Add bless env-var to update changed tests automatically

Author: Nemo157

src/test/mod.rs

@@ -834,6 +834,12 @@ fn handle_result(
 
         // Ignore LF and CRLF difference for Windows.
         if !string_eq_ignore_newline_repr(&fmt_text, &text) {
+            if std::env::var("RUSTFMT_BLESS").ok().as_deref() == Some("1") {
+                let write_error = format!("couldn't write target {target:?}");
+                std::fs::write(&target, &fmt_text).expect(&write_error);
+                continue;
+            }
+
             let diff = make_diff(&text, &fmt_text, DIFF_CONTEXT_SIZE);
             assert!(
                 !diff.is_empty(),