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A general description is here.
The internal ‘untar()’ (as used by default by ‘R CMD INSTALL’) now knows about some ‘pax’ headers which ‘bsdtar’ (e.g., the default ‘tar’ for Mac OS >= 10.6) can incorrectly include in ‘tar’ files, and will skip them with a warning.
PCRE has been upgraded to version 8.21: as well as bug fixes and greater Perl compatibility, this adds a JIT pattern compiler, about which PCRE's news says ‘large performance benefits can be had in many situations’.
Function ‘compactPDF()’ in package ‘tools’ now takes the default for argument ‘gs_quality’ from environment variable ‘GS_QUALITY’: there is a new value ‘"none"’, the ultimate default, which prevents GhostScript being used in preference to ‘qpdf’ just because environment variable ‘R_GSCMD’ is set.