codegen: Refuse to codegen if there are zero scenarios

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

2 files changed, 11 insertions, 1 deletions

diff --git a/src/codegen.rs b/src/codegen.rs
index 89ea591..b67d94a 100644
--- a/src/codegen.rs
+++ b/src/codegen.rs
@@ -39,7 +39,8 @@ pub fn generate_test_program(
 
 fn context(doc: &mut Document, template: &str) -> Result<Context, SubplotError> {
     let mut context = Context::new();
-    context.insert("scenarios", &doc.matched_scenarios(template)?);
+    let scenarios = doc.matched_scenarios(template)?;
+    context.insert("scenarios", &scenarios);
     context.insert("files", doc.files());
 
     let funcs_filenames = doc.meta().functions_filenames();
@@ -51,6 +52,11 @@ fn context(doc: &mut Document, template: &str) -> Result<Context, SubplotError>
     }
     context.insert("functions", &funcs);
 
+    // Any of the above could fail for more serious reasons, but if we get this far
+    // and our context would have no scenarios in it, then we complain.
+    if scenarios.is_empty() {
+        return Err(SubplotError::NoScenariosMatched(template.to_string()));
+    }
     Ok(context)
 }
 
diff --git a/src/error.rs b/src/error.rs
index e69bffe..c041eaf 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -233,6 +233,10 @@ pub enum SubplotError {
     #[error("failure rendering pikchr diagram: {0}")]
     PikchrRenderError(String),
 
+    /// When attempting to codegen, no scenarios matched the desired template language
+    #[error("no scenarios were found matching the `{0}` template")]
+    NoScenariosMatched(String),
+
     /// I/O error
     ///
     /// Subplot did some I/O, and it failed. This is a generic wrapper