Fixes paths thing (#920)

* fixes paths thing 
* add @nilehmann 's fix to check for empty strings!

crates/flux-config/src/lib.rs

@@ -48,7 +48,7 @@ pub fn is_cache_enabled() -> bool {
 }
 
 pub fn is_checked_file(file: &str) -> bool {
-    CONFIG.check_files.is_empty() || CONFIG.check_files.contains(file)
+    CONFIG.check_files.is_checked_file(file)
 }
 
 pub fn cache_path() -> PathBuf {
@@ -85,13 +85,44 @@ struct Config {
     catch_bugs: bool,
     pointer_width: PointerWidth,
     check_def: String,
-    check_files: String,
+    check_files: Paths,
     cache: bool,
     cache_file: String,
     check_overflow: bool,
     scrape_quals: bool,
 }
 
+#[derive(Default)]
+struct Paths {
+    paths: Option<Vec<PathBuf>>,
+}
+
+impl Paths {
+    fn is_checked_file(&self, file: &str) -> bool {
+        self.paths
+            .as_ref()
+            .map_or(true, |p| p.iter().any(|p| p.to_str().unwrap() == file))
+    }
+}
+
+impl<'de> Deserialize<'de> for Paths {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let paths: Vec<PathBuf> = String::deserialize(deserializer)?
+            .split(",")
+            .map(str::trim)
+            .filter(|s| !s.is_empty())
+            .map(PathBuf::from)
+            .collect();
+
+        let paths = if paths.is_empty() { None } else { Some(paths) };
+
+        Ok(Paths { paths })
+    }
+}
+
 #[derive(Copy, Clone, Deserialize)]
 #[serde(try_from = "u8")]
 pub enum PointerWidth {

crates/flux-driver/src/callbacks.rs

@@ -164,7 +164,7 @@ impl<'genv, 'tcx> CrateChecker<'genv, 'tcx> {
                 return config::is_checked_file(&file);
             }
         }
-        return true;
+        true
     }
 
     fn check_def_catching_bugs(&mut self, def_id: LocalDefId) -> Result<(), ErrorGuaranteed> {