feat: add a depends only devshell

[josibake]

Add a depends devshell. This shell only includes packages needed for doing a depends build and some basic debugging tools.

This needs to be a separate shell because depends building will fail if there are system packages installed. Conceptually, its also cleaner to only include the necessary dependencies for a depends build since this is precisely the scenario in which depends is useful.

Closes #19

[josibake]

Note: will need to also update the CI depends job once this merges to use the new shell, but I didn't want to mix CI changes in this one and in theory this new one shouldn't break the old one.

[willcl-ark]

This shim is pretty neat conceptually, but personally I'd make it a bit more "nixxy", like this:

diff --git a/flake.nix b/flake.nix
index 7201ec9..8c5a88a 100644
--- a/flake.nix
+++ b/flake.nix
@@ -55,6 +55,20 @@
           linuxPackages.bpftrace
         ];
 
+      # Alias gcc/g++ to clang/clang++
+      clangShim = pkgs.stdenv.mkDerivation {
+        name = "clang-shim";
+        buildInputs = [llvmTools.clang];
+        phases = ["installPhase"];
+        installPhase = ''
+          mkdir -p $out/bin
+          ln -s ${llvmTools.clang}/bin/clang $out/bin/gcc
+          ln -s ${llvmTools.clang}/bin/clang++ $out/bin/g++
+          ln -s ${llvmTools.clang}/bin/clang $out/bin/clang
+          ln -s ${llvmTools.clang}/bin/clang++ $out/bin/clang++
+        '';
+      };
+
       # Will only exist in the build environment and includes everything needed
       # for a depends build
       dependsNativeBuildInputs = with pkgs;
@@ -64,8 +78,8 @@
           cmake
           curlMinimal
           llvmTools.bintools
-          llvmTools.clang
           llvmTools.clang-tools
+          clangShim
           ninja
           pkg-config
           xz
@@ -112,9 +126,6 @@
 
           inherit (env) CMAKE_GENERATOR LD_LIBRARY_PATH LOCALE_ARCHIVE;
         };
-        # TODO: this could be ported to a gcc shell in the future to bring this closer in line
-        # with our guix depends build. This would allow us to remove the shellHook hack that shims
-        # in clang to masquerade as gcc
         depends = pkgs.mkShellNoCC {
           nativeBuildInputs = dependsNativeBuildInputs;
           buildInputs = usdtPkgs;
@@ -123,19 +134,7 @@
             # build system, so we unset this in the depends devshell
             unset SOURCE_DATE_EPOCH
 
-            # its not super easy to override compiler defaults in
-            # depends, so as a hack we just shim in clang and pretend
-            # its gcc. this is done in a tmp dir to avoid polluting the
-            # users shell and is cleaned up when the shell is exited.
-            TMP_GCC_SHIM=$(mktemp -d "$TMPDIR/clang-shim.XXXXXX")
-
-            ln -sf $(command -v clang++) "$TMP_GCC_SHIM/g++"
-            ln -sf $(command -v clang) "$TMP_GCC_SHIM/gcc"
-
-            export PATH="$TMP_GCC_SHIM:$PATH"
-            echo "Using fake gcc/g++ -> clang in: $TMP_GCC_SHIM"
-
-            trap "rm -rf $TMP_GCC_SHIM" EXIT
+            echo "gcc/g++ are shimmed to llvm tools"
           '';
           inherit (env) CMAKE_GENERATOR LOCALE_ARCHIVE;
         };

LMK what you think?

[willcl-ark]

Did a CI run using the shell in https://github.com/bitcoin-dev-tools/bix/actions/runs/15854794287/job/44697246541?pr=33

Feel free to cherry-pick that commit here if you like

[josibake]

Thanks for the review, @willcl-ark , will update tomorrow! I much prefer your clangShim approach, will go with that instead. Alternatively, since we have separate devshells, maybe we just make the devShell.depends use the same gcc stdenv as our current depends build? That would at least make this dev environment one step close to mirroring our guix depends builds.

Also fine to go with this one for now and we can work on squaring up with guix builds as a separate task.

[willcl-ark]

Well in my testing the shim seemed to break nixpkgs builds: https://github.com/bitcoin-dev-tools/bix/actions/runs/15859413754/job/44712638536?pr=33

but depends build worked ok: https://github.com/bitcoin-dev-tools/bix/actions/runs/15859413749/job/44712638535?pr=33

Not sure what happened. I can perhaps look more closely later, but just a heads-up

[josibake]

Updated to use clangShim. It wasn't clear to me what was going on with the CI failure you linked, so if the CI here fails I'll dig in and try to determine whats going on.