vfmt: support comments after interface methods

vlib/v/ast/ast.v

@@ -273,6 +273,7 @@ pub:
 pub mut:
 	stmts         []Stmt
 	return_type   table.Type
+	comments      []Comment // comments *after* the header, but *before* `{`; used for InterfaceDecl
 }
 
 // break, continue

vlib/v/fmt/fmt.v

@@ -688,7 +688,13 @@ pub fn (mut f Fmt) interface_decl(node ast.InterfaceDecl) {
 	f.comments_after_last_field(node.pre_comments)
 	for method in node.methods {
 		f.write('\t')
-		f.writeln(method.stringify(f.table, f.cur_mod).after('fn '))
+		f.write(method.stringify(f.table, f.cur_mod).after('fn '))
+		f.comments(method.comments, {
+			inline: true
+			has_nl: false
+			level: .indent
+		})
+		f.writeln('')
 	}
 	f.writeln('}\n')
 }

vlib/v/fmt/tests/interface_declaration_comments_keep.vv

@@ -0,0 +1,4 @@
+pub interface ReaderWriter {
+	read(mut buf []byte) ?int // from Reader
+	write(buf []byte) ?int // from Writer
+}

vlib/v/parser/struct.v

@@ -423,6 +423,7 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 		if p.tok.kind.is_start_of_type() && p.tok.line_nr == line_nr {
 			method.return_type = p.parse_type()
 		}
+		method.comments = p.eat_comments()
 		methods << method
 		// println('register method $name')
 		return_type_sym := p.table.get_type_symbol(method.return_type)