Add an is_public_assigned predicate.

Add an `is_public_assigned` predicate, which tests whether a given
`char` is assigned (`General_Category` != `Unassigned`) in the currently
supported version of Unicode, and not Private-Use (`General_Category`
!= `Private_Use`).

This comes up in some use cases sensitive to the stability of NFC over
Unicode version changes. An unassigned codepoint could become assigned in
the future, and new normalizations could apply to it.

For further details, see
 - <https://unicode.org/reports/tr15/#Versioning>

scripts/unicode.py

@@ -98,6 +98,10 @@ class UnicodeData(object):
         self.compat_decomp = {}
         self.canon_decomp = {}
         self.general_category_mark = []
+        self.general_category_public_assigned = []
+
+        assigned_start = 0;
+        prev_char_int = -1;
 
         for line in self._fetch("UnicodeData.txt").splitlines():
             # See ftp://ftp.unicode.org/Public/3.0-Update/UnicodeData-3.0.0.html
@@ -120,6 +124,15 @@ class UnicodeData(object):
             if category == 'M' or 'M' in expanded_categories.get(category, []):
                 self.general_category_mark.append(char_int)
 
+            assert category != 'Cn', "Unexpected: Unassigned codepoint in UnicodeData.txt"
+            if category not in ['Co', 'Cs']:
+                if char_int != prev_char_int + 1:
+                    self.general_category_public_assigned.append((assigned_start, prev_char_int))
+                    assigned_start = char_int
+                prev_char_int = char_int
+
+        self.general_category_public_assigned.append((assigned_start, prev_char_int))
+
     def _load_cjk_compat_ideograph_variants(self):
         for line in self._fetch("StandardizedVariants.txt").splitlines():
             strip_comments = line.split('#', 1)[0].strip()
@@ -418,6 +431,30 @@ def gen_combining_mark(general_category_mark, out):
     gen_mph_data('combining_mark', general_category_mark, 'u32',
         lambda k: '0x{:04x}'.format(k))
 
+def gen_public_assigned(general_category_public_assigned, out):
+    # This could be done as a hash but the table is somewhat small.
+    out.write("#[inline]\n")
+    out.write("pub fn is_public_assigned(c: char) -> bool {\n")
+    out.write("    match c {\n")
+
+    start = True
+    for first, last in general_category_public_assigned:
+        if start:
+            out.write("        ")
+            start = False
+        else:
+            out.write("        | ")
+        if first == last:
+            out.write("'\\u{%s}'\n" % hexify(first))
+        else:
+            out.write("'\\u{%s}'..='\\u{%s}'\n" % (hexify(first), hexify(last)))
+    out.write("        => true,\n")
+
+    out.write("        _ => false,\n")
+    out.write("    }\n")
+    out.write("}\n")
+    out.write("\n")
+
 def gen_stream_safe(leading, trailing, out):
     # This could be done as a hash but the table is very small.
     out.write("#[inline]\n")
@@ -540,6 +577,9 @@ if __name__ == '__main__':
         gen_combining_mark(data.general_category_mark, out)
         out.write("\n")
 
+        gen_public_assigned(data.general_category_public_assigned, out)
+        out.write("\n")
+
         gen_nfc_qc(data.norm_props, out)
         out.write("\n")