installworld and sources

[b. f.]

>Compiling a new kernel from source requires /usr/src to be populated, I
>understand that.
>
>The buildworld process for sure needs /usr/src. My question is , is
>/usr/src also used in the installworld process?
>
>Now I have never had to do this type of system RELEASE upgrade before,
>so I just don't know.

If you don't mean "installworld" literally, but instead you mean
installing pre-compiled binaries like those distributed on the FreeBSD
installation disk images, or those intended for use with
freebsd-update(8), then the answer is "no".  But if you mean it
literally -- you intend to run the "installworld" target -- then the
answer is yes, of course.  It is the makefiles in /usr/src, from
/usr/src/Makefile on down, that are responsible for telling make(1)
what to do during "installworld".

Even after you are done installing, you may still wish to keep part or
all of the system sources around, in case of emergencies (you need to
rebuild/re-install part of the base system), or for reference, or for
subsequent base system updates, or because /sys may be sym-linked to
the headers in /usr/src/sys.

b.