parser: check invalid struct name in struct_init()

vlib/v/checker/tests/wrong_struct_init_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/wrong_struct_init_err.vv:4:33: error: struct name must begin with capital letter
+    2 | 
+    3 | import net.http
+    4 | _ := http.Response{header: http.new_header{} }
+      |                                 ~~~~~~~~~~
+    5 |

vlib/v/checker/tests/wrong_struct_init_err.vv

@@ -0,0 +1,5 @@
+module main
+
+import net.http
+_ := http.Response{header: http.new_header{} }
+

vlib/v/parser/struct.v

@@ -553,6 +553,13 @@ fn (mut p Parser) struct_init(typ_str string, kind ast.StructInitKind, is_option
 	first_pos := (if kind == .short_syntax && p.prev_tok.kind == .lcbr { p.prev_tok } else { p.tok }).pos()
 	p.init_generic_types = []ast.Type{}
 	mut typ := if kind == .short_syntax { ast.void_type } else { p.parse_type() }
+	sym := p.table.sym(typ)
+	struct_name := sym.name.all_after_last('.')
+	if sym.kind == .placeholder && struct_name.len > 0 && !struct_name[0].is_capital()
+		&& !sym.name.starts_with('C.') {
+		p.error_with_pos('struct name must begin with capital letter', first_pos)
+		return ast.StructInit{}
+	}
 	struct_init_generic_types := p.init_generic_types.clone()
 	if is_option {
 		typ = typ.set_flag(.option)