Add indexing expression parsing and tests

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

src/syn/ast.rs

@@ -25,6 +25,7 @@ pub enum Expr {
     Bin(Box<BinExpr>),
     Un(Box<UnExpr>),
     FunCall(Box<FunCallExpr>),
+    Index(Box<IndexExpr>),
     Base(BaseExpr),
 }
 
@@ -35,6 +36,7 @@ impl Spanned for Expr {
             Expr::Bin(b) => b.span(),
             Expr::Un(u) => u.span(),
             Expr::FunCall(f) => f.span(),
+            Expr::Index(i) => i.span(),
         }
     }
 }
@@ -103,6 +105,21 @@ impl Spanned for FunCallExpr {
     }
 }
 
+#[derive(Derivative, Clone, PartialEq, Eq)]
+#[derivative(Debug)]
+pub struct IndexExpr {
+    pub expr: Expr,
+    pub index: Expr,
+    #[derivative(Debug = "ignore")]
+    pub span: Span,
+}
+
+impl Spanned for IndexExpr {
+    fn span(&self) -> Span {
+        self.span
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum BaseExprKind {
     Ident,