parse: Ensure continuation keywords cannot be used too early

We switch from assuming a continuation of `Given` by default
to passing Option<StepKind> around and ensuring that we do not
succeed in parsing a step if that is None.

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

1 files changed, 21 insertions, 11 deletions

diff --git a/src/steps.rs b/src/steps.rs
index b5d1d33..2c6f34f 100644
--- a/src/steps.rs
+++ b/src/steps.rs
@@ -40,15 +40,15 @@ impl ScenarioStep {
     ///
     /// If the step uses the "and" or "but" keyword, use the default
     /// step kind instead.
-    pub fn new_from_str(text: &str, default: StepKind) -> Result<ScenarioStep> {
+    pub fn new_from_str(text: &str, default: Option<StepKind>) -> Result<ScenarioStep> {
         let mut words = text.split_whitespace();
 
         let kind = match words.next() {
             Some("given") => StepKind::Given,
             Some("when") => StepKind::When,
             Some("then") => StepKind::Then,
-            Some("and") => default,
-            Some("but") => default,
+            Some("and") => default.ok_or(SubplotError::ContinuationTooEarly)?,
+            Some("but") => default.ok_or(SubplotError::ContinuationTooEarly)?,
             _ => return Err(SubplotError::UnknownStepKind),
         };
 
@@ -100,42 +100,52 @@ impl fmt::Display for StepKind {
 
 #[cfg(test)]
 mod test {
-    use super::{ScenarioStep, StepKind};
+    use super::{ScenarioStep, StepKind, SubplotError};
 
     #[test]
     fn parses_given() {
-        let step = ScenarioStep::new_from_str("given I am Tomjon", StepKind::Then).unwrap();
+        let step = ScenarioStep::new_from_str("given I am Tomjon", None).unwrap();
         assert_eq!(step.kind(), StepKind::Given);
         assert_eq!(step.text(), "I am Tomjon");
     }
 
     #[test]
     fn parses_given_with_extra_spaces() {
-        let step =
-            ScenarioStep::new_from_str("   given     I   am   Tomjon   ", StepKind::When).unwrap();
+        let step = ScenarioStep::new_from_str("   given     I   am   Tomjon   ", None).unwrap();
         assert_eq!(step.kind(), StepKind::Given);
         assert_eq!(step.text(), "I am Tomjon");
     }
 
     #[test]
     fn parses_when() {
-        let step =
-            ScenarioStep::new_from_str("when I declare myself king", StepKind::Given).unwrap();
+        let step = ScenarioStep::new_from_str("when I declare myself king", None).unwrap();
         assert_eq!(step.kind(), StepKind::When);
         assert_eq!(step.text(), "I declare myself king");
     }
 
     #[test]
     fn parses_then() {
-        let step = ScenarioStep::new_from_str("then everyone accepts it", StepKind::Given).unwrap();
+        let step = ScenarioStep::new_from_str("then everyone accepts it", None).unwrap();
         assert_eq!(step.kind(), StepKind::Then);
         assert_eq!(step.text(), "everyone accepts it");
     }
 
     #[test]
     fn parses_and() {
-        let step = ScenarioStep::new_from_str("and everyone accepts it", StepKind::Then).unwrap();
+        let step =
+            ScenarioStep::new_from_str("and everyone accepts it", Some(StepKind::Then)).unwrap();
         assert_eq!(step.kind(), StepKind::Then);
         assert_eq!(step.text(), "everyone accepts it");
     }
+
+    #[test]
+    fn fails_to_parse_and() {
+        let step = ScenarioStep::new_from_str("and everyone accepts it", None);
+        assert!(step.is_err());
+        match step.err() {
+            None => unreachable!(),
+            Some(SubplotError::ContinuationTooEarly) => (),
+            Some(e) => panic!("Incorrect error: {}", e),
+        }
+    }
 }