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59abdb6a7e
Based on discussion from https://internals.rust-lang.org/t/can-the-standard-library-shrink-option-file/12768, the file descriptor -1 is chosen based on the POSIX API designs that use it as a sentinel to report errors. A bigger niche could've been chosen, particularly on Linux, but would not necessarily be portable. This PR also adds a test case to ensure that the -1 niche (which is kind of hacky and has no obvious test case) works correctly. It requires the "upper" bound, which is actually -1, to be expressed in two's complement.