all: `for mut val in vals {` (part 1)

vlib/v/ast/ast.v

@@ -600,6 +600,8 @@ pub:
 	high      Expr // `10` in `for i in 0..10 {`
 	stmts     []Stmt
 	pos       token.Position
+	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:
 	key_type  table.Type
 	val_type  table.Type

vlib/v/checker/checker.v

@@ -2357,17 +2357,24 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 					node.key_type = key_type
 					scope.update_var_type(node.key_var, key_type)
 				}
-				value_type := c.table.value_type(typ)
+				mut value_type := c.table.value_type(typ)
 				if value_type == table.void_type || typ.has_flag(.optional) {
 					if typ != table.void_type {
 						c.error('for in: cannot index `${c.table.type_to_str(typ)}`',
 							node.cond.position())
 					}
 				}
+				// if node.val_is_mut {
+				// value_type = value_type.to_ptr()
+				// }
 				node.cond_type = typ
 				node.kind = sym.kind
 				node.val_type = value_type
-				scope.update_var_type(node.val_var, value_type)
+				if node.val_is_mut {
+					scope.update_var_type(node.val_var, value_type.to_ptr())
+				} else {
+					scope.update_var_type(node.val_var, value_type)
+				}
 			}
 			c.stmts(node.stmts)
 			c.in_for_count--

vlib/v/parser/for.v

@@ -25,10 +25,11 @@ fn (mut p Parser) for_stmt() ast.Stmt {
 			pos: pos
 			is_inf: true
 		}
-	} else if p.tok.kind == .key_mut {
-		p.error('`mut` is not needed in for loops')
 	} else if p.peek_tok.kind in [.decl_assign, .assign, .semicolon] || p.tok.kind == .semicolon {
 		// `for i := 0; i < 10; i++ {`
+		if p.tok.kind == .key_mut {
+			p.error('`mut` is not needed in `for ;;` loops: use `for i := 0; i < n; i ++ {`')
+		}
 		mut init := ast.Stmt{}
 		mut cond := p.new_true_expr()
 		mut inc := ast.Stmt{}
@@ -68,8 +69,13 @@ fn (mut p Parser) for_stmt() ast.Stmt {
 			inc: inc
 			pos: pos
 		}
-	} else if p.peek_tok.kind in [.key_in, .comma] {
+	} else if p.peek_tok.kind in [.key_in, .comma] ||
+		(p.tok.kind == .key_mut && p.peek_tok2.kind in [.key_in, .comma]) {
 		// `for i in vals`, `for i in start .. end`
+		val_is_mut := p.tok.kind == .key_mut
+		if val_is_mut {
+			p.next()
+		}
 		key_var_pos := p.tok.position()
 		mut val_var_pos := p.tok.position()
 		mut key_var_name := ''
@@ -124,6 +130,7 @@ fn (mut p Parser) for_stmt() ast.Stmt {
 			p.scope.register(val_var_name, ast.Var{
 				name: val_var_name
 				pos: val_var_pos
+				is_mut: val_is_mut
 			})
 		}
 		p.inside_for = false
@@ -138,6 +145,7 @@ fn (mut p Parser) for_stmt() ast.Stmt {
 			high: high_expr
 			is_range: is_range
 			pos: pos
+			val_is_mut: val_is_mut
 		}
 	}
 	// `for cond {`

vlib/v/tests/for_loops_test.v

@@ -54,3 +54,14 @@ fn test_for_char_in_map() {
 	}
 	assert acc == 'a: b, c: d, '
 }
+
+fn test_mut_for() {
+	/*
+	mut vals := [1,2,3]
+	for mut val in vals {
+		(*val)++
+	}
+	assert vals == [2,3,4]
+	println(vals)
+	*/
+}