[MacPorts] #60404: rust: use copy, not system cp -r

[MacPorts]

#60404: rust: use copy, not system cp -r
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  Reporter:  ryandesign  |      Owner:  g5pw
      Type:  defect      |     Status:  assigned
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:  2.6.2
Resolution:              |   Keywords:
      Port:  rust        |
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Comment (by herbygillot):

 So I looked at this while looking at Rust 1.50.0 - cp -R works better than
 copy for this case because the source tarballs are all being extracted to
 the same destination and have overlapping directory paths, even though the
 contents are automatically different.

 TCL copy refuses to copy over existing directories even with -force:

 From the TCL file copy documentation:
 > Trying to overwrite a non-empty directory, overwrite a directory with a
 file, or overwrite a file with a directory will all result in errors even
 if -force was specified.

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Ticket URL: <https://trac.macports.org/ticket/60404#comment:1>
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