safe-string: clarify the relation between configure- and compile-time options

1 files changed, 36 insertions, 1 deletions

diff --git a/configure b/configure
index e854b86217..bbc5b62b80 100755
--- a/configure
+++ b/configure
@@ -64,6 +64,7 @@ TOOLPREF=""
 with_cfi=true
 flambda=false
 safe_string=false
+safe_string_default=true
 afl_instrument=false
 max_testsuite_dir_retries=0
 with_cplugins=false
@@ -214,8 +215,36 @@ while : ; do
         ;; # Ignored for backward compatibility
     -fPIC|--fPIC)
         with_fpic=true;;
+    # There are two configure-time string safety option,
+    # (un)safe-string and (un)safe-string-default that interact
+    # with a compile-time (un)safe-string option.
+    #
+    # If -safe-string is set at configure time, then the compiler will
+    # always enforce that string and bytes are distinct: the
+    # compile-time -unsafe-string option is disabled. This lets us
+    # assume pervasive string immutability, for code optimizations and
+    # in the C layer.
+    #
+    # If -safe-string is not set at configure-time (if its
+    # negation -unsafe-string is used), the compiler will use the
+    # compile-time (un)safe-string setting to decide whether string
+    # and bytes are compatible on a per-module basis. The
+    # configure-time options (un)safe-string-default decide which
+    # setting will be chosen by default, if no compile-time option
+    # is explicitly passed.
+    #
+    # The configure-time behavior of OCaml 4.05 and older was
+    # equivalent to -unsafe-string -unsafe-string-default.
+    # then). OCaml 4.06 uses -unsafe-string -safe-string-default. We
+    # expect -safe-string to become the default in the future.
     -safe-string|--safe-string)
         safe_string=true;;
+    -unsafe-string|--unsafe-string)
+        safe_string=false;;
+    -safe-string-default|--safe-string-default)
+        safe_string_default=true;;
+    -unsafe-string-default|--unsafe-string-default)
+        safe_string_default=false;;
     -flat-float-array|--flat-float-array)
         flat_float_array=true;;
     -no-flat-float-array|--no-flat-float-array)
@@ -2101,6 +2130,7 @@ if [ "$ostype" = Cygwin ]; then
 fi
 config FLAMBDA "$flambda"
 config SAFE_STRING "$safe_string"
+config SAFE_STRING_DEFAULT "$safe_string_default"
 config WINDOWS_UNICODE "0"
 config AFL_INSTRUMENT "$afl_instrument"
 config MAX_TESTSUITE_DIR_RETRIES "$max_testsuite_dir_retries"
@@ -2221,10 +2251,15 @@ else
   else
   inf "        using flambda middle-end . no"
   fi
-  if test "$safe_string" = "true"; then
+  if $safe_string; then
   inf "        safe strings ............. yes"
   else
   inf "        safe strings ............. no"
+  if $safe_string_default; then
+  inf "        (-safe-string is the default per-module option)"
+  else
+  inf "        (-unsafe-string is the default per-module option)"
+  fi
   fi
   if $flat_float_array; then
   inf "        flat float arrays ........ yes"