checker: fix mut check bypass with for in loops (#12208)

vlib/v/checker/checker.v

@@ -121,7 +121,8 @@ pub fn new_checker(table &ast.Table, pref &pref.Preferences) &Checker {
 	}
 }
 
-pub fn (mut c Checker) check(ast_file &ast.File) {
+pub fn (mut c Checker) check(ast_file_ &ast.File) {
+	mut ast_file := ast_file_
 	c.change_current_file(ast_file)
 	for i, ast_import in ast_file.imports {
 		for sym in ast_import.syms {
@@ -212,7 +213,7 @@ pub fn (mut c Checker) check_files(ast_files []&ast.File) {
 	mut has_main_fn := false
 	mut files_from_main_module := []&ast.File{}
 	for i in 0 .. ast_files.len {
-		file := unsafe { ast_files[i] }
+		mut file := unsafe { ast_files[i] }
 		c.timers.start('checker_check $file.path')
 		c.check(file)
 		if file.mod.name == 'main' {
@@ -5092,6 +5093,13 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 					ast.MapInit {
 						c.error('map literal is immutable, it cannot be changed', node.cond.pos)
 					}
+					ast.SelectorExpr {
+						root_ident := node.cond.root_ident() or { node.cond.expr as ast.Ident }
+						if !(root_ident.obj as ast.Var).is_mut {
+							c.error('field `$node.cond.field_name` is immutable, it cannot be changed',
+								node.cond.pos)
+						}
+					}
 					else {}
 				}
 			}
@@ -5199,7 +5207,7 @@ fn (mut c Checker) asm_stmt(mut stmt ast.AsmStmt) {
 	mut aliases := c.asm_ios(stmt.output, mut stmt.scope, true)
 	aliases2 := c.asm_ios(stmt.input, mut stmt.scope, false)
 	aliases << aliases2
-	for template in stmt.templates {
+	for mut template in stmt.templates {
 		if template.is_directive {
 			/*
 			align n[,value]

vlib/v/checker/tests/for_in_mut_val_type.out

@@ -40,3 +40,10 @@ vlib/v/checker/tests/for_in_mut_val_type.vv:20:18: error: map literal is immutab
       |                     ~~~~~~~~~~~~~~~~~~
    21 |         j *= 2
    22 |     }
+vlib/v/checker/tests/for_in_mut_val_type.vv:30:17: error: field `a` is immutable, it cannot be changed
+   28 |
+   29 | fn foo(t Test) {
+   30 |     for mut e in t.a {
+      |                    ^
+   31 |         e = 0
+   32 |     }

vlib/v/checker/tests/for_in_mut_val_type.vv

@@ -21,3 +21,13 @@ fn main() {
 		j *= 2
 	}
 }
+
+struct Test {
+	a []int = [1, 2]
+}
+
+fn foo(t Test) {
+	for mut e in t.a {
+		e = 0
+	}
+}

vlib/v/gen/c/cgen.v

@@ -5634,7 +5634,7 @@ fn (mut g Gen) struct_init(struct_init ast.StructInit) {
 		g.go_back_out(3)
 		return
 	}
-	sym := g.table.get_final_type_symbol(g.unwrap_generic(struct_init.typ))
+	mut sym := g.table.get_final_type_symbol(g.unwrap_generic(struct_init.typ))
 	is_amp := g.is_amp
 	is_multiline := struct_init.fields.len > 5
 	g.is_amp = false // reset the flag immediately so that other struct inits in this expr are handled correctly
@@ -5721,7 +5721,7 @@ fn (mut g Gen) struct_init(struct_init ast.StructInit) {
 	// `inited_fields` is a list of fields that have been init'ed, they are skipped
 	mut nr_fields := 1
 	if sym.kind == .struct_ {
-		info := sym.info as ast.Struct
+		mut info := sym.info as ast.Struct
 		nr_fields = info.fields.len
 		if info.is_union && struct_init.fields.len > 1 {
 			verror('union must not have more than 1 initializer')

vlib/v/tests/for_in_mut_reference_selector_val_test.v

@@ -8,7 +8,7 @@ pub mut:
 }
 
 fn test_for_in_mut_reference_selector_val() {
-	bb := BB{
+	mut bb := BB{
 		arr: [&AA{
 			id: 'Test1'
 		}, &AA{

vlib/v/transformer/transformer.v

@@ -15,12 +15,12 @@ pub fn new_transformer(pref &pref.Preferences) &Transformer {
 
 pub fn (t Transformer) transform_files(ast_files []&ast.File) {
 	for i in 0 .. ast_files.len {
-		file := unsafe { ast_files[i] }
-		t.transform(file)
+		mut file := unsafe { ast_files[i] }
+		t.transform(mut file)
 	}
 }
 
-pub fn (t Transformer) transform(ast_file &ast.File) {
+pub fn (t Transformer) transform(mut ast_file ast.File) {
 	for mut stmt in ast_file.stmts {
 		t.stmt(mut stmt)
 	}