Add tests of _ and self variables in format string

tests/test_display.rs

@@ -318,6 +318,15 @@ fn test_keyword() {
     assert("error: 1", Error);
 }
 
+#[test]
+fn test_self() {
+    #[derive(Error, Debug)]
+    #[error("error: {self:?}")]
+    struct Error;
+
+    assert("error: Error", Error);
+}
+
 #[test]
 fn test_str_special_chars() {
     #[derive(Error, Debug)]

tests/ui/display-underscore.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("{_}")]
+pub struct Error;
+
+fn main() {}

tests/ui/display-underscore.stderr

@@ -0,0 +1,7 @@
+error: invalid format string: invalid argument name `_`
+ --> tests/ui/display-underscore.rs:4:11
+  |
+4 | #[error("{_}")]
+  |           ^ invalid argument name in format string
+  |
+  = note: argument name cannot be a single underscore