Make IO#set_encoding with binary external encoding use nil internal encoding

This was already the behavior when a single `'external:internal'`
encoding specifier string was passed.  This makes the behavior
consistent for the case where separate external and internal
encoding specifiers are provided.

While here, fix the IO#set_encoding method documentation to
state that either the first or second argument can be a string
with an encoding name, and describe the behavior when the
external encoding is binary.

Fixes [Bug #18899]

1 files changed, 13 insertions, 2 deletions

diff --git a/io.c b/io.c
index 2e51a46635..dbcfcf3fc0 100644
--- a/io.c
+++ b/io.c
@@ -11573,6 +11573,11 @@ io_encoding_set(rb_io_t *fptr, VALUE v1, VALUE v2, VALUE opt)
                 enc2 = NULL;
             }
         }
+        if (enc2 == rb_ascii8bit_encoding()) {
+            /* If external is ASCII-8BIT, no transcoding */
+            enc = enc2;
+            enc2 = NULL;
+        }
         SET_UNIVERSAL_NEWLINE_DECORATOR_IF_ENC2(enc2, ecflags);
         ecflags = rb_econv_prepare_options(opt, &ecopts, ecflags);
     }
@@ -13393,10 +13398,12 @@ rb_io_internal_encoding(VALUE io)
  *
  *  See {Encodings}[rdoc-ref:File@Encodings].
  *
- *  Argument +ext_enc+, if given, must be an Encoding object;
+ *  Argument +ext_enc+, if given, must be an Encoding object
+ *  or a String with the encoding name;
  *  it is assigned as the encoding for the stream.
  *
- *  Argument +int_enc+, if given, must be an Encoding object;
+ *  Argument +int_enc+, if given, must be an Encoding object
+ *  or a String with the encoding name;
  *  it is assigned as the encoding for the internal string.
  *
  *  Argument <tt>'ext_enc:int_enc'</tt>, if given, is a string
@@ -13404,6 +13411,10 @@ rb_io_internal_encoding(VALUE io)
  *  corresponding Encoding objects are assigned as the external
  *  and internal encodings for the stream.
  *
+ *  If the external encoding of a string is binary/ASCII-8BIT,
+ *  the internal encoding of the string is set to nil, since no
+ *  transcoding is needed.
+ *
  *  Optional keyword arguments +enc_opts+ specify
  *  {Encoding options}[rdoc-ref:encodings.rdoc@Encoding+Options].
  *