flake: fix rust overlay unpack

fix-rust-overlay-unpack.diff

@@ -0,0 +1,29 @@
+diff --git a/lib/rust-bin.nix b/lib/rust-bin.nix
+index 1f5de03f..54ba8d29 100644
+--- a/lib/rust-bin.nix
++++ b/lib/rust-bin.nix
+@@ -132,9 +132,22 @@ let
+       url' = replaceStrings [" "] ["%20"] url; # This is required or download will fail.
+       # Filter names like `llvm-tools-1.34.2 (6c2484dc3 2019-05-13)-aarch64-unknown-linux-gnu.tar.xz`
+       matchParenPart = match ".*/([^ /]*) [(][^)]*[)](.*)" url;
+-      name = if matchParenPart == null then "" else (elemAt matchParenPart 0) + (elemAt matchParenPart 1);
+     in
+-      fetchurl { inherit name sha256; url = url'; };
++     # NB: This seems verbose, but we do not know the default special-cased `name` is.
++     # It used to be "", but changed to `null` later. We also do not know the exact
++     # revision of nixpkgs we are currently using.
++     # See: <http://github.com/NixOS/nixpkgs/commit/1ec0227cc062251c36140ef1e7c37b1cd1b370f1>
++     if matchParenPart != null then
++       fetchurl {
++         name = (elemAt matchParenPart 0) + (elemAt matchParenPart 1);
++         inherit sha256;
++         url = url';
++       }
++     else
++       fetchurl {
++         inherit sha256;
++         url = url';
++      };
+ 
+   # Resolve final components to install from mozilla-overlay style `extensions`, `targets` and `targetExtensions`.
+   #

flake.nix

@@ -54,9 +54,15 @@
     src-migen,
     src-misoc,
   }: let
+    pkgs' = import nixpkgs { system = "x86_64-linux"; };
+    rust-overlay-patched = pkgs'.applyPatches {
+      name = "rust-overlay-patched";
+      src = rust-overlay;
+      patches = [ ./fix-rust-overlay-unpack.diff ];
+    };
     pkgs = import nixpkgs {
       system = "x86_64-linux";
-      overlays = [(import rust-overlay)];
+      overlays = [(import rust-overlay-patched)];
     };
     pkgs-aarch64 = import nixpkgs {system = "aarch64-linux";};