builtin, checker, cgen: implement x = a[k] or { ... } for maps and arrays (#8193)

vlib/builtin/array.v

@@ -229,6 +229,16 @@ fn (a array) get(i int) voidptr {
 	}
 }
 
+// Private function. Used to implement x = a[i] or { ... }
+fn (a array) get_with_check(i int) voidptr {
+	if i < 0 || i >= a.len {
+		return 0
+	}
+	unsafe {
+		return byteptr(a.data) + i * a.element_size
+	}
+}
+
 // first returns the first element of the array.
 pub fn (a array) first() voidptr {
 	$if !no_bounds_checking ? {

vlib/builtin/map.v

@@ -552,6 +552,29 @@ fn (m &map) get_1(key voidptr, zero voidptr) voidptr {
 	return zero
 }
 
+// If `key` matches the key of an element in the container,
+// the method returns a reference to its mapped value.
+// If not, a zero pointer is returned.
+// This is used in `x := m['key'] or { ... }`
+fn (m &map) get_1_check(key voidptr) voidptr {
+	mut index, mut meta := m.key_to_index(key)
+	for {
+		if meta == unsafe { m.metas[index] } {
+			kv_index := int(unsafe { m.metas[index + 1] })
+			pkey := unsafe { m.key_values.key(kv_index) }
+			if m.key_eq_fn(key, pkey) {
+				return unsafe { byteptr(pkey) + m.key_values.key_bytes }
+			}
+		}
+		index += 2
+		meta += probe_inc
+		if meta > unsafe { m.metas[index] } {
+			break
+		}
+	}
+	return 0
+}
+
 // Checks whether a particular key exists in the map.
 fn (m &map) exists_1(key voidptr) bool {
 	mut index, mut meta := m.key_to_index(key)