fix(pkgs/jolt): Make cargo command select the correct binary depending on variable

Author: Syndamia

packages/all-packages.nix

@@ -201,8 +201,8 @@
         inherit corepack-shims;
 
         zkwasm = callPackage ./zkwasm/default.nix args-zkVM;
-        jolt = callPackage ./jolt/default.nix args-zkVM;
         jolt-guest-rust = callPackage ./jolt-guest-rust/default.nix args-zkVM-rust;
+        jolt = callPackage ./jolt/default.nix (args-zkVM // { inherit jolt-guest-rust; });
         zkm = callPackage ./zkm/default.nix args-zkVM;
         nexus = callPackage ./nexus/default.nix args-zkVM;
         sp1-rust = callPackage ./sp1-rust/default.nix args-zkVM-rust;

packages/jolt/default.nix

@@ -5,6 +5,7 @@
   installSourceAndCargo,
   pkg-config,
   openssl,
+  jolt-guest-rust,
   ...
 }:
 let
@@ -49,5 +50,21 @@ in
         sed -i '44,46d' jolt-core/src/host/toolchain.rs
       '';
 
+      # Different toolchain is used when guest has std features
+      # https://github.com/a16z/jolt/blob/fa45507aaddb1815bafd54332e4b14173a7f8699/jolt-core/src/host/mod.rs#L132-L134
+      postInstall = ''
+        rm $out/bin/rustc
+        cat <<EOF > $out/bin/rustc
+        #!/bin/sh
+        if [ -z "\''${RUSTUP_TOOLCHAIN+x}" ]
+        then
+            ${jolt-guest-rust}/rust/build/host/stage2/bin/rustc \$@
+        else
+            ${rust-toolchain}/bin/rustc \$@
+        fi
+        EOF
+        chmod +x $out/bin/rustc
+      '';
+
       doCheck = false;
     })