parser: deprecate `{var |` struct update (#8618)

vlib/v/ast/ast.v

@@ -1054,6 +1054,8 @@ pub:
 	pos       token.Position
 }
 */
+
+// deprecated
 pub struct Assoc {
 pub:
 	var_name string

vlib/v/fmt/fmt.v

@@ -1136,8 +1136,7 @@ pub fn (mut f Fmt) cast_expr(node ast.CastExpr) {
 pub fn (mut f Fmt) assoc(node ast.Assoc) {
 	f.writeln('{')
 	// f.indent++
-	f.writeln('\t$node.var_name |')
-	// TODO StructInit copy pasta
+	f.writeln('\t...$node.var_name')
 	for i, field in node.fields {
 		f.write('\t$field: ')
 		f.expr(node.exprs[i])

vlib/v/fmt/tests/struct_init_keep.vv

@@ -7,5 +7,8 @@ fn main() {
 	u := User{
 		age: 54
 	}
-	println(u)
+	_ = User{
+		...u
+		name: 'hi'
+	}
 }

vlib/v/gen/c/cgen.v

@@ -6240,9 +6240,9 @@ $staticprefix $interface_name* I_${cctype}_to_Interface_${interface_name}_ptr($c
 					//
 					params_start_pos := g.out.len
 					mut params := method.params.clone()
-					first_param := params[0] // workaround, { params[0] | ... } doesn't work
+					// hack to mutate typ
 					params[0] = {
-						first_param |
+						...params[0]
 						typ: params[0].typ.set_nr_muls(1)
 					}
 					fargs, _ := g.fn_args(params, false) // second argument is ignored anyway

vlib/v/parser/pratt.v

@@ -244,7 +244,8 @@ pub fn (mut p Parser) expr(precedence int) ast.Expr {
 				node = p.map_init()
 			} else {
 				// it should be a struct
-				if p.peek_tok.kind == .pipe {
+				if p.tok.kind == .name && p.peek_tok.kind == .pipe {
+					p.warn_with_pos('use e.g. `...struct_var` instead', p.peek_tok.position())
 					node = p.assoc()
 				} else if (p.tok.kind == .name && p.peek_tok.kind == .colon)
 					|| p.tok.kind in [.rcbr, .comment, .ellipsis] {