checker: add mutability checks for interface fields; add tests (#8312)

vlib/v/checker/checker.v

@@ -1147,13 +1147,18 @@ fn (mut c Checker) fail_if_immutable(expr ast.Expr) (string, token.Position) {
 					}
 				}
 				.interface_ {
-					// TODO: mutability checks on interface fields?
 					interface_info := typ_sym.info as table.Interface
-					interface_info.find_field(expr.field_name) or {
+					mut field_info := interface_info.find_field(expr.field_name) or {
 						type_str := c.table.type_to_str(expr.expr_type)
 						c.error('unknown field `${type_str}.$expr.field_name`', expr.pos)
 						return '', pos
 					}
+					if !field_info.is_mut {
+						type_str := c.table.type_to_str(expr.expr_type)
+						c.error('field `$expr.field_name` of interface `$type_str` is immutable',
+							expr.pos)
+					}
+					c.fail_if_immutable(expr.expr)
 				}
 				.array, .string {
 					// This should only happen in `builtin`
@@ -1942,6 +1947,10 @@ fn (mut c Checker) type_implements(typ table.Type, inter_typ table.Type, pos tok
 					c.error('`$styp` incorrectly implements field `$ifield.name` of interface `$inter_sym.name`, expected `$exp`, got `$got`',
 						pos)
 					return false
+				} else if ifield.is_mut && !(field.is_mut || field.is_global) {
+					c.error('`$styp` incorrectly implements interface `$inter_sym.name`, field `$ifield.name` must be mutable',
+						pos)
+					return false
 				}
 				continue
 			}

vlib/v/checker/tests/immutable_interface_field.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/immutable_interface_field.vv:10:4: error: field `i1` of interface `&Bbb` is immutable
+    8 | 
+    9 | fn mutate_interface(mut b Bbb) {
+   10 |     b.i1 = 2
+      |       ~~
+   11 | }
+   12 |

vlib/v/checker/tests/immutable_interface_field.vv

@@ -0,0 +1,16 @@
+struct Aaa {
+	i1 int
+}
+
+interface Bbb {
+	i1 int
+}
+
+fn mutate_interface(mut b Bbb) {
+	b.i1 = 2
+}
+
+fn main() {
+	mut a := Aaa{1}
+	mutate_interface(mut a)
+}

vlib/v/checker/tests/unimplemented_interface_h.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/unimplemented_interface_h.vv:9:13: error: `Cat` doesn't implement field `name` of interface `Animal`
+    7 | fn main() {
+    8 |     mut animals := []Animal{}
+    9 |     animals << Cat{}
+      |                ~~~~~
+   10 | }

vlib/v/checker/tests/unimplemented_interface_h.vv

@@ -0,0 +1,10 @@
+interface Animal {
+	name string
+}
+
+struct Cat {}
+
+fn main() {
+	mut animals := []Animal{}
+	animals << Cat{}
+}

vlib/v/checker/tests/unimplemented_interface_i.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/unimplemented_interface_i.vv:11:13: error: `Cat` incorrectly implements field `name` of interface `Animal`, expected `string`, got `int`
+    9 | fn main() {
+   10 |     mut animals := []Animal{}
+   11 |     animals << Cat{}
+      |                ~~~~~
+   12 | }

vlib/v/checker/tests/unimplemented_interface_i.vv

@@ -0,0 +1,12 @@
+interface Animal {
+	name string
+}
+
+struct Cat {
+	name int
+}
+
+fn main() {
+	mut animals := []Animal{}
+	animals << Cat{}
+}

vlib/v/checker/tests/unimplemented_interface_j.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/unimplemented_interface_j.vv:12:13: error: `Cat` incorrectly implements interface `Animal`, field `name` must be mutable
+   10 | fn main() {
+   11 |     mut animals := []Animal{}
+   12 |     animals << Cat{}
+      |                ~~~~~
+   13 | }

vlib/v/checker/tests/unimplemented_interface_j.vv

@@ -0,0 +1,13 @@
+interface Animal {
+mut:
+	name string
+}
+
+struct Cat {
+	name string
+}
+
+fn main() {
+	mut animals := []Animal{}
+	animals << Cat{}
+}

vlib/v/parser/struct.v

@@ -460,17 +460,17 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	mut methods := []ast.FnDecl{cap: 20}
 	mut is_mut := false
 	for p.tok.kind != .rcbr && p.tok.kind != .eof {
+		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
+		}
 		if p.peek_tok.kind == .lpar {
 			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()
@@ -538,11 +538,14 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 				type_pos: type_pos
 				typ: field_typ
 				comments: comments
+				is_public: true
 			}
 			mut info := ts.info as table.Interface
 			info.fields << table.Field{
 				name: field_name
 				typ: field_typ
+				is_pub: true
+				is_mut: is_mut
 			}
 			ts.info = info
 		}

vlib/v/tests/interface_fields_test.v

@@ -1,4 +1,5 @@
 interface Animal {
+mut:
 	breed string
 }