Auto merge of #8235 - badboy:8234-numbers-in-strings-are-numbers, r=alexcrichton

Auto merge of #8235 - badboy:8234-numbers-in-strings-are-numbers, r=alexcrichton

Expand error message to explain that a string was found

With `opt-level = "3"` this previously said:

    must be an integer, `z`, or `s`, but found: 3 for ...

The error message doesn't make that super clear.
This should now be a bit more clear.

Fixes #8234

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We could even include a bit more saying that `"3"` should become 3 (either unconditionally or after trying to parse `"3"` into an integer?

cc @steveklabnik