Devirtualize double-copied paths

Borrowed from the original lazy-trees branch.

src/libexpr/eval.cc

@@ -2317,6 +2317,9 @@ BackedStringView EvalState::coerceToString(
     }
 
     if (v.type() == nPath) {
+        // FIXME: instead of copying the path to the store, we could
+        // return a virtual store path that lazily copies the path to
+        // the store in devirtualize().
         return
             !canonicalizePath && !copyToStore
             ? // FIXME: hack to preserve path literals that end in a
@@ -2406,7 +2409,7 @@ StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePat
                 *store,
                 path.resolveSymlinks(SymlinkResolution::Ancestors),
                 settings.readOnlyMode ? FetchMode::DryRun : FetchMode::Copy,
-                path.baseName(),
+                computeBaseName(path),
                 ContentAddressMethod::Raw::NixArchive,
                 nullptr,
                 repair);

src/libexpr/include/nix/expr/eval.hh

@@ -586,6 +586,19 @@ public:
 
     StorePath copyPathToStore(NixStringContext & context, const SourcePath & path);
 
+
+    /**
+     * Compute the base name for a `SourcePath`. For non-store paths,
+     * this is just `SourcePath::baseName()`. But for store paths, for
+     * backwards compatibility, it needs to be `<hash>-source`,
+     * i.e. as if the path were copied to the Nix store. This results
+     * in a "double-copied" store path like
+     * `/nix/store/<hash1>-<hash2>-source`. We don't need to
+     * materialize /nix/store/<hash2>-source though. Still, this
+     * requires reading/hashing the path twice.
+     */
+    std::string computeBaseName(const SourcePath & path);
+
     /**
      * Path coercion.
      *

src/libexpr/paths.cc

@@ -52,4 +52,19 @@ std::string EvalState::devirtualize(std::string_view s, const NixStringContext &
     return rewriteStrings(std::string(s), rewrites);
 }
 
+std::string EvalState::computeBaseName(const SourcePath & path)
+{
+    if (path.accessor == rootFS) {
+        if (auto storePath = store->maybeParseStorePath(path.path.abs())) {
+            warn(
+                "Performing inefficient double copy of path '%s' to the store. "
+                "This can typically be avoided by rewriting an attribute like `src = ./.` "
+                "to `src = builtins.path { path = ./.; name = \"source\"; }`.",
+                path);
+            return std::string(fetchToStore(*store, path, FetchMode::DryRun).to_string());
+        }
+    }
+    return std::string(path.baseName());
+}
+
 }

src/libexpr/primops.cc

@@ -2539,7 +2539,7 @@ static void prim_filterSource(EvalState & state, const PosIdx pos, Value * * arg
         "while evaluating the second argument (the path to filter) passed to 'builtins.filterSource'");
     state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.filterSource");
 
-    addPath(state, pos, path.baseName(), path, args[0], ContentAddressMethod::Raw::NixArchive, std::nullopt, v, context);
+    addPath(state, pos, state.computeBaseName(path), path, args[0], ContentAddressMethod::Raw::NixArchive, std::nullopt, v, context);
 }
 
 static RegisterPrimOp primop_filterSource({