parser: support `mut:` section in the interface methods, and a mut interface fn modifier (#8092)

vlib/v/checker/tests/function_arg_mutable_err.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/function_arg_mutable_err.vv:1:18: error: mutable arguments are only allowed for arrays, maps, structs and pointers
+vlib/v/checker/tests/function_arg_mutable_err.vv:1:18: error: mutable arguments are only allowed for arrays, interfaces, maps, pointers and structs
 return values instead: `fn foo(mut n int) {` => `fn foo(n int) int {`
     1 | fn mod_ptr(mut a int) {
       |                  ~~~

vlib/v/checker/tests/mut_int.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/mut_int.vv:1:14: error: mutable arguments are only allowed for arrays, maps, structs and pointers
+vlib/v/checker/tests/mut_int.vv:1:14: error: mutable arguments are only allowed for arrays, interfaces, maps, pointers and structs
 return values instead: `fn foo(mut n int) {` => `fn foo(n int) int {`
     1 | fn foo(mut x int) {
       |              ~~~

vlib/v/parser/fn.v

@@ -700,10 +700,11 @@ fn (mut p Parser) fn_args() ([]table.Param, bool, bool) {
 
 fn (mut p Parser) check_fn_mutable_arguments(typ table.Type, pos token.Position) {
 	sym := p.table.get_type_symbol(typ)
-	if sym.kind !in [.array, .array_fixed, .struct_, .map, .placeholder, .sum_type] &&
+	if sym.kind !in
+		[.array, .array_fixed, .interface_, .map, .placeholder, .struct_, .sum_type] &&
 		!typ.is_ptr() && !typ.is_pointer()
 	{
-		p.error_with_pos('mutable arguments are only allowed for arrays, maps, structs and pointers\n' +
+		p.error_with_pos('mutable arguments are only allowed for arrays, interfaces, maps, pointers and structs\n' +
 			'return values instead: `fn foo(mut n $sym.name) {` => `fn foo(n $sym.name) $sym.name {`',
 			pos)
 	}

vlib/v/parser/struct.v

@@ -462,8 +462,18 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	ts.methods = []table.Fn{cap: 20}
 	// Parse methods
 	mut methods := []ast.FnDecl{cap: 20}
+	mut is_mut := false
 	for p.tok.kind != .rcbr && p.tok.kind != .eof {
 		ts = p.table.get_type_symbol(typ) // removing causes memory bug visible by `v -silent test-fmt`
+		if p.tok.kind == .key_mut {
+			if is_mut {
+				p.error_with_pos('redefinition of `mut` section', p.tok.position())
+				return {}
+			}
+			p.next()
+			p.check(.colon)
+			is_mut = true
+		}
 		method_start_pos := p.tok.position()
 		line_nr := p.tok.line_nr
 		name := p.check_name()
@@ -479,6 +489,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 		args2, _, is_variadic := p.fn_args() // TODO merge table.Param and ast.Arg to avoid this
 		mut args := [table.Param{
 			name: 'x'
+			is_mut: is_mut
 			typ: typ
 			is_hidden: true
 		}]

vlib/v/tests/interface_test.v

@@ -7,7 +7,7 @@ mut:
 	breed string
 }
 
-fn (mut c Cat) name() string {
+fn (c &Cat) name() string {
 	if c.breed != '' {
 		assert c.breed == 'Persian'
 	}
@@ -113,7 +113,7 @@ fn test_perform_speak() {
 	*/
 }
 
-fn change_animal_breed(a &Animal, new string) {
+fn change_animal_breed(mut a Animal, new string) {
 	a.set_breed(new)
 }
 
@@ -122,7 +122,7 @@ fn test_interface_ptr_modification() {
 		breed: 'Persian'
 	}
 	// TODO Should fail and require `mut cat`
-	change_animal_breed(cat, 'Siamese')
+	change_animal_breed(mut cat, 'Siamese')
 	assert cat.breed == 'Siamese'
 }
 
@@ -218,11 +218,9 @@ fn (mut b Boss) return_speaker2() ?Speaker2 {
 	return b
 }
 
-fn return_speaker2(sp Speaker2) Speaker2 {
+fn return_speaker2(mut sp Speaker2) Speaker2 {
 	s := sp.return_speaker()
-	s2 := sp.return_speaker2() or {
+	s2 := sp.return_speaker2() or { return *sp }
-		return sp
-	}
 	s.speak()
 	s2.speak()
 	return s2
@@ -231,7 +229,7 @@ fn return_speaker2(sp Speaker2) Speaker2 {
 fn test_interface_returning_interface() {
 	mut b := Boss{'bob'}
 	assert b.name == 'bob'
-	s2 := return_speaker2(b)
+	s2 := return_speaker2(mut b)
 	if s2 is Boss {
 		assert s2.name == 'boss'
 	}