Issue #20416 has been updated by byroot (Jean Boussier). I git blamed this behavior all the way down to 2007, by @matz: https://github.com/ruby/ruby/commit/0ca7036682dad1caea4e7a4fe8fd908d94c878e8 and it seems deliberate ``` * io.c (read_all): should associate default external encoding. * io.c (io_read): should NOT associate default external encoding. ``` ---------------------------------------- Bug #20416: IO#read doesn't preserve buffer encoding if `maxlen = nil` https://bugs.ruby-lang.org/issues/20416#change-107862 * Author: andrykonchin (Andrew Konchin) * Status: Open * ruby -v: 3.3.0 * Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN ---------------------------------------- `IO#read(nil, buffer)` doesn't change buffer's encoding, but it does when `maxlen` parameter is specified: ```ruby buffer = "".encode(Encoding::ISO_8859_1) p File.open("test.rb").read(1000, buffer).encoding # => #<Encoding:ISO-8859-1> buffer = "".encode(Encoding::ISO_8859_1) p File.open("test.rb").read(nil, buffer).encoding # => #<Encoding:UTF-8> ``` I would expect changing buffer encoding in all the cases. -- https://bugs.ruby-lang.org/