add test for array len inference

src/test/ui/array-slice-vec/infer_array_len.stderr

@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed
+  --> $DIR/infer_array_len.rs:19:9
+   |
+LL |     let [_, _] = a.into();
+   |         ^^^^^^ consider giving this pattern a type
+   |
+   = note: type must be known at this point
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.