errors · 3.1the obvious first try, 497 MB later

oss-cad-suite has no nextpnr-xilinx

You want Yosys and nextpnr on a Mac. oss-cad-suite is YosysHQ's ready-made bundle with a macOS build, so it looks like the whole answer. You download it, unpack it, and there is no nextpnr-xilinx inside.

Why does it happen?

The bundle ships nextpnr for the architectures that live in the main nextpnr repository: ice40, ecp5, nexus and Gowin. Xilinx support lives in a separate project, openXC7/nextpnr-xilinx, and it is not part of the bundle. For the Basys3's Artix-7 you need that one, and Homebrew has no formula for it either.

What is the fix?

Take Yosys, Icarus Verilog and openFPGALoader from Homebrew, and build nextpnr-xilinx from source.

brew install yosys openfpgaloader icarus-verilog cmake ninja eigen pkg-config python@3.14

cd ~/fpga
git clone --recursive https://github.com/openXC7/nextpnr-xilinx.git
cd nextpnr-xilinx
git checkout 3fd78784c7788f93f276358edf5477221cc6c179
git submodule update --init --recursive
cmake -B build -G Ninja -DARCH=xilinx -DCMAKE_BUILD_TYPE=Release \
  -DUSE_OPENMP=OFF -DBUILD_GUI=OFF -DBUILD_PYTHON=OFF -DBUILD_TESTS=OFF
ninja -C build -j3 nextpnr-xilinx bbasm

The build takes about a minute and a half on an M2. Two flags in there each remove a wall of their own: -DUSE_OPENMP=OFF and --recursive, which brings the raw chip data for the chip database.

Sections 3.1 and 3.3 of the guide: Homebrew packages and nextpnr-xilinx.