Add builtins.filterPath

This is like builtins.{filterSource,path}, but returns a virtual path
that applies the filter lazily, rather than copying the result to the
Nix store. Thus filterPath can be composed.

tests/flakes/tree-operators.sh

@@ -0,0 +1,48 @@
+source ./common.sh
+
+flake1Dir=$TEST_ROOT/flake1
+
+mkdir -p $flake1Dir
+
+pwd=$(pwd)
+
+cat > $flake1Dir/flake.nix <<EOF
+{
+  inputs.docs = {
+    url = "path://$pwd/../../doc";
+    flake = false;
+  };
+
+  outputs = { self, docs }: with import ./lib.nix; {
+
+    mdOnly = builtins.filterPath {
+      path = docs;
+      filter =
+        path: type:
+        (type != "regular" || hasSuffix ".md" path);
+    };
+
+    noReleaseNotes = builtins.filterPath {
+      path = self.mdOnly + "/manual";
+      filter =
+        path: type:
+        assert !hasPrefix "/manual" path;
+        (builtins.baseNameOf path != "release-notes");
+    };
+
+  };
+}
+EOF
+
+cp ../lang/lib.nix $flake1Dir/
+
+nix build --out-link $TEST_ROOT/result $flake1Dir#mdOnly
+[[ -e $TEST_ROOT/result/manual/src/quick-start.md ]]
+[[ -e $TEST_ROOT/result/manual/src/release-notes ]]
+(! find $TEST_ROOT/result/ -type f | grep -v '.md$')
+find $TEST_ROOT/result/ -type f | grep release-notes
+
+nix build --out-link $TEST_ROOT/result $flake1Dir#noReleaseNotes
+[[ -e $TEST_ROOT/result/src/quick-start.md ]]
+(! [[ -e $TEST_ROOT/result/src/release-notes ]])
+(! find $TEST_ROOT/result/ -type f | grep release-notes)

tests/lang/lib.nix

@@ -19,6 +19,8 @@ rec {
 
   sum = foldl' (x: y: add x y) 0;
 
+  hasPrefix = pref: str: substring 0 (stringLength pref) str == pref;
+
   hasSuffix = ext: fileName:
     let lenFileName = stringLength fileName;
         lenExt = stringLength ext;

tests/local.mk

@@ -10,6 +10,7 @@ nix_tests = \
   flakes/unlocked-override.sh \
   flakes/absolute-paths.sh \
   flakes/build-paths.sh \
+  flakes/tree-operators.sh \
   ca/gc.sh \
   gc.sh \
   remote-store.sh \