cgen, checker, ast: add syntax support for reference x in for x in &somearrary { and for x in &somemap { loops (#15678)

vlib/v/ast/ast.v

@@ -1048,14 +1048,15 @@ pub:
 	val_is_mut bool // `for mut val in vals {` means that modifying `val` will modify the array
 	// and the array cannot be indexed inside the loop
 pub mut:
-	cond      Expr
-	key_type  Type
-	val_type  Type
-	cond_type Type
-	high_type Type
-	kind      Kind   // array/map/string
-	label     string // `label: for {`
-	scope     &Scope
+	val_is_ref bool // `for val in &arr {` means that value of `val` will be the reference of the value in `arr`
+	cond       Expr
+	key_type   Type
+	val_type   Type
+	cond_type  Type
+	high_type  Type
+	kind       Kind   // array/map/string
+	label      string // `label: for {`
+	scope      &Scope
 }
 
 pub struct ForCStmt {

vlib/v/checker/for.v

@@ -70,6 +70,22 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 		node.scope.update_var_type(node.val_var, node.val_type)
 	} else {
 		sym := c.table.final_sym(typ)
+		if sym.kind != .string {
+			match mut node.cond {
+				ast.PrefixExpr {
+					node.val_is_ref = node.cond.op == .amp
+				}
+				ast.Ident {
+					match mut node.cond.info {
+						ast.IdentVar {
+							node.val_is_ref = !node.cond.is_mut() && node.cond.info.typ.is_ptr()
+						}
+						else {}
+					}
+				}
+				else {}
+			}
+		}
 		if sym.kind == .struct_ {
 			// iterators
 			next_fn := sym.find_method_with_generic_parent('next') or {
@@ -148,6 +164,8 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 					}
 					else {}
 				}
+			} else if node.val_is_ref {
+				value_type = value_type.ref()
 			}
 			node.cond_type = typ
 			node.kind = sym.kind

vlib/v/gen/c/for.v

@@ -178,7 +178,7 @@ fn (mut g Gen) for_in_stmt(node ast.ForInStmt) {
 				// instead of
 				// `int* val = ((int**)arr.data)[i];`
 				// right := if node.val_is_mut { styp } else { styp + '*' }
-				right := if node.val_is_mut {
+				right := if node.val_is_mut || node.val_is_ref {
 					'(($styp)$cond_var${op_field}data) + $i'
 				} else {
 					'(($styp*)$cond_var${op_field}data)[$i]'
@@ -295,7 +295,7 @@ fn (mut g Gen) for_in_stmt(node ast.ForInStmt) {
 			} else {
 				val_styp := g.typ(node.val_type)
 				if node.val_type.is_ptr() {
-					if node.val_is_mut {
+					if node.val_is_mut || node.val_is_ref {
 						g.write('$val_styp ${c_name(node.val_var)} = &(*($val_styp)')
 					} else {
 						g.write('$val_styp ${c_name(node.val_var)} = (*($val_styp*)')

vlib/v/tests/for_in_ref_arr_test.v

@@ -0,0 +1,31 @@
+fn test_for_in_ref_val_ref_arr() {
+	arr := [1, 2, 3, 4, 5]
+	mut rets := []&int{}
+	mut expects := unsafe { []&int{len: 5, init: &arr[it]} }
+
+	for val in &arr {
+		println(val)
+		rets << val
+	}
+
+	for i, val in &arr {
+		assert voidptr(val) == voidptr(rets[i])
+	}
+	assert rets == expects
+}
+
+fn test_for_in_ref_val_ref_arr_ident() {
+	arr_ := [1, 2, 3, 4, 5]
+	arr := &arr_
+	mut rets := []&int{}
+	mut expects := unsafe { []&int{len: 5, init: &arr_[it]} }
+
+	for val in arr {
+		rets << val
+	}
+
+	for i, val in arr {
+		assert voidptr(val) == voidptr(rets[i])
+	}
+	assert rets == expects
+}

vlib/v/tests/for_in_ref_map_test.v

@@ -0,0 +1,48 @@
+fn test_for_in_ref_val_ref_map() {
+	mut mp := {
+		'a': 1
+		'b': 2
+		'c': 3
+		'd': 4
+		'e': 5
+	}
+	mut rets := map[string]&int{}
+	mut expects := map[string]&int{}
+
+	for k, mut val in mp {
+		expects[k] = unsafe { val }
+	}
+	for k, val in &mp {
+		rets[k] = unsafe { val }
+	}
+
+	for k, val in &mp {
+		assert voidptr(val) == voidptr(expects[k])
+	}
+	assert rets == expects
+}
+
+fn test_for_in_ref_val_ref_map_ident() {
+	mut mp_ := {
+		'a': 1
+		'b': 2
+		'c': 3
+		'd': 4
+		'e': 5
+	}
+	mp := &mp_
+	mut rets := map[string]&int{}
+	mut expects := map[string]&int{}
+
+	for k, mut val in mp_ {
+		expects[k] = unsafe { val }
+	}
+	for k, val in mp {
+		rets[k] = unsafe { val }
+	}
+
+	for k, val in mp {
+		assert voidptr(val) == voidptr(expects[k])
+	}
+	assert rets == expects
+}

vlib/v/tests/inout/printing_for_v_in_a.out

@@ -2,10 +2,10 @@
 98
 99
 abc
-23
-77
+&23
+&77
 abc
-> k: abc | v: xyz
-> k: def | v: jkl
+> k: abc | v: &xyz
+> k: def | v: &jkl
 abc
 abc