<ScenarioOutcome>,
-    total_mutants: usize,
-    missed: usize,
-    caught: usize,
-    timeout: usize,
-    unviable: usize,
-    success: usize,
-    failure: usize,
+    pub outcomes: Vec<ScenarioOutcome>,
+    pub total_mutants: usize,
+    pub missed: usize,
+    pub caught: usize,
+    pub timeout: usize,
+    pub unviable: usize,
+    pub success: usize,
+    pub failure: usize,
 }
 
 impl LabOutcome {