Fixed ICE when formatting files outside of toml dir

Author: aznszn

src/ignore_path.rs

@@ -22,10 +22,16 @@ impl IgnorePathSet {
     pub(crate) fn is_match(&self, file_name: &FileName) -> bool {
         match file_name {
             FileName::Stdin => false,
-            FileName::Real(p) => self
-                .ignore_set
-                .matched_path_or_any_parents(p, false)
-                .is_ignore(),
+            FileName::Real(p) => {
+                let ignore_set_root = self.ignore_set.path();
+                if p.is_absolute() && !p.starts_with(ignore_set_root) {
+                    false
+                } else {
+                    self.ignore_set
+                        .matched_path_or_any_parents(p, false)
+                        .is_ignore()
+                }
+            }
         }
     }
 }
@@ -70,4 +76,21 @@ mod test {
         assert!(ignore_path_set.is_match(&FileName::Real(PathBuf::from("bar_dir/baz/a.rs"))));
         assert!(!ignore_path_set.is_match(&FileName::Real(PathBuf::from("bar_dir/baz/what.rs"))));
     }
+    
+    #[nightly_only_test]
+    #[test]
+    fn test_ignore_path_outside_root() {
+        //See: https://github.com/rust-lang/rustfmt/issues/6843
+        use crate::config::{Config, FileName};
+        use crate::ignore_path::IgnorePathSet;
+        use std::path::Path;
+
+        let config =
+            Config::from_toml(r#"ignore = ["bar.rs"]"#, Path::new("./rustfmt.toml")).unwrap();
+        let ignore_path_set = IgnorePathSet::from_ignore_list(&config.ignore()).unwrap();
+
+        let outside = std::env::temp_dir().join("foo.rs");
+        //should not panic
+        assert!(!ignore_path_set.is_match(&FileName::Real(outside)));
+    }
 }