fix(cli): reject invalid scan roots

CHANGELOG.md

@@ -8,6 +8,8 @@ All notable changes to Diskern are documented here. The format follows
 
 ### Fixed
 
+- `diskern scan` now rejects missing and nonexistent roots instead of
+  reporting a successful empty scan
 - A relative scan root no longer hides every finding a root-anchored rule
   would have made. `diskern scan tmp` from `/var` reported nothing to
   clean; roots are resolved to absolute paths before the walk

crates/diskern-cli/src/main.rs

@@ -1,4 +1,4 @@
-use anyhow::{Context, Result};
+use anyhow::{bail, Context, Result};
 use clap::{Parser, Subcommand, ValueEnum};
 use diskern_core::{report, rules::RulesDb, scanner, Category, Finding, Verdict};
 use std::path::PathBuf;
@@ -19,6 +19,7 @@ enum Command {
     /// Read-only scan: find duplicates, caches, and reclaimable space.
     Scan {
         /// Directories to scan
+        #[arg(required = true)]
         roots: Vec<PathBuf>,
         /// Emit full JSON report instead of a summary
         #[arg(long)]
@@ -217,6 +218,15 @@ fn load_rules(path: Option<&std::path::Path>) -> Result<RulesDb> {
     Ok(rules.with_embedded_protected_rules())
 }
 
+fn validate_roots(roots: &[PathBuf]) -> Result<()> {
+    for root in roots {
+        if !root.try_exists()? {
+            bail!("scan root does not exist: {}", root.display());
+        }
+    }
+    Ok(())
+}
+
 fn main() -> Result<()> {
     let cli = Cli::parse();
     match cli.command {
@@ -227,6 +237,7 @@ fn main() -> Result<()> {
             verdict,
             rules,
         } => {
+            validate_roots(&roots)?;
             let external_rules = rules.as_deref();
             let rules_db = load_rules(external_rules)?;
             let opts = scanner::ScanOptions {
@@ -305,7 +316,30 @@ fn main() -> Result<()> {
 
 #[cfg(test)]
 mod tests {
-    use super::{human_bytes, plural};
+    use super::{human_bytes, plural, validate_roots, Cli};
+    use clap::{error::ErrorKind, Parser};
+    use std::path::PathBuf;
+
+    #[test]
+    fn scan_requires_at_least_one_root() {
+        let error = match Cli::try_parse_from(["diskern", "scan"]) {
+            Ok(_) => panic!("scan without roots should be rejected"),
+            Err(error) => error,
+        };
+
+        assert_eq!(error.kind(), ErrorKind::MissingRequiredArgument);
+    }
+
+    #[test]
+    fn nonexistent_scan_root_is_rejected_before_scanning() {
+        let missing = PathBuf::from("diskern-test-root-that-does-not-exist");
+        let error = validate_roots(std::slice::from_ref(&missing)).unwrap_err();
+
+        assert_eq!(
+            error.to_string(),
+            format!("scan root does not exist: {}", missing.display())
+        );
+    }
 
     #[test]
     fn plural_returns_empty_only_for_singular() {