Check that the bench path exists before invoking analysis-stats

xtask/src/metrics.rs

@@ -97,6 +97,7 @@ fn measure_analysis_stats_path(
         name: &str,
         path: &str,
     ) -> anyhow::Result<()> {
+        assert!(Path::new(path).exists(), "unable to find bench in {path}");
         eprintln!("\nMeasuring analysis-stats/{name}");
         let output = cmd!(sh, "./target/release/rust-analyzer -q analysis-stats {path}").read()?;
         for (metric, value, unit) in parse_metrics(&output) {