fix: don't allow empty string in range, test coverage improvements

pomsky-lib/tests/testcases/codepoints/e_surrogate.txt

@@ -0,0 +1,5 @@
+#! expect=error
+U+D801
+-----
+ERROR: This code point is outside the allowed range
+SPAN: 0..6

pomsky-lib/tests/testcases/codepoints/e_too_large.txt

@@ -0,0 +1,5 @@
+#! expect=error
+U+EEEEEE
+-----
+ERROR: This code point is outside the allowed range
+SPAN: 0..8

pomsky-lib/tests/testcases/codepoints/e_too_long.txt

@@ -0,0 +1,5 @@
+#! expect=error
+U+12345678
+-----
+ERROR: This code point is outside the allowed range
+SPAN: 0..10

pomsky-lib/tests/testcases/errors/empty_string_in_set_range.txt

@@ -0,0 +1,5 @@
+#! expect=error
+[''-'z']
+-----
+ERROR: Strings used in ranges can't be empty
+SPAN: 1..3

pomsky-lib/tests/testcases/errors/negated_shorthand_twice.txt

@@ -0,0 +1,6 @@
+#! expect=error
+[!!d]
+-----
+ERROR: A shorthand character class can't be negated more than once
+HELP: The number of exclamation marks is even, so you can remove all of them
+SPAN: 1..3

pomsky-lib/tests/testcases/errors/set_range_descending.txt

@@ -0,0 +1,6 @@
+#! expect=error
+['z'-'g']
+-----
+ERROR: Character range must be in increasing order, but it is U+007A - U+0067
+HELP: Switch the characters: 'g'-'z'
+SPAN: 1..8

pomsky-lib/tests/testcases/ranges/e_empty_string.txt

@@ -0,0 +1,5 @@
+#! expect=error
+range ''-'0'
+-----
+ERROR: cannot parse integer from empty string
+SPAN: 6..8

pomsky-syntax/src/parse/parser_impl.rs

@@ -626,9 +626,7 @@ impl<'i> Parser<'i> {
             }
 
             let hex = trimmed_u.trim_start_matches(|c: char| c == '+' || c.is_whitespace());
-            if hex.len() > 6 {
-                return Err(PEK::InvalidCodePoint.at(span));
-            }
+
             u32::from_str_radix(hex, 16)
                 .ok()
                 .and_then(|n| char::try_from(n).ok())
@@ -751,6 +749,11 @@ impl<'i> Parser<'i> {
             let end = helper::parse_number(helper::strip_first_last(second), radix)
                 .map_err(|k| PEK::from(k).at(span_2))?;
 
+            if start.is_empty() || end.is_empty() {
+                let span = if start.is_empty() { span_1 } else { span_2 };
+                return Err(PEK::Number(NumberError::Empty).at(span));
+            }
+
             if start.len() > end.len() || (start.len() == end.len() && start > end) {
                 return Err(PEK::RangeIsNotIncreasing.at(span_1.join(span_2)));
             }