diff --git a/default.nix b/default.nix
index edaf9775271..e6a595beea6 100644
--- a/default.nix
+++ b/default.nix
@@ -32,9 +32,11 @@ let
     llvmPackages_12.bintools
     rustc-nightly
     cargo-nightly
-    xargo
     wasmtime
     rust-bindgen
+    python3
+  ] ++ pkgs.lib.optional pkgs.stdenv.isDarwin [
+    libiconv
   ];
 
   llvmEnv = ''
@@ -152,46 +154,97 @@ in
 rec {
   rts =
     let
-      # We run this on motoko-rts-tests, to get the union of all
-      # dependencies
-      rustDeps = nixpkgs.rustPlatform-nightly.fetchCargoTarball {
+      # Build Rust package cargo-vendor-tools
+      cargoVendorTools = nixpkgs.rustPlatform.buildRustPackage rec {
+        name = "cargo-vendor-tools";
+        src = ./rts/cargo-vendor-tools;
+        cargoSha256 = "0zi3fiq9sy6c9dv7fd2xc9lan85d16gfax47n6g6f5q5c1zb5r47";
+      };
+
+      # Path to vendor-rust-std-deps, provided by cargo-vendor-tools
+      vendorRustStdDeps = "${cargoVendorTools}/bin/vendor-rust-std-deps";
+
+      # SHA256 of Rust std deps
+      rustStdDepsHash = "0wxx8prh66i19vd5078iky6x5bzs6ppz7c1vbcyx9h4fg0f7pfj6";
+
+      # Vendor directory for Rust std deps
+      rustStdDeps = nixpkgs.stdenvNoCC.mkDerivation {
+        name = "rustc-std-deps";
+
+        nativeBuildInputs = with nixpkgs; [
+          curl
+        ];
+
+        buildCommand = ''
+          mkdir $out
+          cd $out
+          ${vendorRustStdDeps} ${nixpkgs.rustc-nightly} .
+        '';
+
+        outputHash = rustStdDepsHash;
+        outputHashAlgo = "sha256";
+        outputHashMode = "recursive";
+      };
+
+      # Vendor tarball of the RTS
+      rtsDeps = nixpkgs.rustPlatform.fetchCargoTarball {
         name = "motoko-rts-deps";
         src = subpath ./rts;
         sourceRoot = "rts/motoko-rts-tests";
-        sha256 = "0jyp3j8n5bj5cy1fd26d7h55zmc4v14qc2w8adxqwmsv5riqz41g";
+        sha256 = "129gfmn96vm7di903pxirg7zybl83q6nkwiqr3rsy7l1q8667kxx";
         copyLockfile = true;
       };
+
+      # Unpacked RTS deps
+      rtsDepsUnpacked = nixpkgs.stdenvNoCC.mkDerivation {
+        name = rtsDeps.name + "-unpacked";
+        buildCommand = ''
+          tar xf ${rtsDeps}
+          mv *.tar.gz $out
+        '';
+      };
+
+      # All dependencies needed to build the RTS, including Rust std deps, to
+      # allow `cargo -Zbuild-std`. (rust-lang/wg-cargo-std-aware#23)
+      allDeps = nixpkgs.stdenvNoCC.mkDerivation {
+          name = "merged-rust-deps";
+
+          buildCommand = ''
+            mkdir -p $out
+            cp -r ${rtsDepsUnpacked}/* $out/
+            cp -r ${rustStdDeps}/* $out/
+          '';
+        };
     in
 
     stdenv.mkDerivation {
       name = "moc-rts";
 
       src = subpath ./rts;
-      nativeBuildInputs = [ nixpkgs.makeWrapper nixpkgs.removeReferencesTo ];
+
+      nativeBuildInputs = [ nixpkgs.makeWrapper nixpkgs.removeReferencesTo nixpkgs.cacert ];
 
       buildInputs = rtsBuildInputs;
 
       preBuild = ''
-        export XARGO_HOME=$PWD/xargo-home
         export CARGO_HOME=$PWD/cargo-home
 
-        # this replicates logic from nixpkgs’ pkgs/build-support/rust/default.nix
+        # This replicates logic from nixpkgs’ pkgs/build-support/rust/default.nix
         mkdir -p $CARGO_HOME
-        echo "Using vendored sources from ${rustDeps}"
-        unpackFile ${rustDeps}
+        echo "Using vendored sources from ${rtsDeps}"
+        unpackFile ${allDeps}
         cat > $CARGO_HOME/config <<__END__
           [source."crates-io"]
           "replace-with" = "vendored-sources"
 
           [source."vendored-sources"]
-          "directory" = "$(stripHash ${rustDeps})"
+          "directory" = "$(stripHash ${allDeps})"
         __END__
 
         ${llvmEnv}
         export TOMMATHSRC=${nixpkgs.sources.libtommath}
         export MUSLSRC=${nixpkgs.sources.musl-wasi}/libc-top-half/musl
         export MUSL_WASI_SYSROOT=${musl-wasi-sysroot}
-
       '';
 
       doCheck = true;
@@ -206,11 +259,16 @@ rec {
         cp mo-rts-debug.wasm $out/rts
       '';
 
-      # This needs to be self-contained. Remove mention of
-      # nix path in debug message.
+      # This needs to be self-contained. Remove mention of nix path in debug
+      # message.
       preFixup = ''
-        remove-references-to -t ${nixpkgs.rustc-nightly} -t ${rustDeps} $out/rts/mo-rts.wasm $out/rts/mo-rts-debug.wasm
+        remove-references-to \
+          -t ${nixpkgs.rustc-nightly} \
+          -t ${rtsDeps} \
+          -t ${rustStdDeps} \
+          $out/rts/mo-rts.wasm $out/rts/mo-rts-debug.wasm
       '';
+
       allowedRequisites = [];
     };
 
@@ -440,7 +498,6 @@ rec {
 
   filecheck = nixpkgs.linkFarm "FileCheck"
     [ { name = "bin/FileCheck"; path = "${nixpkgs.llvm}/bin/FileCheck";} ];
-  inherit (nixpkgs) xargo;
 
   # gitMinimal is used by nix/gitSource.nix; building it here warms the nix cache
   inherit (nixpkgs) gitMinimal;
diff --git a/nix/default.nix b/nix/default.nix
index 7e68f71465e..3a64fed5524 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -46,7 +46,7 @@ let
         # Rust nightly
         (self: super: let
           moz_overlay = import self.sources.nixpkgs-mozilla self super;
-          rust-channel = moz_overlay.rustChannelOf { date = "2020-07-22"; channel = "nightly"; };
+          rust-channel = moz_overlay.rustChannelOf { date = "2021-10-25"; channel = "nightly"; };
         in rec {
           rustc-nightly = rust-channel.rust.override {
             targets = [
@@ -62,7 +62,6 @@ let
             rustc = rustc-nightly;
             cargo = cargo-nightly;
           };
-          xargo = self.callPackage ./xargo.nix {};
         })
 
         # wasm-profiler
diff --git a/nix/xargo.nix b/nix/xargo.nix
deleted file mode 100644
index c84995221b6..00000000000
--- a/nix/xargo.nix
+++ /dev/null
@@ -1,31 +0,0 @@
-# xargo is used to build motoko-rts for wasm32. We need to make a shared Wasm
-# library for the RTS (that's what moc-ld supports) but Rust ships wasm32
-# libraries (core and std) without PIC relocation model, so we use xargo to make
-# PIC versions of core and std.
-
-{ rustPlatform-nightly, fetchFromGitHub, lib }:
-
-rustPlatform-nightly.buildRustPackage rec {
-  name = "xargo";
-
-  src = fetchFromGitHub {
-    owner = "japaric";
-    repo = "${name}";
-    rev = "16035a7c401262824edcb87e1401fe4b05a5ccc0";
-    sha256 = "0m1dg7vwmmlpqp20p219gsm7zbnnii6lik6hc2vvfsdmnygf271l";
-    fetchSubmodules = true;
-  };
-
-  cargoSha256 = "171a7xm47qdcdd6n67plyvnsxp00hn6skx5vzxd8a2kmblfqn5gy";
-
-  doCheck = false;
-  USER = "nobody"; # for xargo tests (if we would run them)
-
-  meta = with lib; {
-    description = "The sysroot manager that lets you build and customize std";
-    homepage = "https://github.com/japaric/xargo";
-    license = licenses.mit;
-    maintainers = [];
-    platforms = platforms.unix;
-  };
-}
diff --git a/rts/Makefile b/rts/Makefile
index 415881dc19a..eb5fd2efac1 100644
--- a/rts/Makefile
+++ b/rts/Makefile
@@ -223,11 +223,11 @@ $(TOMMATH_BINDINGS_RS): | _build
 
 
 $(RTS_RUST_WASM_A): $(TOMMATH_BINDINGS_RS) $(RTS_RUST_FILES) $(RTS_CARGO_FILES) | _build/wasm
-	cd motoko-rts && xargo build --release --target=wasm32-unknown-emscripten
+	cd motoko-rts && cargo build --release --target=wasm32-unknown-emscripten -Zbuild-std=core,alloc
 	cp motoko-rts/target/wasm32-unknown-emscripten/release/libmotoko_rts.a $@
 
 $(RTS_RUST_DEBUG_WASM_A): $(TOMMATH_BINDINGS_RS) $(RTS_RUST_FILES) $(RTS_CARGO_FILES) | _build/wasm
-	cd motoko-rts && xargo build --target=wasm32-unknown-emscripten
+	cd motoko-rts && cargo build --target=wasm32-unknown-emscripten -Zbuild-std=core,alloc
 	cp motoko-rts/target/wasm32-unknown-emscripten/debug/libmotoko_rts.a $@
 
 #
@@ -286,5 +286,4 @@ clean:
 	  mo-rts-debug.wasm \
 	  motoko-rts/target \
 	  motoko-rts-tests/target \
-	  motoko-rts/xargo-home \
 	  motoko-rts/cargo-home
diff --git a/rts/README.md b/rts/README.md
index 3e464e8b6fd..d1ba8ffc1df 100644
--- a/rts/README.md
+++ b/rts/README.md
@@ -58,31 +58,36 @@ See `motoko-rts/src/bigint.rs` for the technical details.
 Rust build
 ----------
 
-The Rust parts are built from `motoko-rts`, using `xargo` and `cargo`.
+To build Motoko RTS in nix we need pre-fetch Rust dependencies. This works in
+`nix-build` by:
 
-To build this in nix, we need pre-fetch some dependencies (currently
-`compiler_builtins` and `libc`). This works in `nix-build` by:
-
- * Adding`compiler_builtins` as a dependency in `Cargo.toml` (even though not
-   needed), so that it shows up with a hash in `Cargo.lock`
-
- * Building a directory with vendored sources in `default.nix` (see `rustDeps`)
+ * Building a directory with vendored sources in `default.nix`
 
  * Configuring `cargo` to use that vendored directory (see `preBuild`)
 
 If you change dependencies (e.g. bump versions, add more crates),
 
- 1. Add them to `Cargo.toml`
- 2. Make sure that `Cargo.lock` is up to date
- 3. In `default.nix`, invalidate the `sha256` of `rustDeps` (e.g. change one
+ 1. Make sure that `motoko-rts-tests/Cargo.lock` is up to date. This can be
+    done by running `cargo build --target=wasm32-wasi` in `motoko-rts-tests/`
+    directory.
+ 2. In `default.nix`, invalidate the `sha256` of `rtsDeps` (e.g. change one
     character)
- 4. Run `nix-build -A rts`. You should get an error message about the actual
+ 3. Run `nix-build -A rts`. You should get an error message about the actual
     checksum.
- 5. Set that as `sha256` of `rustDeps` in `default.nix`
+ 4. Set that as `sha256` of `rtsDeps` in `default.nix`
 
 Warning: nix will happily use a stale version of the dependencies if you do not
 do step 3.
 
+**Updating rustc**:
+
+1. Update Rust version in `nix/default.nix`, in the line with
+   `moz_overlay.rustChannelOf { ... }`.
+2. Invalidate `rustStdDepsHash` in `default.nix`.
+3. Run `nix-build -A rts`. You should get an error message about the expected
+   value of `rustStdDepsHash`.
+4. Update `rustStdDepsHash` with the expected value in the error message.
+
 Running RTS tests
 -----------------
 
diff --git a/rts/cargo-vendor-tools/Cargo.lock b/rts/cargo-vendor-tools/Cargo.lock
new file mode 100644
index 00000000000..e372c4c585a
--- /dev/null
+++ b/rts/cargo-vendor-tools/Cargo.lock
@@ -0,0 +1,230 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "ansi_term"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ee49baf6cb617b853aa8d93bf420db2383fab46d314482ca2803b40d5fde979b"
+dependencies = [
+ "winapi",
+]
+
+[[package]]
+name = "atty"
+version = "0.2.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d9b39be18770d11421cdb1b9947a45dd3f37e93092cbf377614828a319d5fee8"
+dependencies = [
+ "hermit-abi",
+ "libc",
+ "winapi",
+]
+
+[[package]]
+name = "bitflags"
+version = "1.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bef38d45163c2f1dde094a7dfd33ccf595c92905c8f8f4fdc18d06fb1037718a"
+
+[[package]]
+name = "block-buffer"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4152116fd6e9dadb291ae18fc1ec3575ed6d84c29642d97890f4b4a3417297e4"
+dependencies = [
+ "generic-array",
+]
+
+[[package]]
+name = "cargo-vendor-tools"
+version = "0.1.0"
+dependencies = [
+ "clap",
+ "serde_json",
+ "sha2",
+ "toml",
+]
+
+[[package]]
+name = "cfg-if"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
+
+[[package]]
+name = "clap"
+version = "2.33.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "37e58ac78573c40708d45522f0d80fa2f01cc4f9b4e2bf749807255454312002"
+dependencies = [
+ "ansi_term",
+ "atty",
+ "bitflags",
+ "strsim",
+ "textwrap",
+ "unicode-width",
+ "vec_map",
+]
+
+[[package]]
+name = "cpufeatures"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "95059428f66df56b63431fdb4e1947ed2190586af5c5a8a8b71122bdf5a7f469"
+dependencies = [
+ "libc",
+]
+
+[[package]]
+name = "digest"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d3dd60d1080a57a05ab032377049e0591415d2b31afd7028356dbf3cc6dcb066"
+dependencies = [
+ "generic-array",
+]
+
+[[package]]
+name = "generic-array"
+version = "0.14.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "501466ecc8a30d1d3b7fc9229b122b2ce8ed6e9d9223f1138d4babb253e51817"
+dependencies = [
+ "typenum",
+ "version_check",
+]
+
+[[package]]
+name = "hermit-abi"
+version = "0.1.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "62b467343b94ba476dcb2500d242dadbb39557df889310ac77c5d99100aaac33"
+dependencies = [
+ "libc",
+]
+
+[[package]]
+name = "itoa"
+version = "0.4.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b71991ff56294aa922b450139ee08b3bfc70982c6b2c7562771375cf73542dd4"
+
+[[package]]
+name = "libc"
+version = "0.2.105"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "869d572136620d55835903746bcb5cdc54cb2851fd0aeec53220b4bb65ef3013"
+
+[[package]]
+name = "opaque-debug"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "624a8340c38c1b80fd549087862da4ba43e08858af025b236e509b6649fc13d5"
+
+[[package]]
+name = "ryu"
+version = "1.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "71d301d4193d031abdd79ff7e3dd721168a9572ef3fe51a1517aba235bd8f86e"
+
+[[package]]
+name = "serde"
+version = "1.0.130"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f12d06de37cf59146fbdecab66aa99f9fe4f78722e3607577a5375d66bd0c913"
+
+[[package]]
+name = "serde_json"
+version = "1.0.68"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0f690853975602e1bfe1ccbf50504d67174e3bcf340f23b5ea9992e0587a52d8"
+dependencies = [
+ "itoa",
+ "ryu",
+ "serde",
+]
+
+[[package]]
+name = "sha2"
+version = "0.9.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b69f9a4c9740d74c5baa3fd2e547f9525fa8088a8a958e0ca2409a514e33f5fa"
+dependencies = [
+ "block-buffer",
+ "cfg-if",
+ "cpufeatures",
+ "digest",
+ "opaque-debug",
+]
+
+[[package]]
+name = "strsim"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8ea5119cdb4c55b55d432abb513a0429384878c15dde60cc77b1c99de1a95a6a"
+
+[[package]]
+name = "textwrap"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d326610f408c7a4eb6f51c37c330e496b08506c9457c9d34287ecc38809fb060"
+dependencies = [
+ "unicode-width",
+]
+
+[[package]]
+name = "toml"
+version = "0.5.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a31142970826733df8241ef35dc040ef98c679ab14d7c3e54d827099b3acecaa"
+dependencies = [
+ "serde",
+]
+
+[[package]]
+name = "typenum"
+version = "1.14.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b63708a265f51345575b27fe43f9500ad611579e764c79edbc2037b1121959ec"
+
+[[package]]
+name = "unicode-width"
+version = "0.1.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3ed742d4ea2bd1176e236172c8429aaf54486e7ac098db29ffe6529e0ce50973"
+
+[[package]]
+name = "vec_map"
+version = "0.8.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f1bddf1187be692e79c5ffeab891132dfb0f236ed36a43c7ed39f1165ee20191"
+
+[[package]]
+name = "version_check"
+version = "0.9.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5fecdca9a5291cc2b8dcf7dc02453fee791a280f3743cb0905f8822ae463b3fe"
+
+[[package]]
+name = "winapi"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c839a674fcd7a98952e593242ea400abe93992746761e38641405d28b00f419"
+dependencies = [
+ "winapi-i686-pc-windows-gnu",
+ "winapi-x86_64-pc-windows-gnu",
+]
+
+[[package]]
+name = "winapi-i686-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"
+
+[[package]]
+name = "winapi-x86_64-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
diff --git a/rts/cargo-vendor-tools/Cargo.toml b/rts/cargo-vendor-tools/Cargo.toml
new file mode 100644
index 00000000000..7b63e2002cc
--- /dev/null
+++ b/rts/cargo-vendor-tools/Cargo.toml
@@ -0,0 +1,15 @@
+[package]
+name = "cargo-vendor-tools"
+version = "0.1.0"
+authors = ["dfinity <sdk@dfinity.org>"]
+edition = "2018"
+
+[[bin]]
+name = "vendor-rust-std-deps"
+path = "src/vendor_rust_std_deps.rs"
+
+[dependencies]
+clap = "2.33.3"
+serde_json = "1.0.64"
+sha2 = "0.9.5"
+toml = "0.5.8"
diff --git a/rts/cargo-vendor-tools/rustfmt.toml b/rts/cargo-vendor-tools/rustfmt.toml
new file mode 100644
index 00000000000..66aa257537b
--- /dev/null
+++ b/rts/cargo-vendor-tools/rustfmt.toml
@@ -0,0 +1 @@
+struct_lit_width = 40
diff --git a/rts/cargo-vendor-tools/src/vendor_rust_std_deps.rs b/rts/cargo-vendor-tools/src/vendor_rust_std_deps.rs
new file mode 100644
index 00000000000..3c3bf7e7848
--- /dev/null
+++ b/rts/cargo-vendor-tools/src/vendor_rust_std_deps.rs
@@ -0,0 +1,235 @@
+use std::io::Write;
+use std::path::{Path, PathBuf};
+use std::process::{Command, Output, Stdio};
+
+use clap::{App, Arg};
+use serde_json as json;
+use sha2::{Digest, Sha256};
+
+static DESCR: &str = "
+Given a Rust toolchain path and a vendor directory, this program vendors Rust std dependencies in
+the given vendor directory. This directory can then be used to build packages with `-Zbuild-std`
+parameters in nix.
+";
+
+fn main() {
+    let args = App::new("vendor-rust-std-deps")
+        .long_about(DESCR)
+        .arg(
+            Arg::with_name("rust_install_path")
+                .value_name("RUST_INSTALL_PATH")
+                .takes_value(true)
+                .required(true),
+        )
+        .arg(
+            Arg::with_name("vendor_dir")
+                .value_name("VENDOR_DIR")
+                .takes_value(true)
+                .required(true),
+        )
+        .get_matches();
+
+    let rust_install_path = args.value_of("rust_install_path").unwrap();
+    let vendor_dir = args.value_of("vendor_dir").unwrap();
+
+    let compiler_cargo_lock_path = format!(
+        "{}/{}",
+        rust_install_path, "lib/rustlib/src/rust/Cargo.lock"
+    );
+
+    println!("Reading compiler Cargo.lock ...");
+    let compiler_deps = cargo_lock_deps(&compiler_cargo_lock_path);
+
+    println!(
+        "Fetching and unpacking compiler dependencies in {} ...",
+        vendor_dir
+    );
+
+    let total_deps = compiler_deps.len();
+    for (dep_idx, Dep { url, name, tarball_checksum }) in compiler_deps.into_iter().enumerate() {
+        println!("  {} ({}/{})", name, dep_idx + 1, total_deps);
+
+        //
+        // Fetch the package tarball
+        //
+
+        // curl --location <url>
+        let Output { status, stdout: curl_stdout, stderr } = Command::new("curl")
+            .args(&["--location", &url, "--insecure"])
+            .output()
+            .unwrap();
+
+        if !status.success() {
+            panic!(
+                "`curl --location {}` returned {:?}:\n{}",
+                url,
+                status.code(),
+                String::from_utf8_lossy(&stderr)
+            );
+        }
+
+        let package_vendor_dir = format!("{}/{}", vendor_dir, name);
+
+        //
+        // Create package vendor dir
+        //
+
+        std::fs::create_dir_all(&package_vendor_dir).unwrap();
+
+        //
+        // Unpack package
+        //
+
+        let mut tar_cmd = Command::new("tar")
+            .args(&["xz"])
+            .current_dir(&vendor_dir)
+            .stdin(Stdio::piped())
+            .stdout(Stdio::piped())
+            .stderr(Stdio::piped())
+            .spawn()
+            .unwrap();
+
+        let mut tar_stdin = tar_cmd.stdin.take().unwrap();
+        tar_stdin.write_all(&curl_stdout).unwrap();
+        drop(tar_stdin);
+
+        let Output { status, stdout: _, stderr } = tar_cmd.wait_with_output().unwrap();
+
+        if !status.success() {
+            panic!(
+                "`tar xz` returned {:?}:\n{}",
+                status.code(),
+                String::from_utf8_lossy(&stderr)
+            );
+        }
+
+        //
+        // Generate file checksums
+        //
+
+        let mut file_checksums: json::map::Map<String, json::Value> = Default::default();
+        generate_file_checksums(
+            &package_vendor_dir,
+            PathBuf::from(&package_vendor_dir).as_ref(),
+            &mut file_checksums,
+        );
+
+        let mut json_map = json::map::Map::new();
+        json_map.insert("files".to_owned(), json::Value::Object(file_checksums));
+        json_map.insert(
+            "package".to_owned(),
+            json::Value::String(tarball_checksum.to_owned()),
+        );
+
+        let mut checksum_file =
+            std::fs::File::create(format!("{}/.cargo-checksum.json", package_vendor_dir)).unwrap();
+        json::ser::to_writer_pretty(&mut checksum_file, &json_map).unwrap();
+    }
+}
+
+#[derive(Debug)]
+struct Dep {
+    /// URL to fetch the package tarball
+    url: String,
+
+    /// <package-name>-<version>
+    name: String,
+
+    /// Expected checksum (SHA256) of the tarball
+    tarball_checksum: String,
+}
+
+fn cargo_lock_deps(lock_file_path: &str) -> Vec<Dep> {
+    let mut deps = vec![];
+
+    let lock_file = std::fs::read_to_string(lock_file_path).unwrap();
+
+    let toml_value: toml::value::Value = toml::de::from_str(&lock_file).unwrap();
+
+    let packages = toml_value.as_table().unwrap().get("package").unwrap();
+
+    for package in packages.as_array().unwrap() {
+        let package_tbl = package.as_table().unwrap();
+        if let Some(source) = package_tbl.get("source") {
+            if source.as_str().unwrap() == "registry+https://github.com/rust-lang/crates.io-index" {
+                let name = package_tbl.get("name").unwrap().as_str().unwrap();
+                let version = package_tbl.get("version").unwrap().as_str().unwrap();
+                let checksum = package_tbl.get("checksum").unwrap().as_str().unwrap();
+                deps.push(Dep {
+                    url: format!(
+                        "https://crates.io/api/v1/crates/{}/{}/download",
+                        name, version
+                    ),
+                    name: format!("{}-{}", name, version),
+                    tarball_checksum: checksum.to_owned(),
+                });
+            }
+        }
+    }
+
+    deps
+}
+
+static IGNORED_FILES: [&str; 7] = [
+    ".",
+    "..",
+    ".gitattributes",
+    ".gitignore",
+    ".cargo-ok",
+    ".cargo-checksum.json",
+    ".cargo_vcs_info.json",
+];
+
+fn generate_file_checksums(
+    root: &str,
+    dir: &Path,
+    file_checksums: &mut json::map::Map<String, json::Value>,
+) {
+    for entry in std::fs::read_dir(dir).unwrap() {
+        let entry = entry.unwrap();
+        let file_name = entry.file_name();
+
+        if IGNORED_FILES
+            .iter()
+            .any(|ignored_file| *ignored_file == file_name)
+        {
+            continue;
+        }
+
+        match entry.file_type() {
+            Err(err) => {
+                eprintln!(
+                    "Error while getting type of file {:?}: {:?}",
+                    file_name, err
+                );
+            }
+            Ok(file_type) => {
+                if file_type.is_dir() {
+                    let mut dir_path = PathBuf::new();
+                    dir_path.push(dir);
+                    dir_path.push(entry.file_name());
+                    generate_file_checksums(root, &dir_path, file_checksums);
+                } else {
+                    let mut file_path = PathBuf::new();
+                    file_path.push(dir);
+                    file_path.push(entry.file_name());
+
+                    let rel_file_path = file_path.strip_prefix(root).unwrap();
+
+                    let file_contents = std::fs::read(&file_path).unwrap();
+
+                    let mut hasher = Sha256::new();
+                    hasher.update(&file_contents);
+                    let hash = hasher.finalize();
+                    let hash_str = format!("{:x}", hash);
+
+                    let old = file_checksums.insert(
+                        rel_file_path.to_string_lossy().to_string(),
+                        json::Value::String(hash_str),
+                    );
+                    assert!(old.is_none());
+                }
+            }
+        }
+    }
+}
diff --git a/rts/motoko-rts-tests/Cargo.lock b/rts/motoko-rts-tests/Cargo.lock
index 6b60a63f4e4..834253a1868 100644
--- a/rts/motoko-rts-tests/Cargo.lock
+++ b/rts/motoko-rts-tests/Cargo.lock
@@ -1,5 +1,7 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+version = 3
+
 [[package]]
 name = "autocfg"
 version = "1.0.1"
@@ -24,12 +26,6 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
-[[package]]
-name = "compiler_builtins"
-version = "0.1.39"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3748f82c7d366a0b4950257d19db685d4958d2fa27c6d164a3f069fec42b748b"
-
 [[package]]
 name = "fxhash"
 version = "0.2.1"
@@ -66,7 +62,6 @@ checksum = "1482821306169ec4d07f6aca392a4681f66c75c9918aa49641a2595db64053cb"
 name = "motoko-rts"
 version = "0.1.0"
 dependencies = [
- "compiler_builtins",
  "libc",
  "motoko-rts-macros",
  "static_assertions",
diff --git a/rts/motoko-rts-tests/src/main.rs b/rts/motoko-rts-tests/src/main.rs
index f18df6ac569..9f77525e034 100644
--- a/rts/motoko-rts-tests/src/main.rs
+++ b/rts/motoko-rts-tests/src/main.rs
@@ -1,4 +1,4 @@
-#![feature(ptr_offset_from, map_first_last, clamp)]
+#![feature(map_first_last)]
 
 mod bigint;
 mod bitmap;
diff --git a/rts/motoko-rts/Cargo.lock b/rts/motoko-rts/Cargo.lock
index 1e4ffb001c7..dc5205045cf 100644
--- a/rts/motoko-rts/Cargo.lock
+++ b/rts/motoko-rts/Cargo.lock
@@ -1,10 +1,6 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
-[[package]]
-name = "compiler_builtins"
-version = "0.1.39"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3748f82c7d366a0b4950257d19db685d4958d2fa27c6d164a3f069fec42b748b"
+version = 3
 
 [[package]]
 name = "libc"
@@ -16,7 +12,6 @@ checksum = "1482821306169ec4d07f6aca392a4681f66c75c9918aa49641a2595db64053cb"
 name = "motoko-rts"
 version = "0.1.0"
 dependencies = [
- "compiler_builtins",
  "libc",
  "motoko-rts-macros",
  "static_assertions",
diff --git a/rts/motoko-rts/Cargo.toml b/rts/motoko-rts/Cargo.toml
index 575b327b100..b7eaa74f564 100644
--- a/rts/motoko-rts/Cargo.toml
+++ b/rts/motoko-rts/Cargo.toml
@@ -23,20 +23,6 @@ libc = { version = "0.2.81", default_features = false }
 motoko-rts-macros = { path = "../motoko-rts-macros" }
 static_assertions = "1.1.0"
 
-# Added here so that it ends up in Cargo.lock, so that nix will pre-fetch it
-[dependencies.compiler_builtins]
-version = "0.1.39"
-# Without this feature we get dozens of duplicate symbol errors when generating
-# the final shared .wasm file:
-#
-#     wasm-ld: error: duplicate symbol: __multi3
-#     >>> defined in _build/wasm/libmotoko_rts.a(compiler_builtins-d709bd899857aa61.compiler_builtins.3abndchk-cgu.0.rcgu.o)
-#     >>> defined in _build/wasm/libmotoko_rts.a(compiler_builtins-06d1ead628e1f468.compiler_builtins.6moz1ltd-cgu.0.rcgu.o)
-#
-# It seems like we're linking multiple versions of compiler_builtins in the same
-# shared library, which we should fix at some point. TODO
-features = ["mangled-names"]
-
 [profile.dev]
 panic = "abort"
 
diff --git a/rts/motoko-rts/Xargo.toml b/rts/motoko-rts/Xargo.toml
deleted file mode 100644
index d3910abede1..00000000000
--- a/rts/motoko-rts/Xargo.toml
+++ /dev/null
@@ -1,6 +0,0 @@
-[dependencies.core]
-stage = 0
-
-[dependencies.compiler_builtins]
-stage = 1
-version = "0.1.39"
diff --git a/rts/motoko-rts/native/Cargo.toml b/rts/motoko-rts/native/Cargo.toml
index 9c8913e32ff..5e22f655e31 100644
--- a/rts/motoko-rts/native/Cargo.toml
+++ b/rts/motoko-rts/native/Cargo.toml
@@ -13,10 +13,6 @@ libc = { version = "0.2.73", default_features = false }
 motoko-rts-macros = { path = "../../motoko-rts-macros" }
 static_assertions = "1.1.0"
 
-[dependencies.compiler_builtins]
-version = "0.1.39"
-features = ["mangled-names"]
-
 [profile.dev]
 panic = "abort"
 
diff --git a/rts/motoko-rts/src/lib.rs b/rts/motoko-rts/src/lib.rs
index df68ab06b74..e036b0d3096 100644
--- a/rts/motoko-rts/src/lib.rs
+++ b/rts/motoko-rts/src/lib.rs
@@ -2,13 +2,7 @@
 
 #![no_std]
 // TODO (osa): Some of these are stabilized, we need to update rustc
-#![feature(
-    arbitrary_self_types,
-    assoc_char_funcs,
-    core_intrinsics,
-    panic_info_message,
-    ptr_offset_from
-)]
+#![feature(arbitrary_self_types, core_intrinsics, panic_info_message)]
 
 #[macro_use]
 mod print;