Render lazy tree paths in messages withouth the/nix/store/hash... prefix in substituted source trees

src/libfetchers/fetchers.cc

@@ -5,6 +5,7 @@
 #include "nix/util/json-utils.hh"
 #include "nix/fetchers/store-path-accessor.hh"
 #include "nix/fetchers/fetch-settings.hh"
+#include "nix/util/forwarding-source-accessor.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -293,6 +294,21 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessor(ref<Store> store) const
     }
 }
 
+/**
+ * Helper class that ensures that paths in substituted source trees
+ * are rendered as `«input»/path` rather than
+ * `«input»/nix/store/<hash>-source/path`.
+ */
+struct SubstitutedSourceAccessor : ForwardingSourceAccessor
+{
+    using ForwardingSourceAccessor::ForwardingSourceAccessor;
+
+    std::string showPath(const CanonPath & path) override
+    {
+        return displayPrefix + path.abs() + displaySuffix;
+    }
+};
+
 std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> store) const
 {
     // FIXME: cache the accessor
@@ -320,10 +336,12 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> sto
             debug("using substituted/cached input '%s' in '%s'",
                 to_string(), store->printStorePath(storePath));
 
-            auto accessor = makeStorePathAccessor(store, storePath);
+            auto accessor = make_ref<SubstitutedSourceAccessor>(makeStorePathAccessor(store, storePath));
 
             accessor->fingerprint = getFingerprint(store);
 
+            // FIXME: ideally we would use the `showPath()` of the
+            // "real" accessor for this fetcher type.
             accessor->setPathDisplay("«" + to_string() + "»");
 
             return {accessor, *this};

src/libutil/include/nix/util/forwarding-source-accessor.hh

@@ -0,0 +1,57 @@
+#pragma once
+
+#include "source-accessor.hh"
+
+namespace nix {
+
+/**
+ * A source accessor that just forwards every operation to another
+ * accessor. This is not useful in itself but can be used as a
+ * superclass for accessors that do change some operations.
+ */
+struct ForwardingSourceAccessor : SourceAccessor
+{
+    ref<SourceAccessor> next;
+
+    ForwardingSourceAccessor(ref<SourceAccessor> next)
+        : next(next)
+    {
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        return next->readFile(path);
+    }
+
+    void readFile(const CanonPath & path, Sink & sink, std::function<void(uint64_t)> sizeCallback) override
+    {
+        next->readFile(path, sink, sizeCallback);
+    }
+
+    std::optional<Stat> maybeLstat(const CanonPath & path) override
+    {
+        return next->maybeLstat(path);
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        return next->readDirectory(path);
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        return next->readLink(path);
+    }
+
+    std::string showPath(const CanonPath & path) override
+    {
+        return next->showPath(path);
+    }
+
+    std::optional<std::filesystem::path> getPhysicalPath(const CanonPath & path) override
+    {
+        return next->getPhysicalPath(path);
+    }
+};
+
+}

src/libutil/include/nix/util/meson.build

@@ -34,6 +34,7 @@ headers = files(
   'file-system.hh',
   'finally.hh',
   'fmt.hh',
+  'forwarding-source-accessor.hh',
   'fs-sink.hh',
   'git.hh',
   'hash.hh',