Hacking on the Fil-C compiler without Nix rebuilds

nix build .#filcc builds clang (filc0) from scratch whenever its source changes. For compiler work, use the filc-llvm dev shell instead: it builds clang incrementally with ninja in a git worktree and gives you clang/clang++ wrappers that run that clang with the pinned Nix-built Fil-C runtime (libpizlo, glibc, libc++, headers).

# One-time: a sparse worktree of fil-c at the revision filnix pins.
nix develop .#filc-llvm -c filc-llvm-worktree ~/fil-c ~/fil-c-dev my-branch

export FILC_SRC=~/fil-c-dev              # default: ~/fil-c-dev
nix develop .#filc-llvm
filc-llvm-build                          # configure if needed, then ninja clang
filc-llvm-build clang opt llvm-dis       # any ninja targets
clang++ -std=c++20 test.cpp -o test      # dev clang + pinned runtime

The build directory is $FILC_DEV_LLVM (default $FILC_SRC/build-filnix). It uses the CMake options from compiler/filc0.nix, but links with lld and split DWARF so relinking clang after an edit takes seconds rather than minutes. filc-llvm-configure reruns CMake and forwards extra -D options.

The wrappers read $FILC_DEV_LLVM when they run and disable ccache. ccache keys on the wrapper, which stays the same when the dev clang changes. The runtime libraries were compiled by the pinned clang, which is fine for pass and CodeGen changes that keep the ABI.

filc-llvm starts from the official release. For fork experiments, use nix develop .#filc-llvm-staging instead, including for the worktree creation command. It uses the staging revision and pinned staging runtime. Worktree creation fetches from the selected pin's repository, not an assumed origin. Choose a separate worktree/build directory when working on both variants.

When a change works, commit it on the worktree branch and push it to the fork. Update only the staging pin; do not replace the ordinary release pin:

python3 scripts/update-filc-source-hashes.py --variant staging \
  --repo ~/fil-c --rev <commit>

Sources whose files did not change keep their hashes and are not rebuilt. A pin change that touches filc0-src rebuilds clang from scratch in Nix: its build tree (RelWithDebInfo with assertions) peaks around 165 GB, because the debug-info binaries are installed twice. Build such a pin on a machine with well over that much free space (swa, not igloo), and cover all the checks in one nix build so filc0 is built once. Pins that change only libpas-src or other runtime sources reuse the built filc0. Staging changes to compiler semantics, capability tracking, GC or runtime still need soundness review and targeted regressions. A successful package build alone is not that review. ABI-changing experiments must rebuild their runtime and libraries; the development compiler substitution deliberately does not do so.