check index on placeholder (#1005)

src/eval/mod.rs

@@ -1268,7 +1268,13 @@ impl ast::Pattern {
                 ast::DestructuringKind::Named(named) => {
                     bail!(named.span(), "cannot destructure named elements from an array")
                 }
-                ast::DestructuringKind::Placeholder(_) => i += 1,
+                ast::DestructuringKind::Placeholder(underscore) => {
+                    if i < value.len() {
+                        i += 1
+                    } else {
+                        bail!(underscore.span(), "not enough elements to destructure")
+                    }
+                }
             }
         }
         if i < value.len() {

tests/typ/compiler/let.typ

@@ -177,6 +177,12 @@ Three
 #let (a, ..) = (a: 1, b: 2)
 #test(a, 1)
 
+---
+// Trailing placeholders.
+// Error: 10-11 not enough elements to destructure
+#let (a, _, _, _, _) = (1,)
+#test(a, 1)
+
 ---
 // Error: 10-13 expected identifier, found string
 // Error: 18-19 expected identifier, found integer