feat(nix/readTree): give better error message when not a function r/2224

When a file is added to the depot tree that is picked up by read-tree,
but it’s not a function like ({...}: {}), `readTree` will fail on the
function application, leading to a bad error message.

We can do slightly better, by checking the type and throwing a nicer
trace message.

`assertMsg` is copied from `nixpkgs/lib/assert.nix`, since at this
point we don’t have a reference to the lib.

There is another evaluation failure that can happen, which is when the
function we try to call does not have dots; however, nix does not
provide any inflection capabilies for checking whether a function
attrset is open (`builtins.functionArgs` only tells us the attrs it
mentions explicitly). Maybe the locality of the error could be
improved somehow.

Change-Id: Ibe38ce78bb56902075f7c31f2eeeb93485b34be3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2469
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

1 files changed, 3 insertions, 0 deletions

diff --git a/nix/readTree/tests/test-wrong-no-dots/no-dots-in-function.nix b/nix/readTree/tests/test-wrong-no-dots/no-dots-in-function.nix
new file mode 100644
index 000000000000..4681253af831
--- /dev/null
+++ b/nix/readTree/tests/test-wrong-no-dots/no-dots-in-function.nix
@@ -0,0 +1,3 @@
+{}:
+
+"This is a function, but readTree wants to pass a bunch of arguments, and not having dots means we depend on exactly which arguments."