remove usage of IFD in nix packages

Lix builders have IFD disabled, and the changes needed to support this
are minimal.

We do need to do slightly more manual work to keep rust-toolchain.toml
in sync with flake.nix, but it's not significant.

flake.nix

@@ -8,6 +8,12 @@
     flake-utils.url = "github:numtide/flake-utils?ref=main";
     nix-filter.url = "github:numtide/nix-filter?ref=main";
     nixpkgs.url = "github:NixOS/nixpkgs?ref=nixos-unstable";
+
+    rust-manifest = {
+      # Keep version in sync with rust-toolchain.toml
+      url = "https://static.rust-lang.org/dist/channel-rust-1.78.0.toml";
+      flake = false;
+    };
   };
 
   outputs = inputs:
@@ -29,16 +35,27 @@
         shell = self.callPackage ./nix/shell.nix {};
 
         # The Rust toolchain to use
-        toolchain = inputs
-          .fenix
-          .packages
-          .${pkgs.pkgsBuildHost.system}
-          .fromToolchainFile {
-            file = ./rust-toolchain.toml;
-
-            # See also `rust-toolchain.toml`
-            sha256 = "sha256-opUgs6ckUQCyDxcB9Wy51pqhd0MPGHUVbwRKKPGiwZU=";
-          };
+        # Using fromManifestFile and parsing the toolchain file with importTOML
+        # instead of fromToolchainFile to avoid IFD
+        toolchain = let
+          toolchainFile = pkgs.lib.importTOML ./rust-toolchain.toml;
+          defaultProfileComponents = [
+            "rustc"
+            "cargo"
+            "rust-docs"
+            "rustfmt"
+            "clippy"
+          ];
+          components = defaultProfileComponents ++
+            toolchainFile.toolchain.components;
+          targets = toolchainFile.toolchain.targets;
+          fenix = inputs.fenix.packages.${pkgs.pkgsBuildHost.system};
+        in
+          fenix.combine (builtins.map
+            (target:
+              (fenix.targets.${target}.fromManifestFile inputs.rust-manifest)
+              .withComponents components)
+            targets);
       });
     in
     inputs.flake-utils.lib.eachDefaultSystem (system: