Optimize searching for cargo configuration files

* Rather than recursively look up all paths in a given project and then
  filtering for cargo-specific files, we now use our own customized
  traversal method to only capture the results we want.

docs/API.md

@@ -380,6 +380,18 @@ vendoring.
 * `source`: the source key recorded in the Cargo.lock file
 * `version`: the version of the crate
 
+### `lib.findCargoFiles`
+
+`findCargoFiles :: path -> set of lists`
+
+Given a path, recursively search it for any `Cargo.toml`, `.cargo/config` or
+`.cargo/config.toml` files.
+
+```nix
+lib.findCargoFiles ./src
+# { cargoTomls = [ "..." ]; cargoConfigs = [ "..." ]; }
+```
+
 ### `lib.fromTOML`
 
 `fromTOML :: String -> set`

lib/default.nix

@@ -21,6 +21,7 @@ lib.makeScope newScope (self:
     cleanCargoToml = callPackage ./cleanCargoToml.nix { };
     crateNameFromCargoToml = callPackage ./crateNameFromCargoToml.nix { };
     downloadCargoPackage = callPackage ./downloadCargoPackage.nix { };
+    findCargoFiles = callPackage ./findCargoFiles.nix { };
     mkCargoDerivation = callPackage ./mkCargoDerivation.nix { };
     mkDummySrc = callPackage ./mkDummySrc.nix { };
     urlForCargoPackage = callPackage ./urlForCargoPackage.nix { };

lib/findCargoFiles.nix

@@ -0,0 +1,41 @@
+{ lib
+}:
+
+src:
+let
+  inherit (lib)
+    flatten
+    lists
+    mapAttrs
+    mapAttrsToList;
+
+  # A specialized form of lib.listFilesRecursive except it will only look
+  # for Cargo.toml and config.toml files to keep the intermediate results lean
+  listFilesRecursive = parentIsDotCargo: dir: flatten (mapAttrsToList
+    (name: type:
+      let
+        cur = dir + "/${name}";
+        isConfig = parentIsDotCargo && (name == "config" || name == "config.toml");
+        isCargoToml = name == "Cargo.toml";
+      in
+      if type == "directory"
+      then listFilesRecursive (name == ".cargo") cur
+      else if isCargoToml
+      then [{ path = cur; type = "cargoTomls"; }]
+      else if isConfig
+      then [{ path = cur; type = "cargoConfigs"; }]
+      else [ ]
+    )
+    (builtins.readDir dir));
+
+  foundFiles = listFilesRecursive false src;
+  grouped = builtins.groupBy (x: x.type) foundFiles;
+  cleaned = mapAttrs (_: map (y: y.path)) grouped;
+
+  # Ensure we have a well typed result
+  default = {
+    cargoTomls = [ ];
+    cargoConfigs = [ ];
+  };
+in
+default // cleaned

lib/mkDummySrc.nix

@@ -1,4 +1,5 @@
 { cleanCargoToml
+, findCargoFiles
 , lib
 , runCommand
 , writeText
@@ -13,12 +14,10 @@ let
   inherit (builtins)
     dirOf
     concatStringsSep
-    filter
     hasAttr
     pathExists;
 
   inherit (lib)
-    hasSuffix
     optionalString
     removePrefix;
 
@@ -34,22 +33,9 @@ let
     cp -f ${dummyrs} ${prefix}/${path}
   '';
 
-  allPaths = lib.filesystem.listFilesRecursive src;
+  inherit (findCargoFiles src)
-
+    cargoTomls
-  isCargoToml = path: hasSuffix "Cargo.toml" path;
+    cargoConfigs;
-  isCargoConfig = path:
-    let
-      p = toString path;
-      matches = s: hasSuffix s p;
-      # Cargo accepts one of two file names for its configuration.
-      # Just copy whatever we find and let cargo sort it out.
-      # https://doc.rust-lang.org/cargo/reference/config.html
-      isMatch = matches ".cargo/config" || matches ".cargo/config.toml";
-    in
-    isMatch;
-
-  cargoConfigs = filter isCargoConfig allPaths;
-  cargoTomls = filter isCargoToml allPaths;
 
   basePath = (toString src) + "/";
   copyAllCargoConfigs = concatStrings (map