NLL: Invalid "cannot borrow as mutable more than once" error related to array of mutable references

[lqd]

Found while triaging the nll-1 crater run, extracted from fscm-0.1.1.

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Accessing the first element in the array generates an error about the second element borrowing _ more than once at a time. Mutating the second element compiles fine though.

[matthewjasper]

This looks like it was fixed by #51894

[lqd]

Confirmed on the latest nightly