I don’t know to which component to report this bug, so I thought I’d report it here.
TL;DR mandatory quotation-marks in arguments to PPX rewriter ppx_expect (due to ppxlib’s use of Ocaml’s AST parser to parse command-line arguments) breaks ability to invoke ppx_expect rewriter via ocamlfind/make/ocamlc (unless the user is willing to double-quote-then-single-quote-then-single-quote, escaping all the way).
Here’s a Git repo that repros the problem: https://github.com/chetmurthy/dune-ocaml-ppx-bug
In the repro, you (a) run with dune, then (b)run a script that directly invokes the PPX rewriters, then © run a script that does what should work using ocamlfind (but does not). I haven’t bothered to figure out how to do the dq/sq/sq-escaping I describe above, b/c I use camlp5-based PPX rewriters, but I figured I should report this bug, nonetheless.
Also, thank you to @Kakadu for testing out this repro and finding a number of environmental defects.