Make the Derived Path family of types inductive for dynamic derivations

We want to be able to write down `foo.drv^bar.drv^baz`:
`foo.drv^bar.drv` is the dynamic derivation (since it is itself a
derivation output, `bar.drv` from `foo.drv`).

To that end, we create `Single{Derivation,BuiltPath}` types, that are
very similar except instead of having multiple outputs (in a set or
map), they have a single one. This is for everything to the left of the
rightmost `^`.

`NixStringContextElem` has an analogous change, and now can reuse
`SingleDerivedPath` at the top level. In fact, if we ever get rid of
`DrvDeep`, `NixStringContextElem` could be replaced with
`SingleDerivedPath` entirely!

Important note: some JSON formats have changed.

We already can *produce* dynamic derivations, but we can't refer to them
directly. Today, we can merely express building or example at the top
imperatively over time by building `foo.drv^bar.drv`, and then with a
second nix invocation doing `<result-from-first>^baz`, but this is not
declarative. The ethos of Nix of being able to write down the full plan
everything you want to do, and then execute than plan with a single
command, and for that we need the new inductive form of these types.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

9 files changed, 191 insertions, 112 deletions

diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index 9e734e654..6a60ba87b 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -599,7 +599,7 @@ string_t AttrCursor::getStringWithContext()
                             return d.drvPath;
                         },
                         [&](const NixStringContextElem::Built & b) -> const StorePath & {
-                            return b.drvPath;
+                            return b.drvPath->getBaseStorePath();
                         },
                         [&](const NixStringContextElem::Opaque & o) -> const StorePath & {
                             return o.path;
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index a8e6baea6..3e521b732 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1042,7 +1042,7 @@ void EvalState::mkOutputString(
             : DownstreamPlaceholder::unknownCaOutput(drvPath, outputName, xpSettings).render(),
         NixStringContext {
             NixStringContextElem::Built {
-                .drvPath = drvPath,
+                .drvPath = makeConstantStorePathRef(drvPath),
                 .output = outputName,
             }
         });
@@ -2299,7 +2299,7 @@ StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, NixStringCon
 }
 
 
-std::pair<DerivedPath, std::string_view> EvalState::coerceToDerivedPathUnchecked(const PosIdx pos, Value & v, std::string_view errorCtx)
+std::pair<SingleDerivedPath, std::string_view> EvalState::coerceToSingleDerivedPathUnchecked(const PosIdx pos, Value & v, std::string_view errorCtx)
 {
     NixStringContext context;
     auto s = forceString(v, context, pos, errorCtx);
@@ -2310,21 +2310,16 @@ std::pair<DerivedPath, std::string_view> EvalState::coerceToDerivedPathUnchecked
             s, csize)
             .withTrace(pos, errorCtx).debugThrow<EvalError>();
     auto derivedPath = std::visit(overloaded {
-        [&](NixStringContextElem::Opaque && o) -> DerivedPath {
-            return DerivedPath::Opaque {
-                .path = std::move(o.path),
-            };
+        [&](NixStringContextElem::Opaque && o) -> SingleDerivedPath {
+            return std::move(o);
         },
-        [&](NixStringContextElem::DrvDeep &&) -> DerivedPath {
+        [&](NixStringContextElem::DrvDeep &&) -> SingleDerivedPath {
             error(
                 "string '%s' has a context which refers to a complete source and binary closure. This is not supported at this time",
                 s).withTrace(pos, errorCtx).debugThrow<EvalError>();
         },
-        [&](NixStringContextElem::Built && b) -> DerivedPath {
-            return DerivedPath::Built {
-                .drvPath = std::move(b.drvPath),
-                .outputs = OutputsSpec::Names { std::move(b.output) },
-            };
+        [&](NixStringContextElem::Built && b) -> SingleDerivedPath {
+            return std::move(b);
         },
     }, ((NixStringContextElem &&) *context.begin()).raw());
     return {
@@ -2334,12 +2329,12 @@ std::pair<DerivedPath, std::string_view> EvalState::coerceToDerivedPathUnchecked
 }
 
 
-DerivedPath EvalState::coerceToDerivedPath(const PosIdx pos, Value & v, std::string_view errorCtx)
+SingleDerivedPath EvalState::coerceToSingleDerivedPath(const PosIdx pos, Value & v, std::string_view errorCtx)
 {
-    auto [derivedPath, s_] = coerceToDerivedPathUnchecked(pos, v, errorCtx);
+    auto [derivedPath, s_] = coerceToSingleDerivedPathUnchecked(pos, v, errorCtx);
     auto s = s_;
     std::visit(overloaded {
-        [&](const DerivedPath::Opaque & o) {
+        [&](const SingleDerivedPath::Opaque & o) {
             auto sExpected = store->printStorePath(o.path);
             if (s != sExpected)
                 error(
@@ -2347,25 +2342,27 @@ DerivedPath EvalState::coerceToDerivedPath(const PosIdx pos, Value & v, std::str
                     s, sExpected)
                     .withTrace(pos, errorCtx).debugThrow<EvalError>();
         },
-        [&](const DerivedPath::Built & b) {
-            // TODO need derived path with single output to make this
-            // total. Will add as part of RFC 92 work and then this is
-            // cleaned up.
-            auto output = *std::get<OutputsSpec::Names>(b.outputs).begin();
-
-            auto drv = store->readDerivation(b.drvPath);
-            auto i = drv.outputs.find(output);
-            if (i == drv.outputs.end())
-                throw Error("derivation '%s' does not have output '%s'", store->printStorePath(b.drvPath), output);
-            auto optOutputPath = i->second.path(*store, drv.name, output);
-            // This is testing for the case of CA derivations
-            auto sExpected = optOutputPath
-                ? store->printStorePath(*optOutputPath)
-                : DownstreamPlaceholder::unknownCaOutput(b.drvPath, output).render();
+        [&](const SingleDerivedPath::Built & b) {
+            auto sExpected = std::visit(overloaded {
+                [&](const SingleDerivedPath::Opaque & o) {
+                    auto drv = store->readDerivation(o.path);
+                    auto i = drv.outputs.find(b.output);
+                    if (i == drv.outputs.end())
+                        throw Error("derivation '%s' does not have output '%s'", b.drvPath->to_string(*store), b.output);
+                    auto optOutputPath = i->second.path(*store, drv.name, b.output);
+                    // This is testing for the case of CA derivations
+                    return optOutputPath
+                        ? store->printStorePath(*optOutputPath)
+                        : DownstreamPlaceholder::fromSingleDerivedPathBuilt(b).render();
+                },
+                [&](const SingleDerivedPath::Built & o) {
+                    return DownstreamPlaceholder::fromSingleDerivedPathBuilt(b).render();
+                },
+            }, b.drvPath->raw());
             if (s != sExpected)
                 error(
                     "string '%s' has context with the output '%s' from derivation '%s', but the string is not the right placeholder for this derivation output. It should be '%s'",
-                    s, output, store->printStorePath(b.drvPath), sExpected)
+                    s, b.output, b.drvPath->to_string(*store), sExpected)
                     .withTrace(pos, errorCtx).debugThrow<EvalError>();
         }
     }, derivedPath.raw());
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 29d0f05a1..0268a2a12 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -22,7 +22,7 @@ namespace nix {
 class Store;
 class EvalState;
 class StorePath;
-struct DerivedPath;
+struct SingleDerivedPath;
 enum RepairFlag : bool;
 
 
@@ -532,12 +532,12 @@ public:
     StorePath coerceToStorePath(const PosIdx pos, Value & v, NixStringContext & context, std::string_view errorCtx);
 
     /**
-     * Part of `coerceToDerivedPath()` without any store IO which is exposed for unit testing only.
+     * Part of `coerceToSingleDerivedPath()` without any store IO which is exposed for unit testing only.
      */
-    std::pair<DerivedPath, std::string_view> coerceToDerivedPathUnchecked(const PosIdx pos, Value & v, std::string_view errorCtx);
+    std::pair<SingleDerivedPath, std::string_view> coerceToSingleDerivedPathUnchecked(const PosIdx pos, Value & v, std::string_view errorCtx);
 
     /**
-     * Coerce to `DerivedPath`.
+     * Coerce to `SingleDerivedPath`.
      *
      * Must be a string which is either a literal store path or a
      * "placeholder (see `DownstreamPlaceholder`).
@@ -551,7 +551,7 @@ public:
      * source of truth, and ultimately tells us what we want, and then
      * we ensure the string corresponds to it.
      */
-    DerivedPath coerceToDerivedPath(const PosIdx pos, Value & v, std::string_view errorCtx);
+    SingleDerivedPath coerceToSingleDerivedPath(const PosIdx pos, Value & v, std::string_view errorCtx);
 
 public:
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 430607214..54943b481 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -56,7 +56,7 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
                     .drvPath = b.drvPath,
                     .outputs = OutputsSpec::Names { b.output },
                 });
-                ensureValid(b.drvPath);
+                ensureValid(b.drvPath->getBaseStorePath());
             },
             [&](const NixStringContextElem::Opaque & o) {
                 auto ctxS = store->printStorePath(o.path);
@@ -77,7 +77,7 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
     if (!evalSettings.enableImportFromDerivation)
         debugThrowLastTrace(Error(
             "cannot build '%1%' during evaluation because the option 'allow-import-from-derivation' is disabled",
-            store->printStorePath(drvs.begin()->drvPath)));
+            drvs.begin()->to_string(*store)));
 
     /* Build/substitute the context. */
     std::vector<DerivedPath> buildReqs;
@@ -95,7 +95,11 @@ StringMap EvalState::realiseContext(const NixStringContext & context)
             /* Get all the output paths corresponding to the placeholders we had */
             if (experimentalFeatureSettings.isEnabled(Xp::CaDerivations)) {
                 res.insert_or_assign(
-                    DownstreamPlaceholder::unknownCaOutput(drv.drvPath, outputName).render(),
+                    DownstreamPlaceholder::fromSingleDerivedPathBuilt(
+                        SingleDerivedPath::Built {
+                            .drvPath = drv.drvPath,
+                            .output = outputName,
+                        }).render(),
                     store->printStorePath(outputPath)
                 );
             }
@@ -1251,7 +1255,10 @@ drvName, Bindings * attrs, Value & v)
                 }
             },
             [&](const NixStringContextElem::Built & b) {
-                drv.inputDrvs[b.drvPath].insert(b.output);
+                if (auto * p = std::get_if<DerivedPath::Opaque>(&*b.drvPath))
+                    drv.inputDrvs[p->path].insert(b.output);
+                else
+                    throw UnimplementedError("Dependencies on the outputs of dynamic derivations are not yet supported");
             },
             [&](const NixStringContextElem::Opaque & o) {
                 drv.inputSrcs.insert(o.path);
diff --git a/src/libexpr/primops/context.cc b/src/libexpr/primops/context.cc
index 8b3468009..bfc731744 100644
--- a/src/libexpr/primops/context.cc
+++ b/src/libexpr/primops/context.cc
@@ -106,7 +106,10 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
                 contextInfos[std::move(d.drvPath)].allOutputs = true;
             },
             [&](NixStringContextElem::Built && b) {
-                contextInfos[std::move(b.drvPath)].outputs.emplace_back(std::move(b.output));
+                // FIXME should eventually show string context as is, no
+                // resolving here.
+                auto drvPath = resolveDerivedPath(*state.store, *b.drvPath);
+                contextInfos[std::move(drvPath)].outputs.emplace_back(std::move(b.output));
             },
             [&](NixStringContextElem::Opaque && o) {
                 contextInfos[std::move(o.path)].path = true;
@@ -222,7 +225,7 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
             for (auto elem : iter->value->listItems()) {
                 auto outputName = state.forceStringNoCtx(*elem, iter->pos, "while evaluating an output name within a string context");
                 context.emplace(NixStringContextElem::Built {
-                    .drvPath = namePath,
+                    .drvPath = makeConstantStorePathRef(namePath),
                     .output = std::string { outputName },
                 });
             }
diff --git a/src/libexpr/tests/derived-path.cc b/src/libexpr/tests/derived-path.cc
index c713fe28a..2a5ca64f6 100644
--- a/src/libexpr/tests/derived-path.cc
+++ b/src/libexpr/tests/derived-path.cc
@@ -21,12 +21,12 @@ TEST_F(DerivedPathExpressionTest, force_init)
 RC_GTEST_FIXTURE_PROP(
     DerivedPathExpressionTest,
     prop_opaque_path_round_trip,
-    (const DerivedPath::Opaque & o))
+    (const SingleDerivedPath::Opaque & o))
 {
     auto * v = state.allocValue();
     state.mkStorePathString(o.path, *v);
-    auto d = state.coerceToDerivedPath(noPos, *v, "");
-    RC_ASSERT(DerivedPath { o } == d);
+    auto d = state.coerceToSingleDerivedPath(noPos, *v, "");
+    RC_ASSERT(SingleDerivedPath { o } == d);
 }
 
 // TODO use DerivedPath::Built for parameter once it supports a single output
@@ -46,12 +46,12 @@ RC_GTEST_FIXTURE_PROP(
 
     auto * v = state.allocValue();
     state.mkOutputString(*v, drvPath, outputName.name, std::nullopt, mockXpSettings);
-    auto [d, _] = state.coerceToDerivedPathUnchecked(noPos, *v, "");
-    DerivedPath::Built b {
-        .drvPath = drvPath,
-        .outputs = OutputsSpec::Names { outputName.name },
+    auto [d, _] = state.coerceToSingleDerivedPathUnchecked(noPos, *v, "");
+    SingleDerivedPath::Built b {
+        .drvPath = makeConstantStorePathRef(drvPath),
+        .output = outputName.name,
     };
-    RC_ASSERT(DerivedPath { b } == d);
+    RC_ASSERT(SingleDerivedPath { b } == d);
 }
 
 RC_GTEST_FIXTURE_PROP(
@@ -61,12 +61,12 @@ RC_GTEST_FIXTURE_PROP(
 {
     auto * v = state.allocValue();
     state.mkOutputString(*v, drvPath, outputName.name, outPath);
-    auto [d, _] = state.coerceToDerivedPathUnchecked(noPos, *v, "");
-    DerivedPath::Built b {
-        .drvPath = drvPath,
-        .outputs = OutputsSpec::Names { outputName.name },
+    auto [d, _] = state.coerceToSingleDerivedPathUnchecked(noPos, *v, "");
+    SingleDerivedPath::Built b {
+        .drvPath = makeConstantStorePathRef(drvPath),
+        .output = outputName.name,
     };
-    RC_ASSERT(DerivedPath { b } == d);
+    RC_ASSERT(SingleDerivedPath { b } == d);
 }
 
 } /* namespace nix */
diff --git a/src/libexpr/tests/value/context.cc b/src/libexpr/tests/value/context.cc
index 0d9381577..c56b50b59 100644
--- a/src/libexpr/tests/value/context.cc
+++ b/src/libexpr/tests/value/context.cc
@@ -8,6 +8,8 @@
 
 namespace nix {
 
+// Test a few cases of invalid string context elements.
+
 TEST(NixStringContextElemTest, empty_invalid) {
     EXPECT_THROW(
         NixStringContextElem::parse(""),
@@ -38,6 +40,10 @@ TEST(NixStringContextElemTest, slash_invalid) {
         BadStorePath);
 }
 
+/**
+ * Round trip (string <-> data structure) test for
+ * `NixStringContextElem::Opaque`.
+ */
 TEST(NixStringContextElemTest, opaque) {
     std::string_view opaque = "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x";
     auto elem = NixStringContextElem::parse(opaque);
@@ -47,6 +53,10 @@ TEST(NixStringContextElemTest, opaque) {
     ASSERT_EQ(elem.to_string(), opaque);
 }
 
+/**
+ * Round trip (string <-> data structure) test for
+ * `NixStringContextElem::DrvDeep`.
+ */
 TEST(NixStringContextElemTest, drvDeep) {
     std::string_view drvDeep = "=g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv";
     auto elem = NixStringContextElem::parse(drvDeep);
@@ -56,28 +66,62 @@ TEST(NixStringContextElemTest, drvDeep) {
     ASSERT_EQ(elem.to_string(), drvDeep);
 }
 
-TEST(NixStringContextElemTest, built) {
+/**
+ * Round trip (string <-> data structure) test for a simpler
+ * `NixStringContextElem::Built`.
+ */
+TEST(NixStringContextElemTest, built_opaque) {
     std::string_view built = "!foo!g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv";
     auto elem = NixStringContextElem::parse(built);
     auto * p = std::get_if<NixStringContextElem::Built>(&elem);
     ASSERT_TRUE(p);
     ASSERT_EQ(p->output, "foo");
-    ASSERT_EQ(p->drvPath, StorePath { built.substr(5) });
+    ASSERT_EQ(*p->drvPath, ((SingleDerivedPath) SingleDerivedPath::Opaque {
+        .path = StorePath { built.substr(5) },
+    }));
     ASSERT_EQ(elem.to_string(), built);
 }
 
+/**
+ * Round trip (string <-> data structure) test for a more complex,
+ * inductive `NixStringContextElem::Built`.
+ */
+TEST(NixStringContextElemTest, built_built) {
+    /**
+     * We set these in tests rather than the regular globals so we don't have
+     * to worry about race conditions if the tests run concurrently.
+     */
+    ExperimentalFeatureSettings mockXpSettings;
+    mockXpSettings.set("experimental-features", "dynamic-derivations ca-derivations");
+
+    std::string_view built = "!foo!bar!g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv";
+    auto elem = NixStringContextElem::parse(built, mockXpSettings);
+    auto * p = std::get_if<NixStringContextElem::Built>(&elem);
+    ASSERT_TRUE(p);
+    ASSERT_EQ(p->output, "foo");
+    auto * drvPath = std::get_if<SingleDerivedPath::Built>(&*p->drvPath);
+    ASSERT_TRUE(drvPath);
+    ASSERT_EQ(drvPath->output, "bar");
+    ASSERT_EQ(*drvPath->drvPath, ((SingleDerivedPath) SingleDerivedPath::Opaque {
+        .path = StorePath { built.substr(9) },
+    }));
+    ASSERT_EQ(elem.to_string(), built);
 }
 
-namespace rc {
-using namespace nix;
+/**
+ * Without the right experimental features enabled, we cannot parse a
+ * complex inductive string context element.
+ */
+TEST(NixStringContextElemTest, built_built_xp) {
+    ASSERT_THROW(
+        NixStringContextElem::parse("!foo!bar!g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv"),        MissingExperimentalFeature);
+}
 
-Gen<NixStringContextElem::Opaque> Arbitrary<NixStringContextElem::Opaque>::arbitrary()
-{
-    return gen::just(NixStringContextElem::Opaque {
-        .path = *gen::arbitrary<StorePath>(),
-    });
 }
 
+namespace rc {
+using namespace nix;
+
 Gen<NixStringContextElem::DrvDeep> Arbitrary<NixStringContextElem::DrvDeep>::arbitrary()
 {
     return gen::just(NixStringContextElem::DrvDeep {
@@ -85,14 +129,6 @@ Gen<NixStringContextElem::DrvDeep> Arbitrary<NixStringContextElem::DrvDeep>::arb
     });
 }
 
-Gen<NixStringContextElem::Built> Arbitrary<NixStringContextElem::Built>::arbitrary()
-{
-    return gen::just(NixStringContextElem::Built {
-        .drvPath = *gen::arbitrary<StorePath>(),
-        .output = (*gen::arbitrary<StorePathName>()).name,
-    });
-}
-
 Gen<NixStringContextElem> Arbitrary<NixStringContextElem>::arbitrary()
 {
     switch (*gen::inRange<uint8_t>(0, std::variant_size_v<NixStringContextElem::Raw>)) {
diff --git a/src/libexpr/value/context.cc b/src/libexpr/value/context.cc
index f76fc76e4..d8116011e 100644
--- a/src/libexpr/value/context.cc
+++ b/src/libexpr/value/context.cc
@@ -4,29 +4,52 @@
 
 namespace nix {
 
-NixStringContextElem NixStringContextElem::parse(std::string_view s0)
+NixStringContextElem NixStringContextElem::parse(
+    std::string_view s0,
+    const ExperimentalFeatureSettings & xpSettings)
 {
     std::string_view s = s0;
 
+    std::function<SingleDerivedPath()> parseRest;
+    parseRest = [&]() -> SingleDerivedPath {
+        // Case on whether there is a '!'
+        size_t index = s.find("!");
+        if (index == std::string_view::npos) {
+            return SingleDerivedPath::Opaque {
+                .path = StorePath { s },
+            };
+        } else {
+            std::string output { s.substr(0, index) };
+            // Advance string to parse after the '!'
+            s = s.substr(index + 1);
+            auto drv = make_ref<SingleDerivedPath>(parseRest());
+            drvRequireExperiment(*drv, xpSettings);
+            return SingleDerivedPath::Built {
+                .drvPath = std::move(drv),
+                .output = std::move(output),
+            };
+        }
+    };
+
     if (s.size() == 0) {
         throw BadNixStringContextElem(s0,
             "String context element should never be an empty string");
     }
+
     switch (s.at(0)) {
     case '!': {
-        s = s.substr(1); // advance string to parse after first !
-        size_t index = s.find("!");
-        // This makes index + 1 safe. Index can be the length (one after index
-        // of last character), so given any valid character index --- a
-        // successful find --- we can add one.
-        if (index == std::string_view::npos) {
+        // Advance string to parse after the '!'
+        s = s.substr(1);
+
+        // Find *second* '!'
+        if (s.find("!") == std::string_view::npos) {
             throw BadNixStringContextElem(s0,
                 "String content element beginning with '!' should have a second '!'");
         }
-        return NixStringContextElem::Built {
-            .drvPath = StorePath { s.substr(index + 1) },
-            .output = std::string(s.substr(0, index)),
-        };
+
+        return std::visit(
+            [&](auto x) -> NixStringContextElem { return std::move(x); },
+            parseRest());
     }
     case '=': {
         return NixStringContextElem::DrvDeep {
@@ -34,33 +57,51 @@ NixStringContextElem NixStringContextElem::parse(std::string_view s0)
         };
     }
     default: {
-        return NixStringContextElem::Opaque {
-            .path = StorePath { s },
-        };
+        // Ensure no '!'
+        if (s.find("!") != std::string_view::npos) {
+            throw BadNixStringContextElem(s0,
+                "String content element not beginning with '!' should not have a second '!'");
+        }
+        return std::visit(
+            [&](auto x) -> NixStringContextElem { return std::move(x); },
+            parseRest());
     }
     }
 }
 
-std::string NixStringContextElem::to_string() const {
-    return std::visit(overloaded {
+std::string NixStringContextElem::to_string() const
+{
+    std::string res;
+
+    std::function<void(const SingleDerivedPath &)> toStringRest;
+    toStringRest = [&](auto & p) {
+        std::visit(overloaded {
+            [&](const SingleDerivedPath::Opaque & o) {
+                res += o.path.to_string();
+            },
+            [&](const SingleDerivedPath::Built & o) {
+                res += o.output;
+                res += '!';
+                toStringRest(*o.drvPath);
+            },
+        }, p.raw());
+    };
+
+    std::visit(overloaded {
         [&](const NixStringContextElem::Built & b) {
-            std::string res;
-            res += '!';
-            res += b.output;
             res += '!';
-            res += b.drvPath.to_string();
-            return res;
+            toStringRest(b);
+        },
+        [&](const NixStringContextElem::Opaque & o) {
+            toStringRest(o);
         },
         [&](const NixStringContextElem::DrvDeep & d) {
-            std::string res;
             res += '=';
             res += d.drvPath.to_string();
-            return res;
-        },
-        [&](const NixStringContextElem::Opaque & o) {
-            return std::string { o.path.to_string() };
         },
     }, raw());
+
+    return res;
 }
 
 }
diff --git a/src/libexpr/value/context.hh b/src/libexpr/value/context.hh
index 287ae08a9..a1b71695b 100644
--- a/src/libexpr/value/context.hh
+++ b/src/libexpr/value/context.hh
@@ -3,7 +3,7 @@
 
 #include "util.hh"
 #include "comparator.hh"
-#include "path.hh"
+#include "derived-path.hh"
 
 #include <variant>
 
@@ -31,11 +31,7 @@ public:
  *
  * Encoded as just the path: ‘<path>’.
  */
-struct NixStringContextElem_Opaque {
-    StorePath path;
-
-    GENERATE_CMP(NixStringContextElem_Opaque, me->path);
-};
+typedef SingleDerivedPath::Opaque NixStringContextElem_Opaque;
 
 /**
  * Path to a derivation and its entire build closure.
@@ -57,12 +53,7 @@ struct NixStringContextElem_DrvDeep {
  *
  * Encoded in the form ‘!<output>!<drvPath>’.
  */
-struct NixStringContextElem_Built {
-    StorePath drvPath;
-    std::string output;
-
-    GENERATE_CMP(NixStringContextElem_Built, me->drvPath, me->output);
-};
+typedef SingleDerivedPath::Built NixStringContextElem_Built;
 
 using _NixStringContextElem_Raw = std::variant<
     NixStringContextElem_Opaque,
@@ -93,8 +84,12 @@ struct NixStringContextElem : _NixStringContextElem_Raw {
      * - ‘<path>’
      * - ‘=<path>’
      * - ‘!<name>!<path>’
+     *
+     * @param xpSettings Stop-gap to avoid globals during unit tests.
      */
-    static NixStringContextElem parse(std::string_view s);
+    static NixStringContextElem parse(
+        std::string_view s,
+        const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
     std::string to_string() const;
 };