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A general description is here.
Source package installation is by default ‘staged’: the package is installed into a temporary location under the final library directory and moved into place once the installation is complete. The benefit is that partially-installed packages are hidden from other R sessions.
The overall default is set by environment variable ‘R_INSTALL_STAGED’. ‘R CMD INSTALL’ has new options ‘--staged-install’ and ‘--no-staged-install’, and packages can use the ‘StagedInstall’ field in their ‘DESCRIPTION’ file to opt out.
The interpretation of source code with extension ‘.f’ is changing. Previously this denoted FORTRAN 77 code, but current compilers no longer have a FORTRAN 77 mode and interpret it as ‘fixed-form’ Fortran 90 (or later where supported) code. Extensions ‘.f90’ and ‘.f95’ continue to indicate ‘free-form’ Fortran code.
Legal FORTRAN 77 code is also legal fixed-form Fortran 9x; however this change legitimizes the use of later features, in particular to replace features marked ‘obsolescent’ in Fortran 95 which ‘gfortran’ 8.x warns about.
‘axTicks()’ no longer returns small “almost zero” numbers (in exponential format) instead of zero, fixing Ilario Gelmetti's PR#17534.