improve errors around dots

Replicates graphql/graphql-js@67afee4

Author: Cito

src/graphql/language/lexer.py

@@ -144,8 +144,29 @@ def read_next_token(self, start: int) -> Token:
             if is_name_start(char):
                 return self.read_name(position)
 
-            if char == "." and body[position + 1 : position + 3] == "..":
-                return self.create_token(TokenKind.SPREAD, position, position + 3)
+            if char == ".":
+                next_char = body[position + 1 : position + 2]
+                if next_char == ".":
+                    if body[position + 2 : position + 3] == ".":
+                        return self.create_token(
+                            TokenKind.SPREAD, position, position + 3
+                        )
+                    raise GraphQLSyntaxError(
+                        self.source,
+                        position,
+                        "Unexpected '..', did you mean '...'?",
+                    )
+                if is_digit(next_char):
+                    end = position + 1
+                    while end < body_length and is_digit(body[end]):
+                        end += 1
+                    digits = body[position + 1 : end]
+                    raise GraphQLSyntaxError(
+                        self.source,
+                        position,
+                        f"Invalid number, expected digit before '.'"
+                        f", did you mean '0.{digits}'?",
+                    )
 
             message = (
                 "Unexpected single quote character ('),"

tests/language/test_lexer.py

@@ -73,6 +73,9 @@ def skips_whitespace_and_comments():
         token = lex_one(",,,foo,,,")
         assert token == Token(TokenKind.NAME, 3, 6, 1, 4, "foo")
 
+    def reports_unexpected_characters():
+        assert_syntax_error(".", "Unexpected character: '.'.", (1, 1))
+
     def errors_respect_whitespace():
         with pytest.raises(GraphQLSyntaxError) as exc_info:
             lex_one("\n\n ~\n")
@@ -454,7 +457,11 @@ def lex_reports_useful_number_errors():
         assert_syntax_error(
             "1.e1", "Invalid number, expected digit but got: 'e'.", (1, 3)
         )
-        assert_syntax_error(".123", "Unexpected character: '.'.", (1, 1))
+        assert_syntax_error(
+            ".123",
+            "Invalid number, expected digit before '.', did you mean '0.123'?",
+            (1, 1),
+        )
         assert_syntax_error(
             "1.A", "Invalid number, expected digit but got: 'A'.", (1, 3)
         )
@@ -520,7 +527,7 @@ def lexes_punctuation():
         assert lex_one("|") == Token(TokenKind.PIPE, 0, 1, 1, 1, None)
 
     def lex_reports_useful_unknown_character_error():
-        assert_syntax_error("..", "Unexpected character: '.'.", (1, 1))
+        assert_syntax_error("..", "Unexpected '..', did you mean '...'?", (1, 1))
         assert_syntax_error("~", "Unexpected character: '~'.", (1, 1))
         assert_syntax_error("\x00", "Unexpected character: U+0000.", (1, 1))
         assert_syntax_error("\b", "Unexpected character: U+0008.", (1, 1))