diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index 21e62e47a5..3963072860 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -624,7 +624,6 @@ pub struct Stmt {
 pub struct Var {
 pub:
 	name            string
-	expr            Expr
 	share           ShareType
 	is_mut          bool
 	is_autofree_tmp bool
@@ -632,6 +631,7 @@ pub:
 	is_auto_deref   bool
 	is_inherited    bool
 pub mut:
+	expr       Expr
 	typ        Type
 	orig_type  Type   // original sumtype type; 0 if it's not a sumtype
 	smartcasts []Type // nested sum types require nested smart casting, for that a list of types is needed
@@ -997,7 +997,6 @@ pub struct ForInStmt {
 pub:
 	key_var    string
 	val_var    string
-	cond       Expr
 	is_range   bool
 	high       Expr // `10` in `for i in 0..10 {`
 	stmts      []Stmt
@@ -1005,6 +1004,7 @@ 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
diff --git a/vlib/v/checker/assign.v b/vlib/v/checker/assign.v
index f7ac0f3ac7..e418d724a4 100644
--- a/vlib/v/checker/assign.v
+++ b/vlib/v/checker/assign.v
@@ -16,7 +16,7 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 	node.left_types = []
 	mut right_len := node.right.len
 	mut right_type0 := ast.void_type
-	for i, right in node.right {
+	for i, mut right in node.right {
 		if right in [ast.CallExpr, ast.IfExpr, ast.LockExpr, ast.MatchExpr] {
 			if right in [ast.IfExpr, ast.MatchExpr] && node.left.len == node.right.len && !is_decl
 				&& node.left[i] in [ast.Ident, ast.SelectorExpr] && !node.left[i].is_blank_ident() {
@@ -41,18 +41,18 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 				right_len = 0
 			}
 		}
-		if right is ast.InfixExpr {
+		if mut right is ast.InfixExpr {
 			if right.op == .arrow {
 				c.error('cannot use `<-` on the right-hand side of an assignment, as it does not return any values',
 					right.pos)
 			}
 		}
-		if right is ast.Ident {
+		if mut right is ast.Ident {
 			if right.is_mut {
 				c.error('unexpected `mut` on right-hand side of assignment', right.mut_pos)
 			}
 		}
-		if right is ast.None {
+		if mut right is ast.None {
 			c.error('you can not assign a `none` value to a variable', right.pos)
 		}
 	}
@@ -112,9 +112,9 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 				node.right_types << c.check_expr_opt_call(node.right[i], right_type)
 			}
 		}
-		right := if i < node.right.len { node.right[i] } else { node.right[0] }
+		mut right := if i < node.right.len { node.right[i] } else { node.right[0] }
 		mut right_type := node.right_types[i]
-		if right is ast.Ident {
+		if mut right is ast.Ident {
 			right_sym := c.table.sym(right_type)
 			if right_sym.info is ast.Struct {
 				if right_sym.info.generic_types.len > 0 {
@@ -128,12 +128,12 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 		}
 		if is_decl {
 			// check generic struct init and return unwrap generic struct type
-			if right is ast.StructInit {
+			if mut right is ast.StructInit {
 				if right.typ.has_flag(.generic) {
 					c.expr(right)
 					right_type = right.typ
 				}
-			} else if right is ast.PrefixExpr {
+			} else if mut right is ast.PrefixExpr {
 				if right.op == .amp && right.right is ast.StructInit {
 					right_type = c.expr(right)
 				}
@@ -144,7 +144,7 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 				left_type = ast.mktyp(right_type)
 			}
 			if left_type == ast.int_type {
-				if right is ast.IntegerLiteral {
+				if mut right is ast.IntegerLiteral {
 					mut is_large := right.val.len > 13
 					if !is_large && right.val.len > 8 {
 						val := right.val.i64()
@@ -244,7 +244,7 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 									}
 								}
 								if left_type in ast.unsigned_integer_type_idxs {
-									if right is ast.IntegerLiteral {
+									if mut right is ast.IntegerLiteral {
 										if right.val[0] == `-` {
 											c.error('Cannot assign negative value to unsigned integer type',
 												right.pos)
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 228d2306f8..82bd9a2684 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -3280,10 +3280,11 @@ pub fn (mut c Checker) concat_expr(mut node ast.ConcatExpr) ast.Type {
 }
 
 // smartcast takes the expression with the current type which should be smartcasted to the target type in the given scope
-fn (mut c Checker) smartcast(expr ast.Expr, cur_type ast.Type, to_type_ ast.Type, mut scope ast.Scope) {
+fn (mut c Checker) smartcast(expr_ ast.Expr, cur_type ast.Type, to_type_ ast.Type, mut scope ast.Scope) {
 	sym := c.table.sym(cur_type)
 	to_type := if sym.kind == .interface_ { to_type_.ref() } else { to_type_ }
-	match expr {
+	mut expr := unsafe { expr_ }
+	match mut expr {
 		ast.SelectorExpr {
 			mut is_mut := false
 			mut smartcasts := []ast.Type{}
diff --git a/vlib/v/checker/for.v b/vlib/v/checker/for.v
index b4209e23ea..94bbea266a 100644
--- a/vlib/v/checker/for.v
+++ b/vlib/v/checker/for.v
@@ -102,7 +102,7 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 			}
 			if node.val_is_mut {
 				value_type = value_type.ref()
-				match node.cond {
+				match mut node.cond {
 					ast.Ident {
 						if mut node.cond.obj is ast.Var {
 							if !node.cond.obj.is_mut {
diff --git a/vlib/v/checker/if.v b/vlib/v/checker/if.v
index ee0f45f147..4660ac3287 100644
--- a/vlib/v/checker/if.v
+++ b/vlib/v/checker/if.v
@@ -309,12 +309,17 @@ fn (mut c Checker) smartcast_if_conds(node ast.Expr, mut scope ast.Scope) {
 					c.error('cannot use type `$expect_str` as type `$expr_str`', node.pos)
 				}
 				if node.left in [ast.Ident, ast.SelectorExpr] && node.right is ast.TypeNode {
-					is_variable := if mut node.left is ast.Ident {
+					is_variable := if node.left is ast.Ident {
 						node.left.kind == .variable
 					} else {
 						true
 					}
 					if is_variable {
+						if (node.left is ast.Ident && (node.left as ast.Ident).is_mut)
+							|| (node.left is ast.SelectorExpr
+							&& (node.left as ast.SelectorExpr).is_mut) {
+							c.fail_if_immutable(node.left)
+						}
 						if left_sym.kind in [.interface_, .sum_type] {
 							c.smartcast(node.left, node.left_type, right_type, mut scope)
 						}
diff --git a/vlib/v/checker/match.v b/vlib/v/checker/match.v
index b6371783ff..81e9ef937f 100644
--- a/vlib/v/checker/match.v
+++ b/vlib/v/checker/match.v
@@ -237,7 +237,7 @@ fn (mut c Checker) match_exprs(mut node ast.MatchExpr, cond_type_sym ast.TypeSym
 						}
 					}
 				}
-			} else if mut cond_type_sym.info is ast.SumType {
+			} else if cond_type_sym.info is ast.SumType {
 				if expr_type !in cond_type_sym.info.variants {
 					expr_str := c.table.type_to_str(expr_type)
 					expect_str := c.table.type_to_str(node.cond_type)
diff --git a/vlib/v/checker/tests/if_mut_with_immutable_var_err.out b/vlib/v/checker/tests/if_mut_with_immutable_var_err.out
new file mode 100644
index 0000000000..aca242045e
--- /dev/null
+++ b/vlib/v/checker/tests/if_mut_with_immutable_var_err.out
@@ -0,0 +1,14 @@
+vlib/v/checker/tests/if_mut_with_immutable_var_err.vv:5:9: error: `i` is immutable, declare it with `mut` to make it mutable
+    3 | fn main() {
+    4 |     i := Int(0)
+    5 |     if mut i is int {
+      |            ^
+    6 |         i = 1
+    7 |     } else if mut i is byte {
+vlib/v/checker/tests/if_mut_with_immutable_var_err.vv:7:16: error: `i` is immutable, declare it with `mut` to make it mutable
+    5 |     if mut i is int {
+    6 |         i = 1
+    7 |     } else if mut i is byte {
+      |                   ^
+    8 |         i = 2
+    9 |     }
diff --git a/vlib/v/checker/tests/if_mut_with_immutable_var_err.vv b/vlib/v/checker/tests/if_mut_with_immutable_var_err.vv
new file mode 100644
index 0000000000..4c84c91420
--- /dev/null
+++ b/vlib/v/checker/tests/if_mut_with_immutable_var_err.vv
@@ -0,0 +1,11 @@
+type Int = byte | int
+
+fn main() {
+	i := Int(0)
+	if mut i is int {
+		i = 1
+	} else if mut i is byte {
+		i = 2
+	}
+	println(i)
+}
diff --git a/vlib/v/fmt/fmt.v b/vlib/v/fmt/fmt.v
index cee30af7df..1a25649238 100644
--- a/vlib/v/fmt/fmt.v
+++ b/vlib/v/fmt/fmt.v
@@ -1803,7 +1803,7 @@ pub fn (mut f Fmt) enum_val(node ast.EnumVal) {
 }
 
 pub fn (mut f Fmt) ident(node ast.Ident) {
-	if mut node.info is ast.IdentVar {
+	if node.info is ast.IdentVar {
 		if node.info.is_mut {
 			f.write(node.info.share.str() + ' ')
 		}
diff --git a/vlib/v/gen/c/assign.v b/vlib/v/gen/c/assign.v
index 6f401871a8..33ac7403de 100644
--- a/vlib/v/gen/c/assign.v
+++ b/vlib/v/gen/c/assign.v
@@ -7,7 +7,8 @@ import v.ast
 import v.util
 import v.token
 
-fn (mut g Gen) gen_assign_stmt(node ast.AssignStmt) {
+fn (mut g Gen) gen_assign_stmt(node_ ast.AssignStmt) {
+	mut node := unsafe { node_ }
 	if node.is_static {
 		g.write('static ')
 	}
@@ -88,7 +89,7 @@ fn (mut g Gen) gen_assign_stmt(node ast.AssignStmt) {
 		g.checker_bug('node.left_types.len < node.left.len', node.pos)
 	}
 
-	for i, left in node.left {
+	for i, mut left in node.left {
 		mut is_auto_heap := false
 		mut var_type := node.left_types[i]
 		mut val_type := node.right_types[i]
@@ -163,7 +164,7 @@ fn (mut g Gen) gen_assign_stmt(node ast.AssignStmt) {
 					g.expr(left)
 					g.is_assign_lhs = false
 					g.is_arraymap_set = false
-					if left is ast.IndexExpr {
+					if mut left is ast.IndexExpr {
 						sym := g.table.sym(left.left_type)
 						if sym.kind in [.map, .array] {
 							g.expr(val)
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index dd269e29f4..d576ede838 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -3235,7 +3235,7 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 							dot := if field.typ.is_ptr() { '->' } else { '.' }
 							sum_type_deref_field += ')$dot'
 						}
-						if mut cast_sym.info is ast.Aggregate {
+						if cast_sym.info is ast.Aggregate {
 							agg_sym := g.table.sym(cast_sym.info.types[g.aggregate_type_idx])
 							sum_type_deref_field += '_$agg_sym.cname'
 						} else {
@@ -3695,7 +3695,7 @@ fn (mut g Gen) ident(node ast.Ident) {
 							}
 						}
 						dot := if is_ptr || is_auto_heap { '->' } else { '.' }
-						if mut cast_sym.info is ast.Aggregate {
+						if cast_sym.info is ast.Aggregate {
 							sym := g.table.sym(cast_sym.info.types[g.aggregate_type_idx])
 							g.write('${dot}_$sym.cname')
 						} else {
diff --git a/vlib/v/gen/c/fn.v b/vlib/v/gen/c/fn.v
index 2e3c8ef03d..445da57c66 100644
--- a/vlib/v/gen/c/fn.v
+++ b/vlib/v/gen/c/fn.v
@@ -1342,7 +1342,7 @@ fn (mut g Gen) fn_call(node ast.CallExpr) {
 									}
 								}
 								dot := if is_ptr { '->' } else { '.' }
-								if mut cast_sym.info is ast.Aggregate {
+								if cast_sym.info is ast.Aggregate {
 									sym := g.table.sym(cast_sym.info.types[g.aggregate_type_idx])
 									g.write('${dot}_$sym.cname')
 								} else {
diff --git a/vlib/v/gen/c/infix_expr.v b/vlib/v/gen/c/infix_expr.v
index d844cd277a..af061cf77d 100644
--- a/vlib/v/gen/c/infix_expr.v
+++ b/vlib/v/gen/c/infix_expr.v
@@ -373,7 +373,7 @@ fn (mut g Gen) infix_expr_in_op(node ast.InfixExpr) {
 	}
 	if right.unaliased_sym.kind == .array {
 		if left.sym.kind in [.sum_type, .interface_] {
-			if mut node.right is ast.ArrayInit {
+			if node.right is ast.ArrayInit {
 				if node.right.exprs.len > 0 {
 					mut infix_exprs := []ast.InfixExpr{}
 					for i in 0 .. node.right.exprs.len {
@@ -392,7 +392,7 @@ fn (mut g Gen) infix_expr_in_op(node ast.InfixExpr) {
 				}
 			}
 		}
-		if mut node.right is ast.ArrayInit {
+		if node.right is ast.ArrayInit {
 			if node.right.exprs.len > 0 {
 				// `a in [1,2,3]` optimization => `a == 1 || a == 2 || a == 3`
 				// avoids an allocation
diff --git a/vlib/v/gen/c/str_intp.v b/vlib/v/gen/c/str_intp.v
index 9817e4abdb..6890fb897c 100644
--- a/vlib/v/gen/c/str_intp.v
+++ b/vlib/v/gen/c/str_intp.v
@@ -200,7 +200,7 @@ fn (mut g Gen) string_inter_literal(node ast.StringInterLiteral) {
 	// fn (mut g Gen) str_int2(node ast.StringInterLiteral) {
 	if g.inside_comptime_for_field {
 		mut node_ := unsafe { node }
-		for i, expr in node_.exprs {
+		for i, mut expr in node_.exprs {
 			if mut expr is ast.Ident {
 				if mut expr.obj is ast.Var {
 					node_.expr_types[i] = expr.obj.typ
diff --git a/vlib/v/parser/comptime.v b/vlib/v/parser/comptime.v
index 5a6f5858f8..6d07664354 100644
--- a/vlib/v/parser/comptime.v
+++ b/vlib/v/parser/comptime.v
@@ -259,10 +259,10 @@ fn (mut p Parser) comptime_call() ast.ComptimeCall {
 	mut file := parse_comptime(v_code, p.table, p.pref, scope)
 	file.path = tmpl_path
 	// copy vars from current fn scope into vweb_tmpl scope
-	for stmt in file.stmts {
+	for mut stmt in file.stmts {
 		if mut stmt is ast.FnDecl {
 			if stmt.name == 'main.vweb_tmpl_$tmp_fn_name' {
-				for _, obj in p.scope.objects {
+				for _, mut obj in p.scope.objects {
 					if mut obj is ast.Var {
 						stmt.scope.register(ast.Var{
 							...obj