[fix] osh -n respects --one-pass-parse

It's not always on

TODO: I think we can get rid of this flag once we get rid of the Arena.

It's only useful for the "arena invariant", which test/arena.sh tests,
and ysh-ify uses.

core/shell.py

@@ -467,12 +467,9 @@ def Main(lang, arg_r, environ, login_shell, loader, readline):
     if flag.one_pass_parse and not exec_opts.noexec():
         raise error.Usage('--one-pass-parse requires noexec (-n)', loc.Missing)
 
-    # -n is equivalent to --tool syntax-tree
-    # flag.tool is '' if nothing is passed
-    tool_name = 'syntax-tree' if exec_opts.noexec() else flag.tool
-
     # Tools always use one pass parse
-    one_pass_parse = True if len(tool_name) else flag.one_pass_parse
+    # Note: osh --tool syntax-tree is like osh -n --one-pass-parse
+    one_pass_parse = True if len(flag.tool) else flag.one_pass_parse
 
     parse_ctx = parse_lib.ParseContext(arena,
                                        parse_opts,
@@ -921,6 +918,10 @@ def Main(lang, arg_r, environ, login_shell, loader, readline):
     # Tools that use the OSH/YSH parsing mode, etc.
     #
 
+    # flag.tool is '' if nothing is passed
+    # osh --tool syntax-tree is equivalent to osh -n --one-pass-parse
+    tool_name = 'syntax-tree' if exec_opts.noexec() else flag.tool
+
     if len(tool_name):
         arena.SaveTokens()
 

frontend/flag_def.py

@@ -218,7 +218,10 @@ def _AddShellOptions(spec):
 MAIN_SPEC.LongFlag('--completion-display', ['minimal', 'nice'], default='nice')
 # TODO: Add option for YSH prompt style?  RHS prompt?
 
-# Don't reparse a[x+1] and ``.  Only valid in -n mode.
+# $SH -n won't reparse a[x+1] and ``.  Note that $SH --tool automatically turns
+# it on.
+# TODO: Do we only need this for the "arena invariant"?  e.g. test/arena.sh I
+# think we can REMOVE it if we get rid of the arena.
 MAIN_SPEC.LongFlag('--one-pass-parse')
 
 MAIN_SPEC.LongFlag('--print-status')  # TODO: Replace with a shell hook