3 files changed, 96 insertions, 6 deletions

diff --git a/src/config.rs b/src/config.rs
index 06aa088..5962103 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -22,17 +22,78 @@ fn read_default_feature_name() -> String {
         Err(_) => return "async".to_string(),
     };
 
-    let value: toml::Value = match content.parse() {
-        Ok(v) => v,
-        Err(_) => return "async".to_string(),
-    };
+    parse_feature_name_from_toml(&content).unwrap_or_else(|| "async".to_string())
+}
 
+/// Pure function: parse `default_feature_name` from TOML content.
+/// Returns `None` if the key is missing, unreadable, or not a string.
+fn parse_feature_name_from_toml(content: &str) -> Option<String> {
+    let value: toml::Value = content.parse().ok()?;
     value
         .get("package")
         .and_then(|p| p.get("metadata"))
         .and_then(|m| m.get("might_be_async"))
         .and_then(|a| a.get("default_feature_name"))
         .and_then(|v| v.as_str())
-        .unwrap_or("async")
-        .to_string()
+        .map(|s| s.to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parses_custom_feature_name() {
+        let toml = r#"
+            [package]
+            [package.metadata.might_be_async]
+            default_feature_name = "my_async"
+        "#;
+        assert_eq!(
+            parse_feature_name_from_toml(toml),
+            Some("my_async".to_string())
+        );
+    }
+
+    #[test]
+    fn missing_metadata_returns_none() {
+        let toml = "[package]\nname = \"foo\"\n";
+        assert_eq!(parse_feature_name_from_toml(toml), None);
+    }
+
+    #[test]
+    fn missing_might_be_async_section_returns_none() {
+        let toml = r#"
+            [package]
+            [package.metadata]
+            some_other_key = "value"
+        "#;
+        assert_eq!(parse_feature_name_from_toml(toml), None);
+    }
+
+    #[test]
+    fn empty_string_returns_none() {
+        assert_eq!(parse_feature_name_from_toml(""), None);
+    }
+
+    #[test]
+    fn invalid_toml_returns_none() {
+        assert_eq!(parse_feature_name_from_toml("not valid toml {{"), None);
+    }
+
+    #[test]
+    fn value_is_not_string_returns_none() {
+        let toml = r#"
+            [package.metadata.might_be_async]
+            default_feature_name = 42
+        "#;
+        assert_eq!(parse_feature_name_from_toml(toml), None);
+    }
+
+    #[test]
+    fn env_default_returns_foo_async() {
+        // In `cargo test`, CARGO_MANIFEST_DIR is set to this crate's directory,
+        // whose Cargo.toml has `default_feature_name = "foo_async"`.
+        assert_eq!(default_feature_name(), "foo_async");
+    }
 }
diff --git a/src/func.rs b/src/func.rs
index 5557992..a502e36 100644
--- a/src/func.rs
+++ b/src/func.rs
@@ -78,4 +78,16 @@ mod tests {
         let args: FuncArgs = syn::parse2(input).unwrap();
         assert_eq!(args.feature_name, "custom_name");
     }
+
+    #[test]
+    fn integer_literal_is_rejected() {
+        let input: proc_macro2::TokenStream = "42".parse().unwrap();
+        assert!(syn::parse2::<FuncArgs>(input).is_err());
+    }
+
+    #[test]
+    fn multiple_args_are_rejected() {
+        let input: proc_macro2::TokenStream = r#""a" "b""#.parse().unwrap();
+        assert!(syn::parse2::<FuncArgs>(input).is_err());
+    }
 }
diff --git a/src/invoke.rs b/src/invoke.rs
index f7da20a..2ada0b4 100644
--- a/src/invoke.rs
+++ b/src/invoke.rs
@@ -86,4 +86,21 @@ mod tests {
             _ => panic!("expected Explicit variant"),
         }
     }
+
+    #[test]
+    fn explicit_missing_arrow_is_rejected() {
+        let input: proc_macro2::TokenStream = r#""ft" expr()"#.parse().unwrap();
+        assert!(syn::parse2::<InvokeArgs>(input).is_err());
+    }
+
+    #[test]
+    fn integer_parses_as_default_variant() {
+        // integer literal is not a LitStr, so it falls through to Default variant
+        let input: proc_macro2::TokenStream = "42".parse().unwrap();
+        let args: InvokeArgs = syn::parse2(input).unwrap();
+        match args {
+            InvokeArgs::Default(_) => {} // expected
+            _ => panic!("expected Default variant"),
+        }
+    }
 }