[Issue 20140] New: std.string.tr should not be decoding strings

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20140

          Issue ID: 20140
           Summary: std.string.tr should not be decoding strings
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: bugzilla at digitalmars.com

See: https://forum.dlang.org/post/mvkbemmgatrfbxnmjada@forum.dlang.org

In the implementation of std.string.tr the main loop is:

    foreach (dchar c; str)

meaning char and wchar strings are getting decoded. This leads to problems as
reported on the n.g. link.

The correct way to implement it is to just leave it in the encoding of `str`.
This should result in a significant speed improvement.

This leaves open what to do if `from` and `to` contain invalid UTF sequences if
the conversion of them is necessary. The most pragmatic solution is to reject
`from` and `to` arguments to `tr` that are not of the same UTF encoding as
`str`.

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