Make x test src/tools/rustfmt --bless format rustfmt with the freshly built in-tree version

src/bootstrap/test.rs

@@ -430,6 +430,10 @@ fn run(self, builder: &Builder<'_>) {
             &[],
         );
 
+        if builder.config.cmd.bless() {
+            cargo.env("BLESS", "1");
+        }
+
         let dir = testdir(builder, compiler.host);
         t!(fs::create_dir_all(&dir));
         cargo.env("RUSTFMT_TEST_DIR", dir);

src/tools/rustfmt/src/parse/session.rs

@@ -152,15 +152,13 @@ fn default_handler(
             TerminalUrl::No,
         ))
     };
-    Handler::with_emitter(
-        Box::new(SilentOnIgnoredFilesEmitter {
+    Handler::with_emitter(Box::new(SilentOnIgnoredFilesEmitter {
         has_non_ignorable_parser_errors: false,
         source_map,
         emitter,
         ignore_path_set,
         can_reset,
-        }),
-    )
+    }))
 }
 
 impl ParseSess {

src/tools/rustfmt/src/test/mod.rs

@@ -838,6 +838,12 @@ fn handle_result(
 
         // Ignore LF and CRLF difference for Windows.
         if !string_eq_ignore_newline_repr(&fmt_text, &text) {
+            if let Some(bless) = std::env::var_os("BLESS") {
+                if bless != "0" {
+                    std::fs::write(file_name, fmt_text).unwrap();
+                    continue;
+                }
+            }
             let diff = make_diff(&text, &fmt_text, DIFF_CONTEXT_SIZE);
             assert!(
                 !diff.is_empty(),