fix(tvix/eval): consider `let ... inherit ...` in dynamic scopes r/4598

In conditions where no dynamic identifiers exist in a scope,
inheriting is usually a no-op - *unless* the identifier is not
statically known and the scope has a non-empty `with`-stack.

Change-Id: Iff4138d9cd4c56e844bc574203708dacc11c3f73
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6264
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

2 files changed, 16 insertions, 0 deletions

diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.exp
new file mode 100644
index 000000000000..d00491fd7e5b
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.exp
@@ -0,0 +1 @@
+1
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.nix
new file mode 100644
index 000000000000..d335e5363000
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-useless-inherit-with.nix
@@ -0,0 +1,15 @@
+# Normally using an `inherit` without a source attribute set within a
+# `let` is a no-op, *unless* there is a with in-scope that might
+# provide the value.
+
+# Provide a dynamic `x` identifier in the scope.
+with ({ x = 1;});
+
+# inherit this `x` as a static identifier
+let inherit x;
+
+# Provide another dynamic `x` identifier
+in with ({ x = 3; });
+
+# Inherited static identifier should have precedence
+x