Add a special type of context for the result of toString

When you apply `builtins.toString` to a path value representing a path
in the Nix store (as is the case with flake inputs), historically you
got a string without context (e.g. `/nix/store/...-source`). This is
broken, since it allows you to pass a store path to a
derivation/toFile without a proper store reference. This is especially
a problem with lazy trees, since the store path is a virtual path that
doesn't exist and can be different every time.

For backwards compatibility, and to warn users about this unsafe use
of `toString`, we now keep track of such strings as a special type of
context.

src/libexpr/primops.cc

@@ -89,6 +89,9 @@ StringMap EvalState::realiseContext(const NixStringContext & context, StorePathS
                 if (maybePathsOut)
                     maybePathsOut->emplace(d.drvPath);
             },
+            [&](const NixStringContextElem::Path & p) {
+                // FIXME
+            },
         }, c.raw);
     }
 
@@ -1438,6 +1441,9 @@ static void derivationStrictInternal(
             [&](const NixStringContextElem::Opaque & o) {
                 drv.inputSrcs.insert(state.devirtualize(o.path, &rewrites));
             },
+            [&](const NixStringContextElem::Path & p) {
+                // FIXME: do something
+            },
         }, c.raw);
     }
 
@@ -2346,10 +2352,21 @@ static void prim_toFile(EvalState & state, const PosIdx pos, Value * * args, Val
     std::string contents(state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.toFile"));
 
     StorePathSet refs;
+    StringMap rewrites;
 
     for (auto c : context) {
         if (auto p = std::get_if<NixStringContextElem::Opaque>(&c.raw))
             refs.insert(p->path);
+        else if (auto p = std::get_if<NixStringContextElem::Path>(&c.raw)) {
+            if (contents.find(p->storePath.to_string()) != contents.npos) {
+                warn(
+                    "Using 'builtins.toFile' to create a file named '%s' that references the store path '%s' without a proper context. "
+                    "The resulting file will not have a correct store reference, so this is unreliable and may stop working in the future.",
+                    name,
+                    state.store->printStorePath(p->storePath));
+                state.devirtualize(p->storePath, &rewrites);
+            }
+        }
         else
             state.error<EvalError>(
                 "files created by %1% may not reference derivations, but %2% references %3%",
@@ -2359,6 +2376,8 @@ static void prim_toFile(EvalState & state, const PosIdx pos, Value * * args, Val
             ).atPos(pos).debugThrow();
     }
 
+    contents = rewriteStrings(contents, rewrites);
+
     auto storePath = settings.readOnlyMode
         ? state.store->makeFixedOutputPathFromCA(name, TextInfo {
             .hash = hashString(HashAlgorithm::SHA256, contents),