map: add `move` method (#8660)

vlib/builtin/map.v

@@ -207,7 +207,7 @@ pub struct map {
 mut:
 	// Highest even index in the hashtable
 	even_index u32
-	// Number of cached hashbits left for rehasing
+	// Number of cached hashbits left for rehashing
 	cached_hashbits byte
 	// Used for right-shifting out used hashbits
 	shift byte
@@ -346,6 +346,14 @@ fn new_map_init_2(hash_fn MapHashFn, key_eq_fn MapEqFn, clone_fn MapCloneFn, fre
 	return out
 }
 
+pub fn (mut m map) move() map {
+	r := *m
+	unsafe {
+		C.memset(m, 0, sizeof(map))
+	}
+	return r
+}
+
 [inline]
 fn (m &map) key_to_index(pkey voidptr) (u32, u32) {
 	hash := m.hash_fn(pkey)

vlib/v/checker/checker.v

@@ -1442,10 +1442,13 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 			call_expr.return_type = table.int_type
 		}
 		return call_expr.return_type
-	} else if left_type_sym.kind == .map && method_name in ['clone', 'keys'] {
+	} else if left_type_sym.kind == .map && method_name in ['clone', 'keys', 'move'] {
 		mut ret_type := table.void_type
 		match method_name {
-			'clone' {
+			'clone', 'move' {
+				if method_name[0] == `m` {
+					c.fail_if_immutable(call_expr.left)
+				}
 				ret_type = left_type
 			}
 			'keys' {
@@ -2737,9 +2740,9 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 		if left_sym.kind == .map && !c.inside_unsafe && assign_stmt.op in [.assign, .decl_assign]
 			&& right_sym.kind == .map && (left is ast.Ident && !left.is_blank_ident())
 			&& right is ast.Ident {
-			// Do not allow `a = b`, only `a = b.clone()`
+			// Do not allow `a = b`
-			c.error('use `map2 $assign_stmt.op.str() map1.clone()` instead of `map2 $assign_stmt.op.str() map1` (or use `unsafe`)',
+			c.error('cannot copy map: call `move` or `clone` method first (or use `unsafe`)',
-				assign_stmt.pos)
+				right.position())
 		}
 		left_is_ptr := left_type.is_ptr() || left_sym.is_pointer()
 		if left_is_ptr && c.for_in_mut_val_name != left.str() && left.str() !in c.fn_mut_arg_names {

vlib/v/checker/tests/array_or_map_assign_err.out

@@ -12,16 +12,17 @@ vlib/v/checker/tests/array_or_map_assign_err.vv:5:5: error: use `array2 = array1
       |        ^
     6 | 
     7 |     m1 := {'one': 1}
-vlib/v/checker/tests/array_or_map_assign_err.vv:8:5: error: use `map2 := map1.clone()` instead of `map2 := map1` (or use `unsafe`)
+vlib/v/checker/tests/array_or_map_assign_err.vv:8:8: error: cannot copy map: call `move` or `clone` method first (or use `unsafe`)
     6 | 
     7 |     m1 := {'one': 1}
     8 |     m2 := m1
       |           ~~
     9 |     mut m3 := map[string]int{}
    10 |     m3 = m1
-vlib/v/checker/tests/array_or_map_assign_err.vv:10:5: error: use `map2 = map1.clone()` instead of `map2 = map1` (or use `unsafe`)
+vlib/v/checker/tests/array_or_map_assign_err.vv:10:7: error: cannot copy map: call `move` or `clone` method first (or use `unsafe`)
     8 |     m2 := m1
     9 |     mut m3 := map[string]int{}
    10 |     m3 = m1
-      |        ^
+      |          ~~
-   11 | }
+   11 |     
+   12 |     _ = a2

vlib/v/checker/tests/array_or_map_assign_err.vv

@@ -8,4 +8,7 @@ fn main() {
 	m2 := m1
 	mut m3 := map[string]int{}
 	m3 = m1
+	
+	_ = a2
+	_ = m2
 }

vlib/v/checker/tests/immutable_map.out

@@ -0,0 +1,27 @@
+vlib/v/checker/tests/immutable_map.vv:3:2: error: `m` is immutable, declare it with `mut` to make it mutable
+    1 | fn main() {
+    2 |     m := map[string]int
+    3 |     m['test']++
+      |     ^
+    4 |     m['test']--
+    5 |     _ = m.move()
+vlib/v/checker/tests/immutable_map.vv:4:2: error: `m` is immutable, declare it with `mut` to make it mutable
+    2 |     m := map[string]int
+    3 |     m['test']++
+    4 |     m['test']--
+      |     ^
+    5 |     _ = m.move()
+    6 |     m.delete('s')
+vlib/v/checker/tests/immutable_map.vv:5:6: error: `m` is immutable, declare it with `mut` to make it mutable
+    3 |     m['test']++
+    4 |     m['test']--
+    5 |     _ = m.move()
+      |         ^
+    6 |     m.delete('s')
+    7 | }
+vlib/v/checker/tests/immutable_map.vv:6:2: error: `m` is immutable, declare it with `mut` to make it mutable
+    4 |     m['test']--
+    5 |     _ = m.move()
+    6 |     m.delete('s')
+      |     ^
+    7 | }

vlib/v/checker/tests/immutable_map.vv

@@ -2,4 +2,6 @@ fn main() {
 	m := map[string]int
 	m['test']++
 	m['test']--
+	_ = m.move()
+	m.delete('s')
 }

vlib/v/checker/tests/immutable_map_postfix.out

@@ -1,13 +0,0 @@
-vlib/v/checker/tests/immutable_map_postfix.vv:3:2: error: `m` is immutable, declare it with `mut` to make it mutable
-    1 | fn main() {
-    2 |     m := map[string]int
-    3 |     m['test']++
-      |     ^
-    4 |     m['test']--
-    5 | }
-vlib/v/checker/tests/immutable_map_postfix.vv:4:2: error: `m` is immutable, declare it with `mut` to make it mutable
-    2 |     m := map[string]int
-    3 |     m['test']++
-    4 |     m['test']--
-      |     ^
-    5 | }

vlib/v/gen/c/fn.v

@@ -479,7 +479,7 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 		g.gen_str_for_type(node.receiver_type)
 	}
 	mut has_cast := false
-	if left_sym.kind == .map && node.name == 'clone' {
+	if left_sym.kind == .map && node.name in ['clone', 'move'] {
 		receiver_type_name = 'map'
 	}
 	// TODO performance, detect `array` method differently