Add index assignment and augmented assignment

This allows for syntax like `foo['a'] = 1` and more complex assignments
like `foo.bar()[a() + b()] += 1`

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/parser.rs

@@ -587,7 +587,13 @@ impl Parser {
             let expr = self.expr()?;
             let stmt: StmtP;
 
-            if expr.as_any_ref().downcast_ref::<GetExpr>().is_some()
+            // TODO Parser::stmt_wrapped - complex assign statements could probably be cleaner and
+            // probably need their own function at this point.
+
+            let is_get_expr = expr.as_any_ref().downcast_ref::<GetExpr>().is_some();
+            let is_index_expr = expr.as_any_ref().downcast_ref::<IndexExpr>().is_some();
+
+            if (is_get_expr || is_index_expr)
                 && mat!(
                     self,
                     TokenKind::Eq,
@@ -598,15 +604,28 @@ impl Parser {
                 )
             {
                 let op = self.prev.clone().unwrap();
-                let expr = expr.as_any().downcast::<GetExpr>().unwrap();
                 let rhs = self.expr()?;
-                // unpack the GetExpr and turn it into a SetExpr instead
-                stmt = Box::new(SetStmt {
-                    expr: expr.expr,
-                    name: expr.name,
-                    op,
-                    rhs,
-                });
+
+                // unpack the GetExpr or IndexExpr and turn it into a SetExpr instead
+                if is_get_expr {
+                    let expr = expr.as_any().downcast::<GetExpr>().unwrap();
+                    stmt = Box::new(SetStmt {
+                        expr: expr.expr,
+                        name: expr.name,
+                        op,
+                        rhs,
+                    });
+                } else if is_index_expr {
+                    let expr = expr.as_any().downcast::<IndexExpr>().unwrap();
+                    stmt = Box::new(IndexAssignStmt {
+                        expr: expr.expr,
+                        index: expr.index,
+                        op,
+                        rhs,
+                    });
+                } else {
+                    unreachable!()
+                }
             } else {
                 stmt = Box::new(ExprStmt { expr });
             }