parser: proper error when declaring struct embedding at the wrong pos (#7920)

vlib/v/parser/struct.v

@@ -158,7 +158,8 @@ fn (mut p Parser) struct_decl() ast.StructDecl {
 			field_start_pos := p.tok.position()
 			is_embed := ((p.tok.lit.len > 1 && p.tok.lit[0].is_capital()) ||
 				p.peek_tok.kind == .dot) &&
-				language == .v && ast_fields.len == 0 && !(is_field_mut || is_field_mut || is_field_global)
+				language == .v
+			is_on_top := ast_fields.len == 0 && !(is_field_mut || is_field_mut || is_field_global)
 			mut field_name := ''
 			mut typ := table.Type(0)
 			mut type_pos := token.Position{}
@@ -174,6 +175,11 @@ fn (mut p Parser) struct_decl() ast.StructDecl {
 					}
 				}
 				type_pos = type_pos.extend(p.prev_tok.position())
+				if !is_on_top {
+					p.error_with_pos('struct embedding must be declared at the beginning of the struct body',
+						type_pos)
+					return ast.StructDecl{}
+				}
 				sym := p.table.get_type_symbol(typ)
 				if typ in embed_types {
 					p.error_with_pos('cannot embed `$sym.name` more than once', type_pos)

vlib/v/parser/tests/embed_pub_mut_err.out

@@ -1,7 +0,0 @@
-vlib/v/parser/tests/embed_pub_mut_err.vv:6:1: error: expecting type declaration
-    4 | pub mut:
-    5 |     Context
-    6 | }
-      | ^
-    7 |
-    8 | fn main() {

vlib/v/parser/tests/embed_pub_mut_err.vv

@@ -1,9 +0,0 @@
-struct Context {}
-
-struct App {
-pub mut:
-	Context
-}
-
-fn main() {
-}

vlib/v/parser/tests/struct_embed_wrong_pos_long_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/struct_embed_wrong_pos_long_err.vv:4:2: error: struct embedding must be declared at the beginning of the struct body
+    2 | struct Foo2 {
+    3 | mut:
+    4 |     cli.Command
+      |     ~~~~~~~~~~~
+    5 | }
+    6 | fn main() {}

vlib/v/parser/tests/struct_embed_wrong_pos_long_err.vv

@@ -0,0 +1,6 @@
+import cli
+struct Foo2 {
+mut:
+	cli.Command
+}
+fn main() {}

vlib/v/parser/tests/struct_embed_wrong_pos_short_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/struct_embed_wrong_pos_short_err.vv:6:2: error: struct embedding must be declared at the beginning of the struct body
+    4 | struct Foo2 {
+    5 | mut:
+    6 |     Foo
+      |     ~~~
+    7 | }
+    8 | fn main() {}

vlib/v/parser/tests/struct_embed_wrong_pos_short_err.vv

@@ -0,0 +1,8 @@
+struct Foo {
+	foo string
+}
+struct Foo2 {
+mut:
+	Foo
+}
+fn main() {}