tracing-core: relax get_default
's argument to FnOnce
#2292
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Motivation
This change increases the flexibility of
get_default
, by relaxing the bound on its argument fromFnMut
toFnOnce
.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 anFnOnce
and provides an unsafe interface for invoking it in cases where the compiler cannot otherwise prove that it's only invoked at-most once.