strengthen the contract on lexer results

thanks to @LiberalArtist for finding this improvement

Author: rfindler

syntax-color-doc/syntax-color/syntax-color.scrbl

@@ -87,8 +87,9 @@ Parenthesis matching code built on top of @racket[token-tree%].
  This function raises an error unless the following boolean
  expression is true:
  @racketblock[(or (equal? type 'eof)
-                  (and (<= pos-before new-token-start pos-after)
-                       (<= pos-before new-token-end pos-after)))]
+                  (and (= pos-before new-token-start)
+                       (< new-token-start new-token-end)
+                       (= new-token-end pos-after)))]
  but it checks the individual parts of the expression to
  raise a more meaningful error message when some part is not
  true.

syntax-color-lib/syntax-color/lexer-contract.rkt

@@ -229,11 +229,16 @@
 (define (check-colorer-results-match-port-before-and-after
          who type pos-before new-token-start new-token-end pos-after)
   (unless (equal? 'eof type)
-    (unless (<= pos-before new-token-start pos-after)
+    (unless (= pos-before new-token-start)
       (error who
-             "expected the token start to be between ~s and ~s, got ~s"
-             pos-before pos-after new-token-start))
-    (unless (<= pos-before new-token-end pos-after)
+             "token start expected to match port position before reading\n  token-start: ~e\n  port-position: ~s"
+             new-token-start pos-before))
+    (unless (= pos-after new-token-end)
       (error who
-             "expected the token end to be between ~s and ~s, got ~s"
-             pos-before pos-after new-token-end))))
+             "token end expected to match port position afterwards\n  token-end: ~e\n  port-position: ~s"
+             "expected the token end to be ~s, got ~s"
+             new-token-end pos-after))
+    (unless (< new-token-start new-token-end)
+      (error who
+             "expected the token start to be strictly less than the token end\n  token-start: ~e\n  token-end: ~e"
+             new-token-start new-token-end))))

syntax-color-test/tests/syntax-color/module-lexer.rkt

@@ -22,8 +22,9 @@
                  0
                  mode))
         (define-values (_line2 _col2 pos-after) (port-next-location p))
-        (unless (and (or (not token-start) (<= pos-before token-start pos-after))
-                     (or (not token-end) (<= pos-before token-end pos-after)))
+        (unless (and (or (not token-start) (= pos-before token-start))
+                     (or (not token-end) (= token-end pos-after))
+                     (or (not token-start) (not token-end) (equal? type 'eof) (< token-start token-end)))
           (error 'lex
                  (string-append
                   "pos-before, pos-after, token-start, and token-end aren't in the right relationship\n"