tempdir() does not respect TMPDIR

It is too late to set TMPDIR in .Renviron.  It is one of the
environment variables that has to be set prior to launching R.  From
help("tempfile", package = "base"):

The environment variables TMPDIR, TMP and TEMP are checked in turn and
the first found which points to a writable directory is used: if none
succeeds ?/tmp? is used. The path should not contain spaces. **Note
that setting any of these environment variables in the R session has
no effect on tempdir(): the per-session temporary directory is created
before the interpreter is started.**

/Henrik

Hi there,

When I started R by double clicking on Rgui icon (I am on Windows), the
tempdir() returned the tmpdir in the directory I set in .Renviron. If I
started R by double clicking on a *.RData file, the tempdir() return the
tmpdir in the directory setting by Windows system. I don't know whether
it's designed.

 > sessionInfo()

R version 4.0.2 (2020-06-22)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 18363)
...

Best,
Jinsong

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