Rework evaluator `SingleDerivedPath` infra

`EvalState::mkSingleDerivedPathString` previously contained its own
inverse (printing, rather than parsing) in order to validate what was
parsed. Now that is pulled out into its own separate function:
`EvalState::coerceToSingleDerivedPath`.

In additional that pulled out logic is deduplicated with
`EvalState::mkOutputString` via `EvalState::mkOutputStringRaw`, which is
itself deduplicated (and generalized) with
`DownstreamPlaceholder::mkOutputStringRaw`.

All these changes make the unit tests simpler.

(We would ideally write more unit tests for `mkSingleDerivedPathString`
`coerceToSingleDerivedPath` directly, but we cannot yet do that because
the IO in reading the store path won't work when the dummy store cannot
hold anything. Someday we'll have a proper in-memory store which will
work for this.)

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

6 files changed, 122 insertions, 71 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 063e34c56..79e2c4727 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1027,24 +1027,67 @@ void EvalState::mkStorePathString(const StorePath & p, Value & v)
 }
 
 
+std::string EvalState::mkOutputStringRaw(
+    const SingleDerivedPath::Built & b,
+    std::optional<StorePath> optStaticOutputPath,
+    const ExperimentalFeatureSettings & xpSettings)
+{
+    /* In practice, this is testing for the case of CA derivations, or
+       dynamic derivations. */
+    return optStaticOutputPath
+        ? store->printStorePath(*std::move(optStaticOutputPath))
+        /* Downstream we would substitute this for an actual path once
+           we build the floating CA derivation */
+        : DownstreamPlaceholder::fromSingleDerivedPathBuilt(b, xpSettings).render();
+}
+
+
 void EvalState::mkOutputString(
     Value & value,
-    const StorePath & drvPath,
-    const std::string outputName,
+    const SingleDerivedPath::Built & b,
     std::optional<StorePath> optStaticOutputPath,
     const ExperimentalFeatureSettings & xpSettings)
 {
     value.mkString(
-        optStaticOutputPath
-            ? store->printStorePath(*std::move(optStaticOutputPath))
-            /* Downstream we would substitute this for an actual path once
-               we build the floating CA derivation */
-            : DownstreamPlaceholder::unknownCaOutput(drvPath, outputName, xpSettings).render(),
+        mkOutputStringRaw(b, optStaticOutputPath, xpSettings),
+        NixStringContext { b });
+}
+
+
+std::string EvalState::mkSingleDerivedPathStringRaw(
+    const SingleDerivedPath & p)
+{
+    return std::visit(overloaded {
+        [&](const SingleDerivedPath::Opaque & o) {
+            return store->printStorePath(o.path);
+        },
+        [&](const SingleDerivedPath::Built & b) {
+            auto optStaticOutputPath = 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);
+                    return i->second.path(*store, drv.name, b.output);
+                },
+                [&](const SingleDerivedPath::Built & o) -> std::optional<StorePath> {
+                    return std::nullopt;
+                },
+            }, b.drvPath->raw());
+            return mkOutputStringRaw(b, optStaticOutputPath);
+        }
+    }, p.raw());
+}
+
+
+void EvalState::mkSingleDerivedPathString(
+    const SingleDerivedPath & p,
+    Value & v)
+{
+    v.mkString(
+        mkSingleDerivedPathStringRaw(p),
         NixStringContext {
-            NixStringContextElem::Built {
-                .drvPath = makeConstantStorePathRef(drvPath),
-                .output = outputName,
-            }
+            std::visit([](auto && v) -> NixStringContextElem { return v; }, p),
         });
 }
 
@@ -2333,39 +2376,25 @@ SingleDerivedPath EvalState::coerceToSingleDerivedPath(const PosIdx pos, Value &
 {
     auto [derivedPath, s_] = coerceToSingleDerivedPathUnchecked(pos, v, errorCtx);
     auto s = s_;
-    std::visit(overloaded {
-        [&](const SingleDerivedPath::Opaque & o) {
-            auto sExpected = store->printStorePath(o.path);
-            if (s != sExpected)
+    auto sExpected = mkSingleDerivedPathStringRaw(derivedPath);
+    if (s != sExpected) {
+        /* `std::visit` is used here just to provide a more precise
+           error message. */
+        std::visit(overloaded {
+            [&](const SingleDerivedPath::Opaque & o) {
                 error(
                     "path string '%s' has context with the different path '%s'",
                     s, sExpected)
                     .withTrace(pos, errorCtx).debugThrow<EvalError>();
-        },
-        [&](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 optStaticOutputPath = i->second.path(*store, drv.name, b.output);
-                    // This is testing for the case of CA derivations
-                    return optStaticOutputPath
-                        ? store->printStorePath(*optStaticOutputPath)
-                        : DownstreamPlaceholder::fromSingleDerivedPathBuilt(b).render();
-                },
-                [&](const SingleDerivedPath::Built & o) {
-                    return DownstreamPlaceholder::fromSingleDerivedPathBuilt(b).render();
-                },
-            }, b.drvPath->raw());
-            if (s != sExpected)
+            },
+            [&](const SingleDerivedPath::Built & b) {
                 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, b.output, b.drvPath->to_string(*store), sExpected)
                     .withTrace(pos, errorCtx).debugThrow<EvalError>();
-        }
-    }, derivedPath.raw());
+            }
+        }, derivedPath.raw());
+    }
     return derivedPath;
 }
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 662c123b9..0c3eb6505 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -668,37 +668,46 @@ public:
     /**
      * Create a string representing a store path.
      *
-     * The string is the printed store path with a context containing a single
-     * `NixStringContextElem::Opaque` element of that store path.
+     * The string is the printed store path with a context containing a
+     * single `NixStringContextElem::Opaque` element of that store path.
      */
     void mkStorePathString(const StorePath & storePath, Value & v);
 
     /**
-     * Create a string representing a `DerivedPath::Built`.
+     * Create a string representing a `SingleDerivedPath::Built`.
      *
-     * The string is the printed store path with a context containing a single
-     * `NixStringContextElem::Built` element of the drv path and output name.
+     * The string is the printed store path with a context containing a
+     * single `NixStringContextElem::Built` element of the drv path and
+     * output name.
      *
      * @param value Value we are settings
      *
-     * @param drvPath Path the drv whose output we are making a string for
+     * @param b the drv whose output we are making a string for, and the
+     * output
      *
-     * @param outputName Name of the output
-     *
-     * @param optStaticOutputPath Optional output path for that string. Must
-     * be passed if and only if output store object is input-addressed.
-     * Will be printed to form string if passed, otherwise a placeholder
-     * will be used (see `DownstreamPlaceholder`).
+     * @param optStaticOutputPath Optional output path for that string.
+     * Must be passed if and only if output store object is
+     * input-addressed or fixed output. Will be printed to form string
+     * if passed, otherwise a placeholder will be used (see
+     * `DownstreamPlaceholder`).
      *
      * @param xpSettings Stop-gap to avoid globals during unit tests.
      */
     void mkOutputString(
         Value & value,
-        const StorePath & drvPath,
-        const std::string outputName,
+        const SingleDerivedPath::Built & b,
         std::optional<StorePath> optStaticOutputPath,
         const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
 
+    /**
+     * Create a string representing a `SingleDerivedPath`.
+     *
+     * A combination of `mkStorePathString` and `mkOutputString`.
+     */
+    void mkSingleDerivedPathString(
+        const SingleDerivedPath & p,
+        Value & v);
+
     void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx);
 
     /**
@@ -714,6 +723,22 @@ public:
 
 private:
 
+    /**
+     * Like `mkOutputString` but just creates a raw string, not an
+     * string Value, which would also have a string context.
+     */
+    std::string mkOutputStringRaw(
+        const SingleDerivedPath::Built & b,
+        std::optional<StorePath> optStaticOutputPath,
+        const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
+
+    /**
+     * Like `mkSingleDerivedPathStringRaw` but just creates a raw string
+     * Value, which would also have a string context.
+     */
+    std::string mkSingleDerivedPathStringRaw(
+        const SingleDerivedPath & p);
+
     unsigned long nrEnvs = 0;
     unsigned long nrValuesInEnvs = 0;
     unsigned long nrValues = 0;
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 54943b481..be78bca02 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -156,8 +156,10 @@ static void mkOutputString(
 {
     state.mkOutputString(
         attrs.alloc(o.first),
-        drvPath,
-        o.first,
+        SingleDerivedPath::Built {
+            .drvPath = makeConstantStorePathRef(drvPath),
+            .output = o.first,
+        },
         o.second.path(*state.store, Derivation::nameFromPath(drvPath), o.first));
 }
 
diff --git a/src/libexpr/tests/derived-path.cc b/src/libexpr/tests/derived-path.cc
index 2a5ca64f6..d01735db8 100644
--- a/src/libexpr/tests/derived-path.cc
+++ b/src/libexpr/tests/derived-path.cc
@@ -34,8 +34,8 @@ RC_GTEST_FIXTURE_PROP(
 
 RC_GTEST_FIXTURE_PROP(
     DerivedPathExpressionTest,
-    prop_built_path_placeholder_round_trip,
-    (const StorePath & drvPath, const StorePathName & outputName))
+    prop_derived_path_built_placeholder_round_trip,
+    (const SingleDerivedPath::Built & b))
 {
     /**
      * We set these in tests rather than the regular globals so we don't have
@@ -45,27 +45,19 @@ RC_GTEST_FIXTURE_PROP(
     mockXpSettings.set("experimental-features", "ca-derivations");
 
     auto * v = state.allocValue();
-    state.mkOutputString(*v, drvPath, outputName.name, std::nullopt, mockXpSettings);
+    state.mkOutputString(*v, b, std::nullopt, mockXpSettings);
     auto [d, _] = state.coerceToSingleDerivedPathUnchecked(noPos, *v, "");
-    SingleDerivedPath::Built b {
-        .drvPath = makeConstantStorePathRef(drvPath),
-        .output = outputName.name,
-    };
     RC_ASSERT(SingleDerivedPath { b } == d);
 }
 
 RC_GTEST_FIXTURE_PROP(
     DerivedPathExpressionTest,
-    prop_built_path_out_path_round_trip,
-    (const StorePath & drvPath, const StorePathName & outputName, const StorePath & outPath))
+    prop_derived_path_built_out_path_round_trip,
+    (const SingleDerivedPath::Built & b, const StorePath & outPath))
 {
     auto * v = state.allocValue();
-    state.mkOutputString(*v, drvPath, outputName.name, outPath);
+    state.mkOutputString(*v, b, outPath);
     auto [d, _] = state.coerceToSingleDerivedPathUnchecked(noPos, *v, "");
-    SingleDerivedPath::Built b {
-        .drvPath = makeConstantStorePathRef(drvPath),
-        .output = outputName.name,
-    };
     RC_ASSERT(SingleDerivedPath { b } == d);
 }
 
diff --git a/src/libstore/downstream-placeholder.cc b/src/libstore/downstream-placeholder.cc
index 885b3604d..d951b7b7d 100644
--- a/src/libstore/downstream-placeholder.cc
+++ b/src/libstore/downstream-placeholder.cc
@@ -39,16 +39,18 @@ DownstreamPlaceholder DownstreamPlaceholder::unknownDerivation(
 }
 
 DownstreamPlaceholder DownstreamPlaceholder::fromSingleDerivedPathBuilt(
-    const SingleDerivedPath::Built & b)
+    const SingleDerivedPath::Built & b,
+    const ExperimentalFeatureSettings & xpSettings)
 {
     return std::visit(overloaded {
         [&](const SingleDerivedPath::Opaque & o) {
-            return DownstreamPlaceholder::unknownCaOutput(o.path, b.output);
+            return DownstreamPlaceholder::unknownCaOutput(o.path, b.output, xpSettings);
         },
         [&](const SingleDerivedPath::Built & b2) {
             return DownstreamPlaceholder::unknownDerivation(
-                DownstreamPlaceholder::fromSingleDerivedPathBuilt(b2),
-                b.output);
+                DownstreamPlaceholder::fromSingleDerivedPathBuilt(b2, xpSettings),
+                b.output,
+                xpSettings);
         },
     }, b.drvPath->raw());
 }
diff --git a/src/libstore/downstream-placeholder.hh b/src/libstore/downstream-placeholder.hh
index 9372dcd58..d58a2ac14 100644
--- a/src/libstore/downstream-placeholder.hh
+++ b/src/libstore/downstream-placeholder.hh
@@ -84,7 +84,8 @@ public:
      * `SingleDerivedPath::Built.drvPath` chain.
      */
     static DownstreamPlaceholder fromSingleDerivedPathBuilt(
-        const SingleDerivedPath::Built & built);
+        const SingleDerivedPath::Built & built,
+        const ExperimentalFeatureSettings & xpSettings = experimentalFeatureSettings);
 };
 
 }