parser: support short struct update syntax {...ident, (#8613)

doc/docs.md

@@ -1633,6 +1633,8 @@ Only more complex types such as arrays and maps may be modified.
 Use `user.register()` or `user = register(user)`
 instead of `register(mut user)`.
 
+#### Struct update syntax
+
 V makes it easy to return a modified version of an object:
 
 ```v
@@ -1644,7 +1646,7 @@ struct User {
 
 fn register(u User) User {
 	return {
-		u |
+		...u
 		is_registered: true
 	}
 }

vlib/v/parser/pratt.v

@@ -246,7 +246,8 @@ pub fn (mut p Parser) expr(precedence int) ast.Expr {
 				// it should be a struct
 				if p.peek_tok.kind == .pipe {
 					node = p.assoc()
-				} else if p.peek_tok.kind == .colon || p.tok.kind in [.rcbr, .comment] {
+				} else if (p.tok.kind == .name && p.peek_tok.kind == .colon)
+					|| p.tok.kind in [.rcbr, .comment, .ellipsis] {
 					node = p.struct_init(true) // short_syntax: true
 				} else if p.tok.kind == .name {
 					p.next()

vlib/v/tests/struct_test.v

@@ -164,12 +164,14 @@ fn test_assoc_with_vars() {
 	def2 := Def{
 		a: 12
 	}
-	merged := Def{
+	mut merged := Def{
 		...def2
 		a: 42
 	}
 	assert merged.a == 42
 	assert merged.b == 7
+	merged = {...def2, b: 9}
+	assert merged == Def{12, 9}
 
 	def3 := &Def{ 100, 200 }
 	merged1 := Def{...(*def3)}