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A general description is here.
‘capabilities("libxml")’ is now false.
The description of ‘capabilities("http/ftp")’ now reflects that it refers to the default method, no longer the internal one.
‘matrix(x, n, m)’ now warns in more cases where ‘length(x)’ differs from ‘n * m’, as suggested by Abby Spurdle and Wolfgang Huber in Feb 2021 on the R-devel mailing list.
This warning can be turned into an error by setting environment variable ‘_R_CHECK_MATRIX_DATA_’ to ‘TRUE’: ‘R CMD check --as-cran’ does so unless it is already set.
Facilities for accessing ‘ftp://’ sites are no longer tested (except _pro tem_ for ‘curlGetHeaders()’) as modern browsers have removed support.
The (non-default and deprecated) ‘method = "internal"’ for ‘download.file’ and ‘url’ no longer supports ‘ftp://’ URIs.
Similarly on Windows for ‘download.file(method = "wininet")’.
Setting ‘digits = 0’ in ‘format()’, ‘print.default()’ (and hence typically ‘print()’) or ‘options()’ is again invalid. Its behaviour was platform-dependent, and it is unclear what “zero significant digits” should mean (PR#18098).
The non-default ‘method = "internal"’ is deprecated for ‘http://’ and ‘ftp://’ URIs for both ‘download.file’ and ‘url’.
On Windows, ‘method = "wininet"’ is deprecated for ‘http://’, ‘https://’ and ‘ftp://’ URIs for both ‘download.file’ and ‘url’.
For ‘ftp://’ URIs the default method is now ‘"libcurl"’ if available (which it is on CRAN builds).
‘method = "wininet"’ remains the default for ‘http://’ and ‘https://’ URIs but if ‘libcurl’ is available, using ‘method = "libcurl"’ is preferred.
The character-classification functions used (by default) to replace the system ‘iswxxxxx’ functions on Windows, macOS and AIX have been updated to Unicode 13.0.0.
The character-width tables have been updated to include new assignments in Unicode 13.0.0. This included treating all control characters as having zero width.