feat: (WIP) Rudimentary, WIP attempt at building risc0's fork of rustc

flake.nix

@@ -23,6 +23,7 @@
       };
 
       packages = perSystemPkgs (pkgs: {
+        risc0-rust = pkgs.risc0-rust;
         rust-bin-risc0-latest = pkgs.rust-bin.risc0.latest;
         default = inputs.self.packages.${pkgs.system}.rust-bin-risc0-latest;
       });

overlay.nix

@@ -39,4 +39,7 @@ in
         cargoExtraArgs = (args.cargoExtraArgs or "") + " --target riscv32im-risc0-zkvm-elf";
       }
     );
+
+  # NOTE: Test package for building risc0's rust fork from source.
+  risc0-rust = prev.callPackage ./pkgs/rust.nix { };
 }

pkgs/rust.nix

@@ -0,0 +1,52 @@
+# risc0-rust.nix
+{
+  lib,
+  rustc,
+  fetchFromGitHub,
+}:
+
+let
+  version = "1.85.0";
+
+  risc0Src = fetchFromGitHub {
+    owner = "risc0";
+    repo = "rust";
+    rev = "r0.${version}";
+    fetchSubmodules = true;
+    sha256 = "sha256-uxo6Sx9I+x9Gi9/JttRLIzAa+WzTmJ5DTempRwrsp/U=";
+  };
+
+in
+rustc.unwrapped.overrideAttrs (oldAttrs: {
+  pname = "risc0-rustc";
+  src = risc0Src // {
+    # Override the passthru on the source to indicate it's not a release tarball
+    passthru = (risc0Src.passthru or {}) // {
+      isReleaseTarball = false;
+    };
+  };
+  inherit version;
+
+  # Override configure flags to add RISC Zero target
+  configureFlags =
+    let
+      # Remove existing --target flag and add our own with RISC Zero target included
+      flagsWithoutTarget = builtins.filter
+        (flag: !(lib.hasPrefix "--target=" flag))
+        oldAttrs.configureFlags;
+
+      # Get the existing target list and add our target
+      existingTargets = "x86_64-unknown-linux-gnu,wasm32-unknown-unknown,wasm32v1-none,bpfel-unknown-none,bpfeb-unknown-none";
+      newTargets = "${existingTargets},riscv32im-risc0-zkvm-elf";
+    in
+    flagsWithoutTarget ++ [
+      "--target=${newTargets}"
+    ];
+
+  # Add RISC Zero environment variables
+  preBuild =
+    (oldAttrs.preBuild or "")
+    + ''
+      export CARGO_TARGET_RISCV32IM_RISC0_ZKVM_ELF_RUSTFLAGS="-Cpasses=lower-atomic"
+    '';
+})