tracing-core: relax get_default's argument to FnOnce
#2292
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This change increases the flexibility of `get_default`. It was already the case that `get_default`'s argument was only invoked at-most once. However, the compiler could not prove this, and thus required that the argument be `FnMut`. This commit introduces `UnsafeFnMut`, a type that wraps an `FnOnce` and provides an unsafe interface for invoking it in cases where the compiler cannot otherwise prove that it's only invoked at-most once.
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Motivation
This change increases the flexibility of
get_default, by relaxing the bound on its argument fromFnMuttoFnOnce.Solution
It was already the case that
get_default's argument was only invoked at-most once. However, the compiler could not prove this, and thus required that the argument beFnMut. This commit introducesUnsafeFnMut, a type that wraps anFnOnceand provides an unsafe interface for invoking it in cases where the compiler cannot otherwise prove that it's only invoked at-most once.