auto merge of #8561 : kballard/rust/do-block-internal-err-msg, r=thestinger

When using a `do` block to call an internal iterator, if you forgot to
return a value from the body, it would tell you

    error: Do-block body must return bool, but returns () here. Perhaps
    you meant to write a `for`-loop?

This advice no longer applies as `for` loops are now for external
iterators. Delete this message outright and let it use the default error
message

    error: mismatched types: expected `bool` but found `()`

r? @thestinger

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