From type/IO/Handle

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multi method slurp-rest(IO::Handle:D: :$bin! --> Buf)
multi method slurp-rest(IO::Handle:D: :$enc --> Str)

DEPRECATION NOTICE: this method is deprecated in the 6.d version. Do not use it for new code, use Slurps instead.

Returns the remaining content of the file from the current file position (which may have been set by previous reads or by seek.) If the adverb :bin is provided a Buf will be returned; otherwise the return will be a Str with the optional encoding :enc.