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-test-support/include/nix/expr/tests/value/context.hh

@@ -23,6 +23,11 @@ struct Arbitrary<NixStringContextElem::DrvDeep> {
     static Gen<NixStringContextElem::DrvDeep> arbitrary();
 };
 
+template<>
+struct Arbitrary<NixStringContextElem::Path> {
+    static Gen<NixStringContextElem::Path> arbitrary();
+};
+
 template<>
 struct Arbitrary<NixStringContextElem> {
     static Gen<NixStringContextElem> arbitrary();

src/libexpr-test-support/tests/value/context.cc

@@ -15,6 +15,15 @@ Gen<NixStringContextElem::DrvDeep> Arbitrary<NixStringContextElem::DrvDeep>::arb
     });
 }
 
+Gen<NixStringContextElem::Path> Arbitrary<NixStringContextElem::Path>::arbitrary()
+{
+    return gen::map(gen::arbitrary<StorePath>(), [](StorePath storePath) {
+        return NixStringContextElem::Path{
+            .storePath = storePath,
+        };
+    });
+}
+
 Gen<NixStringContextElem> Arbitrary<NixStringContextElem>::arbitrary()
 {
     return gen::mapcat(
@@ -30,6 +39,9 @@ Gen<NixStringContextElem> Arbitrary<NixStringContextElem>::arbitrary()
             case 2:
                 return gen::map(
                     gen::arbitrary<NixStringContextElem::Built>(), [](NixStringContextElem a) { return a; });
+            case 3:
+                return gen::map(
+                    gen::arbitrary<NixStringContextElem::Path>(), [](NixStringContextElem a) { return a; });
             default:
                 assert(false);
             }