toml: add comptime check if a supported type (struct) was passed to toml.decode, when the type has no custom .from_toml method defined (#19317)

Author: ttytm

vlib/toml/tests/encode_and_decode_test.v

@@ -177,3 +177,13 @@ times = [
 	assert toml.encode[Arrs](a) == s
 	assert toml.decode[Arrs](s)! == a
 }
+
+fn test_unsupported_type() {
+	s := 'name = "Peter"'
+	err_msg := 'toml.decode: expected struct, found '
+	toml.decode[string](s) or { assert err.msg() == err_msg + 'string' }
+	toml.decode[[]string](s) or { assert err.msg() == err_msg + '[]string' }
+	toml.decode[int](s) or { assert err.msg() == err_msg + 'int' }
+	toml.decode[[]f32](s) or { assert err.msg() == err_msg + '[]f32' }
+	// ...
+}

vlib/toml/toml.v

@@ -23,6 +23,9 @@ pub fn decode[T](toml_txt string) !T {
 			return typ
 		}
 	}
+	$if T !is $struct {
+		return error('toml.decode: expected struct, found ${T.name}')
+	}
 	decode_struct[T](doc.to_any(), mut typ)
 	return typ
 }