checker: check for an unknown struct name (fix #8007) (#8010)

vlib/v/checker/checker.v

@@ -511,6 +511,9 @@ pub fn (mut c Checker) struct_init(mut struct_init ast.StructInit) table.Type {
 				struct_init.pos)
 		}
 	}
+	if type_sym.name.len == 1 && !c.cur_fn.is_generic {
+		c.error('unknown struct `$type_sym.name`', struct_init.pos)
+	}
 	match type_sym.kind {
 		.placeholder {
 			c.error('unknown struct: $type_sym.name', struct_init.pos)

vlib/v/checker/tests/unknown_struct_name.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/unknown_struct_name.vv:4:7: error: unknown struct `F`
+    2 |
+    3 | fn main() {
+    4 |     _ := F{}
+      |          ~~~
+    5 | }

vlib/v/checker/tests/unknown_struct_name.vv

@@ -0,0 +1,5 @@
+module main
+
+fn main() {
+	_ := F{}
+}