parser: parse `JS.` interfaces properly (#9876)

vlib/v/ast/ast.v

@@ -264,6 +264,7 @@ pub struct InterfaceDecl {
 pub:
 	name         string
 	name_pos     token.Position
+	language     Language
 	field_names  []string
 	is_pub       bool
 	methods      []FnDecl

vlib/v/checker/checker.v

@@ -353,14 +353,18 @@ pub fn (mut c Checker) sum_type_decl(node ast.SumTypeDecl) {
 pub fn (mut c Checker) interface_decl(decl ast.InterfaceDecl) {
 	c.check_valid_pascal_case(decl.name, 'interface name', decl.pos)
 	for method in decl.methods {
-		c.check_valid_snake_case(method.name, 'method name', method.pos)
+		if decl.language == .v {
+			c.check_valid_snake_case(method.name, 'method name', method.pos)
+		}
 		c.ensure_type_exists(method.return_type, method.return_type_pos) or { return }
 		for param in method.params {
 			c.ensure_type_exists(param.typ, param.pos) or { return }
 		}
 	}
 	for i, field in decl.fields {
-		c.check_valid_snake_case(field.name, 'field name', field.pos)
+		if decl.language == .v {
+			c.check_valid_snake_case(field.name, 'field name', field.pos)
+		}
 		c.ensure_type_exists(field.typ, field.pos) or { return }
 		for j in 0 .. i {
 			if field.name == decl.fields[j].name {

vlib/v/fmt/fmt.v

@@ -1149,8 +1149,10 @@ pub fn (mut f Fmt) interface_decl(node ast.InterfaceDecl) {
 	if node.is_pub {
 		f.write('pub ')
 	}
+	f.write('interface ')
+	f.write_language_prefix(node.language)
 	name := node.name.after('.')
-	f.write('interface $name {')
+	f.write('$name {')
 	if node.fields.len > 0 || node.methods.len > 0 || node.pos.line_nr < node.pos.last_line {
 		f.writeln('')
 	}

vlib/v/parser/struct.v

@@ -440,7 +440,19 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 		p.next()
 	}
 	p.next() // `interface`
+	language := if p.tok.lit == 'C' && p.peek_tok.kind == .dot {
+		ast.Language.c
+	} else if p.tok.lit == 'JS' && p.peek_tok.kind == .dot {
+		ast.Language.js
+	} else {
+		ast.Language.v
+	}
+	if language != .v {
+		p.next() // C || JS
+		p.next() // .
+	}
 	name_pos := p.tok.position()
+	p.check_for_impure_v(language, name_pos)
 	interface_name := p.prepend_mod(p.check_name()).clone()
 	// println('interface decl $interface_name')
 	p.check(.lcbr)
@@ -494,7 +506,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 				p.error_with_pos('duplicate method `$name`', method_start_pos)
 				return ast.InterfaceDecl{}
 			}
-			if util.contains_capital(name) {
+			if language == .v && util.contains_capital(name) {
 				p.error('interface methods cannot contain uppercase letters, use snake_case instead')
 				return ast.InterfaceDecl{}
 			}
@@ -575,6 +587,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	pos = pos.extend_with_last_line(p.prev_tok.position(), p.prev_tok.line_nr)
 	return ast.InterfaceDecl{
 		name: interface_name
+		language: language
 		fields: fields
 		methods: methods
 		is_pub: is_pub