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, 6 insertions, 14 deletions

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);
 }