Interpret absolute paths relative to the root FS rather than the current flake

src/libexpr/eval.cc

@@ -472,7 +472,7 @@ EvalState::EvalState(
             : std::nullopt,
             [](const CanonPath & path) -> RestrictedPathError {
                 auto modeInformation = evalSettings.pureEval
-                    ? "in pure eval mode (use '--impure' to override)"
+                    ? "in pure evaluation mode (use '--impure' to override)"
                     : "in restricted mode";
                 throw RestrictedPathError("access to absolute path '%1%' is forbidden %2%", path, modeInformation);
             }))

src/libexpr/parser.y

@@ -508,8 +508,14 @@ string_parts_interpolated
 
 path_start
   : PATH {
-    SourcePath path { data->basePath.accessor, CanonPath({$1.p, $1.l}, data->basePath.path) };
-    $$ = new ExprPath(std::move(path));
+      std::string_view path({$1.p, $1.l});
+      $$ = new ExprPath(
+          /* Absolute paths are always interpreted relative to the
+             root filesystem accessor, rather than the accessor of the
+             current Nix expression. */
+          hasPrefix(path, "/")
+          ? SourcePath{data->state.rootFS, CanonPath(path)}
+          : SourcePath{data->basePath.accessor, CanonPath(path, data->basePath.path)});
   }
   | HPATH {
     if (evalSettings.pureEval) {

tests/flakes/absolute-paths.sh

@@ -0,0 +1,17 @@
+source ./common.sh
+
+requireGit
+
+flake1Dir=$TEST_ROOT/flake1
+flake2Dir=$TEST_ROOT/flake2
+
+createGitRepo $flake1Dir
+cat > $flake1Dir/flake.nix <<EOF
+{
+    outputs = { self }: { x = builtins.readFile $(pwd)/absolute-paths.sh; };
+}
+EOF
+git -C $flake1Dir add flake.nix
+git -C $flake1Dir commit -m Initial
+
+nix eval --impure --json $flake1Dir#x

tests/local.mk

@@ -8,6 +8,7 @@ nix_tests = \
   flakes/bundle.sh \
   flakes/check.sh \
   flakes/unlocked-override.sh \
+  flakes/absolute-paths.sh \
   ca/gc.sh \
   gc.sh \
   remote-store.sh \