derive: don't panic if source+path are missing

rinja_derive/src/input.rs

@@ -53,9 +53,9 @@ impl TemplateInput<'_> {
         // Validate the `source` and `ext` value together, since they are
         // related. In case `source` was used instead of `path`, the value
         // of `ext` is merged into a synthetic `path` value here.
-        let &(ref source, source_span) = source
-            .as_ref()
-            .expect("template path or source not found in attributes");
+        let &(ref source, source_span) = source.as_ref().ok_or_else(|| {
+            CompileError::new("template `path` or `source` not found in attributes", None)
+        })?;
         let path = match (&source, &ext) {
             (Source::Path(path), _) => config.find_template(path, None, None)?,
             (&Source::Source(_), Some(ext)) => {

rinja_derive/src/tests.rs

@@ -538,4 +538,20 @@ fn test_code_in_comment() {
     let generated = build_template(&ast).unwrap();
     assert!(generated.contains("Hello\nworld!"));
     assert!(!generated.contains("compile_error"));
+
+    let ts = "
+        #[template(ext = \"txt\")]
+        /// `````
+        /// ```rinja
+        /// {{bla}}
+        /// ```
+        /// `````
+        struct BlockOnBlock;
+    ";
+    let ast = syn::parse_str(ts).unwrap();
+    let err = build_template(&ast).unwrap_err();
+    assert_eq!(
+        err.to_string(),
+        "template `path` or `source` not found in attributes"
+    );
 }

testing/tests/ui/rinja-block.rs

@@ -19,4 +19,13 @@ struct Unterminated;
 /// ```
 struct SyntaxError;
 
+#[derive(Template)]
+#[template(ext = "txt")]
+/// `````
+/// ```rinja
+/// {{bla}}
+/// ```
+/// `````
+struct BlockOnBlock;
+
 fn main() {}

testing/tests/ui/rinja-block.stderr

@@ -11,3 +11,9 @@ error: failed to parse template source
    |
 15 | /// ```html,rinja
    | ^^^^^^^^^^^^^^^^^
+
+error: template `path` or `source` not found in attributes
+  --> tests/ui/rinja-block.rs:23:3
+   |
+23 | #[template(ext = "txt")]
+   |   ^^^^^^^^