Rust gcc-builder fails with isl error when trying to build latest gcc on Debian 13/trixie

[N7DR]

While waiting for issue #23 to be fixed, I tried the rust version, gcc-builder (on Debian 13/trixie). I believe that I followed the directions correctly.

I executed:
gcc-builder --latest --prefix $HOME/gcc --jobs $(nproc) --build-dir /tmp/gcc-build

and, after everything was downloaded, received:

checking for isl 0.15 or later... no
required isl version is 0.15 or later
configure: error: Unable to find a usable isl. See config.log for details.

I don't know what isl is, or where to find it. apt doesn't seem to find it (there is no Debian isl package), so I think I'm stuck.

[N7DR]

Looking around the internet, I saw that perhaps isl is provided by the libisl package. But that is already installed (as is libisl-dev). The current version of libisl installed on trixie from the debian stable repository is 0.27 (therefore > 0.15).

[N7DR]

In case this bug was something to do with the fact that it was happening an a system that had been upgraded from bookworm to trixie, I built a virtualbox VM from scratch and installed the latest trixie on that, and then followed the instructions to install the rust version of gcc-builder. Unfortunately I ended up with exactly the same error when I tried to execute the newly-installed gcc-builder to create and install the latest version of gcc (version 15.2).

In a way, I suppose it's good that it behaved exactly the same way as the physical machine that had been upgraded to trixie. But it does mean that I am still stuck, with no way forward :-(

FWIW, here is the config.log from the VM. It doesn't tell me anything useful, but perhaps it's of some help to you.

config.log

[N7DR]

So... although it's MUCH more complicated that simply using the rust gcc-builder that fails, I have found that blindly following the instructions located at:
https://tutorialforlinux.com/2025/08/18/how-to-install-gcc-15-from-source-on-debian-trixie-installation/3/
does not complain with the isl error, and does appear to execute to completion without error on a pristine virtual machine running trixie.

That, I fervently hope, is a helpful clue as to what's going wrong with gcc-builder.

[N7DR]

Oh, I forget to mention that there is a minor error in the instructions at the tutorialforlinux.com site:

the step (for c, c++, fortran, objc, obj-c++ and go):
..configure --enable-languages=c,c++,fortran,objc,obj-c++,go --prefix="/usr/local"

should be:
../configure --enable-languages=c,c++,fortran,objc,obj-c++,go --prefix="/usr/local"

I just built C and C++. Probably all I really needed was C++, since that's all I ever write in, but I thought it might be safer to add C.

[N7DR]

I can confirm that the instructions at https://tutorialforlinux.com/2025/08/18/how-to-install-gcc-15-from-source-on-debian-trixie-installation/3/ do seem to work correctly: I now have g++ 15.2 running on trixie, building a sizeable project in C++-26 mode, and all seems as it should. (The code uses some C++-26 features, and will not build on the default g++ compiler included with trixie.)

So I hope that you'll be able to compare your bash build-gcc.sh and your rust gcc-builder with the instructions at tutorialforlinux.com and spot (and fix) why your scripts are currently failing.

Let me know if I can give you any further info.