vec_push returns inserted index

lang/src/vec.asm

@@ -161,7 +161,8 @@ vec_get:
 
 ;; rdi: pointer to Vec struct
 ;; rsi: pointer to data to push
-;; fn vec_push(vec: *mut Vec, data: &[u8])
+;; returns the index at which the item was pushed (the old length of the vector)
+;; fn vec_push(vec: *mut Vec, data: &[u8]) -> usize
 vec_push:
     push rbp
     mov rbp, rsp
@@ -186,8 +187,9 @@ vec_push:
     mov rdx, [rax + 24]     ; item_size
     call memcpy
     ; vec.len += 1;
-    mov rax, [rsp]          ; vec
-    add qword [rax + 8], 1
+    mov rdx, [rsp]          ; vec
+    mov rax, [rdx + 8]
+    add qword [rdx + 8], 1
     add rsp, 24
     pop rbp
     ret