make RUSTC_BLESS entirely an internal thing

src/tools/miri/CONTRIBUTING.md

@@ -322,3 +322,5 @@ binaries, and as such worth documenting:
   perform verbose logging.
 * `MIRI_HOST_SYSROOT` is set by bootstrap to tell `cargo-miri` which sysroot to use for *host*
   operations.
+* `RUSTC_BLESS` is set by `./miri test` (and `./x.py test miri`) to indicate bless-mode to the test
+  harness.

src/tools/miri/README.md

@@ -468,8 +468,6 @@ by all intended entry points, i.e. `cargo miri` and `./miri {test,run}`):
 * `MIRI_NO_STD` makes sure that the target's sysroot is built without libstd. This allows testing
   and running no_std programs. (Miri has a heuristic to detect no-std targets based on the target
   name; this environment variable is only needed when that heuristic fails.)
-* `RUSTC_BLESS` (recognized by `./miri test` and `cargo-miri-test/run-test.py`): overwrite all
-  `stderr` and `stdout` files instead of checking whether the output matches.
 * `MIRI_SKIP_UI_CHECKS` (recognized by `./miri test`): don't check whether the
   `stderr` or `stdout` files match the actual output.
 

src/tools/miri/test-cargo-miri/run-test.py

@@ -36,7 +36,7 @@ def normalize_stdout(str):
     return str
 
 def check_output(actual, path, name):
-    if os.environ.get("RUSTC_BLESS", "0") != "0":
+    if ARGS.bless:
         # Write the output only if bless is set
         open(path, mode='w').write(actual)
         return True
@@ -185,6 +185,7 @@ def test_cargo_miri_test():
 
 args_parser = argparse.ArgumentParser(description='`cargo miri` testing')
 args_parser.add_argument('--target', help='the target to test')
+args_parser.add_argument('--bless', help='bless the reference files', action='store_true')
 ARGS = args_parser.parse_args()
 
 os.chdir(os.path.dirname(os.path.realpath(__file__)))