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>

1 files changed, 2 insertions, 1 deletions

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);
 };
 
 }