Because I need some help on this one.
If you dont want to open link, please, you may just answer these questions:
Thank you for your attention. 
Only valid tokens are allowed inside macro rules. ⟵ is not a valid token.
There are several restrictions around what tokens a fragments can be followed by to ensure that introducing new syntax is not a breaking change. <- happens to not be in the whitelist for pattern fragments.
Roughly it will try to match the first option and if that doesn't work, the second and so on.
Unrelated to your specific questions, but you can find a lot of useful information in the little book of rust macros.
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