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

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>

nix/readTree/tests/default.nix

@@ -79,7 +79,17 @@ let
       (import ./test-tree-traversal/default-nix/can-be-drv/default.nix {}))
   ];
 
+  # these each call readTree themselves because the throws have to happen inside assertThrows
+  wrong = it "cannot read these files and will complain" [
+    (assertThrows "this file is not a function"
+      (depot.nix.readTree {} ./test-wrong-not-a-function).not-a-function)
+    # can’t test for that, assertThrows can’t catch this error
+    # (assertThrows "this file is a function but doesn’t have dots"
+    #   (depot.nix.readTree {} ./test-wrong-no-dots).no-dots-in-function)
+  ];
+
 in runTestsuite "readTree" [
   example
   traversal-logic
+  wrong
 ]

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."

nix/readTree/tests/test-wrong-not-a-function/not-a-function.nix

@@ -0,0 +1 @@
+"This file needs to be a function, otherwise readTree doesn’t like it!"