Implement a function to detect the end of a footnote reference definition with balanced brackets.

src/parser/parser_context.rs

@@ -141,6 +141,18 @@ pub enum ContextElement<'r, 's> {
     /// org-mode document since text needs to be re-parsed to look for
     /// radio links matching the contents of radio targets.
     RadioTarget(Vec<&'r Vec<Object<'s>>>),
+
+    /// Stores the current bracket depth inside a footnote reference's definition.
+    ///
+    /// The definition inside a footnote reference must have balanced
+    /// brackets [] inside the definition, so this stores the amount
+    /// of opening brackets subtracted by the amount of closing
+    /// brackets within the definition.
+    ///
+    /// A reference to the position in the string is also included so
+    /// unbalanced brackets can be detected in the middle of an
+    /// object.
+    FootnoteReferenceDefinition(FootnoteReferenceDefinition<'s>),
 }
 
 pub struct ExitMatcherNode<'r> {
@@ -148,6 +160,12 @@ pub struct ExitMatcherNode<'r> {
     pub class: ExitClass,
 }
 
+#[derive(Debug)]
+pub struct FootnoteReferenceDefinition<'s> {
+    pub position: &'s str,
+    pub depth: usize,
+}
+
 impl<'r> std::fmt::Debug for ExitMatcherNode<'r> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let mut formatter = f.debug_struct("ExitMatcherNode");