Adding sparse protocol

pkgs/build-support/rust/import-cargo-lock.nix

@@ -12,6 +12,8 @@
 
   # Additional registries to pull sources from
   #   { "https://<registry index URL>" = "https://<registry download URL>"; }
+  #   or if the registry is using the new sparse protocol
+  #   { "sparse+https://<registry download URL>" = "https://<registry download URL>"; }
   # where:
   # - "index URL" is the "index" value of the configuration entry for that registry
   #   https://doc.rust-lang.org/cargo/reference/registries.html#using-an-alternate-registry
@@ -117,7 +119,8 @@ let
       gitParts = parseGit pkg.source;
       registryIndexUrl = lib.removePrefix "registry+" pkg.source;
     in
-      if lib.hasPrefix "registry+" pkg.source && builtins.hasAttr registryIndexUrl registries then
+      if (lib.hasPrefix "registry+" pkg.source || lib.hasPrefix "sparse+" pkg.source)
+        && builtins.hasAttr registryIndexUrl registries then
       let
         crateTarball = fetchCrate pkg registries.${registryIndexUrl};
       in runCommand "${pkg.name}-${pkg.version}" {} ''

pkgs/build-support/rust/test/import-cargo-lock/basic-sparse/.cargo/config.toml

@@ -0,0 +1,5 @@
+[registries.sparse-crates-io]
+index = "sparse+https://index.crates.io/"
+
+[registries.crates-io]
+protocol = "git"

pkgs/build-support/rust/test/import-cargo-lock/basic-sparse/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "basic-sparse"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+rand = { version = "0.8", registry = "sparse-crates-io" }

pkgs/build-support/rust/test/import-cargo-lock/basic-sparse/default.nix

@@ -0,0 +1,33 @@
+{ lib, rustPlatform }:
+let
+  fs = lib.fileset;
+in
+rustPlatform.buildRustPackage {
+  pname = "basic-sparse";
+  version = "0.1.0";
+
+  src = fs.toSource {
+    root = ./.;
+    fileset = fs.unions [
+      ./.cargo/config.toml
+      ./Cargo.toml
+      ./Cargo.lock
+      ./src
+    ];
+  };
+
+  cargoLock = {
+    lockFile = ./Cargo.lock;
+    extraRegistries = {
+        "sparse+https://index.crates.io/" = "https://static.crates.io/crates";
+    };
+  };
+
+  doInstallCheck = true;
+  postConfigure = ''
+    cargo metadata --offline
+  '';
+  installCheckPhase = ''
+    $out/bin/basic-sparse
+  '';
+}

pkgs/build-support/rust/test/import-cargo-lock/basic-sparse/src/main.rs

@@ -0,0 +1,9 @@
+use rand::Rng;
+
+fn main() {
+    let mut rng = rand::thread_rng();
+
+    // Always draw zero :).
+    let roll: u8 = rng.gen_range(0..1);
+    assert_eq!(roll, 0);
+}