InstallableFlake::toDerivedPaths(): Support paths and store paths

This makes 'nix build' work on paths (which will be copied to the
store) and store paths (returned as is). E.g. the following flake
output attributes can be built using 'nix build .#foo':

  foo = ./src;
  foo = self.outPath;
  foo = builtins.fetchTarball { ... };
  foo = (builtins.fetchTree { .. }).outPath;
  foo = builtins.fetchTree { .. } + "/README.md";
  foo = builtins.storePath /nix/store/...;

Note that this is potentially risky, e.g.

  foo = /.;

will cause Nix to try to copy the entire file system to the store.

What doesn't work yet:

  foo = self;
  foo = builtins.fetchTree { .. };

because we don't handle attrsets with an outPath attribute in it yet,
and

  foo = builtins.storePath /nix/store/.../README.md;

since result symlinks have to point to a store path currently (rather
than a file inside a store path).

Fixes #7417.

2 files changed, 67 insertions, 0 deletions

diff --git a/tests/flakes/build-paths.sh b/tests/flakes/build-paths.sh
new file mode 100644
index 000000000..08b4d1763
--- /dev/null
+++ b/tests/flakes/build-paths.sh
@@ -0,0 +1,66 @@
+source ./common.sh
+
+flake1Dir=$TEST_ROOT/flake1
+flake2Dir=$TEST_ROOT/flake2
+
+mkdir -p $flake1Dir $flake2Dir
+
+writeSimpleFlake $flake2Dir
+tar cfz $TEST_ROOT/flake.tar.gz -C $TEST_ROOT flake2
+hash=$(nix hash path $flake2Dir)
+
+dep=$(nix store add-path ./common.sh)
+
+cat > $flake1Dir/flake.nix <<EOF
+{
+  inputs.flake2.url = "file://$TEST_ROOT/flake.tar.gz";
+
+  outputs = { self, flake2 }: {
+
+    a1 = builtins.fetchTarball {
+      #type = "tarball";
+      url = "file://$TEST_ROOT/flake.tar.gz";
+      sha256 = "$hash";
+    };
+
+    a2 = ./foo;
+
+    a3 = ./.;
+
+    a4 = self.outPath;
+
+    # FIXME
+    a5 = self;
+
+    a6 = flake2.outPath;
+
+    # FIXME
+    a7 = "${flake2}/config.nix";
+
+    # This is only allowed in impure mode.
+    a8 = builtins.storePath $dep;
+
+    a9 = "$dep";
+  };
+}
+EOF
+
+echo bar > $flake1Dir/foo
+
+nix build --json --out-link $TEST_ROOT/result $flake1Dir#a1
+[[ -e $TEST_ROOT/result/simple.nix ]]
+
+nix build --json --out-link $TEST_ROOT/result $flake1Dir#a2
+[[ $(cat $TEST_ROOT/result) = bar ]]
+
+nix build --json --out-link $TEST_ROOT/result $flake1Dir#a3
+
+nix build --json --out-link $TEST_ROOT/result $flake1Dir#a4
+
+nix build --json --out-link $TEST_ROOT/result $flake1Dir#a6
+[[ -e $TEST_ROOT/result/simple.nix ]]
+
+nix build --impure --json --out-link $TEST_ROOT/result $flake1Dir#a8
+diff common.sh $TEST_ROOT/result
+
+(! nix build --impure --json --out-link $TEST_ROOT/result $flake1Dir#a9)
diff --git a/tests/local.mk b/tests/local.mk
index 2f7f76261..55913e977 100644
--- a/tests/local.mk
+++ b/tests/local.mk
@@ -9,6 +9,7 @@ nix_tests = \
   flakes/check.sh \
   flakes/unlocked-override.sh \
   flakes/absolute-paths.sh \
+  flakes/build-paths.sh \
   ca/gc.sh \
   gc.sh \
   remote-store.sh \