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>

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;