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From: Jesper Harder <harder@myrealbox.com>
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Subject: Re: decode-coding-string and consing
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"Eli Zaretskii" <eliz@is.elta.co.il> writes:

>> ,----[ C-h f decode-coding-string RET ]
>> | decode-coding-string is a built-in function.
>> | (decode-coding-string STRING CODING-SYSTEM &optional NOCOPY)
>> | 
>> | Decode STRING which is encoded in CODING-SYSTEM, and return the result.
>> | Optional arg NOCOPY non-nil means it is OK to return STRING itself
>> | if the decoding operation is trivial.
>> `----
>> 
>> Under what conditions is the decoding operation "trivial"?
>
> If CODING-SYSTEM is nil or such that no encoding is necessary at all.

But I don't understand why any encoding is needed at all in:

    (decode-coding-string "abc" 'us-ascii t)

The input string and the output string are identical.  Why is it
necessary to cons a new string here?