Expand tests to reproduce #7655

The original `builtins.getContext` test from
1d757292d0cb78beec32fcdfe15c2944a4bc4a95 would have caught this. The
problem is that b30be6b450f872f8be6dc8afa28f4b030fa8d1d1 adding
`builtins.appendContext` modified that test to make it test too much at
once, rather than adding a separate test.

We now have isolated tests for both functions, and also a property test
showing everything put together (in the form of an eta rule for strings
with context). This is better coverage and properly reproduces the bug.

1 files changed, 20 insertions, 3 deletions

diff --git a/tests/lang/eval-okay-context-introspection.nix b/tests/lang/eval-okay-context-introspection.nix
index 43178bd2e..50a78d946 100644
--- a/tests/lang/eval-okay-context-introspection.nix
+++ b/tests/lang/eval-okay-context-introspection.nix
@@ -18,7 +18,24 @@ let
     };
   };
 
-  legit-context = builtins.getContext "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+  combo-path = "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+  legit-context = builtins.getContext combo-path;
 
-  constructed-context = builtins.getContext (builtins.appendContext "" desired-context);
-in legit-context == constructed-context
+  reconstructed-path = builtins.appendContext
+    (builtins.unsafeDiscardStringContext combo-path)
+    desired-context;
+
+  # Eta rule for strings with context.
+  etaRule = str:
+    str == builtins.appendContext
+      (builtins.unsafeDiscardStringContext str)
+      (builtins.getContext str);
+
+in [
+  (legit-context == desired-context)
+  (reconstructed-path == combo-path)
+  (etaRule "foo")
+  (etaRule drv.drvPath)
+  (etaRule drv.foo.outPath)
+  (etaRule (builtins.unsafeDiscardOutputDependency drv.drvPath))
+]