From 1298db255280f4799c4a30acae570984255d299c Mon Sep 17 00:00:00 2001
From: "Jonathan D.A. Jewell" <6759885+hyperpolymath@users.noreply.github.com>
Date: Mon, 3 Aug 2026 14:31:31 +0100
Subject: [PATCH] fix(coverage): make bisect_ppx actually instrument the
 library
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The coverage job runs, passes, and measures nothing.

    $ dune runtest --force --instrument-with bisect_ppx
    537 tests OK, exit 0
    $ bisect-ppx-report summary
    Error: no *.coverage files found

Zero .coverage files are written anywhere in the tree.

== Root cause

lib/dune declares `(preprocess (pps ...))` but no instrumentation backend.
`--instrument-with bisect_ppx` only instruments libraries that opt in with an
`(instrumentation (backend bisect_ppx))` stanza; without one, dune instruments
nothing, the tests run uninstrumented, and no coverage data exists to report.

== Why nobody noticed

.github/workflows/ci.yml:213 sets `continue-on-error: true` on the step that
calls bisect-ppx-report. So the report failed on every run, the failure was
swallowed, and the coverage job went green while producing no data. The same
hollow-gate shape as a test that passes without running.

The `continue-on-error` is left in place — it is defensible for a
visibility-only artefact step, and with this fix the step now has something to
report. Worth revisiting separately if coverage ever becomes gating.

== After the fix

    .coverage files: 0 -> 2
    Coverage: 8103/16243 (49.89%)

That number has never been measurable before, so it is a baseline rather than a
regression or an improvement.

== What it says needs attention

Core paths are reasonably covered:

    parser.ml      75.64%   969/1281
    quantity.ml    79.28%   241/304
    typecheck.ml   68.37%  1042/1524
    borrow.ml      68.03%   800/1176
    resolve.ml     59.21%   405/684
    lexer.ml       58.33%    49/84

The gaps are concentrated in backends and tooling, not the front end:

    wasm_gc.ml      0.00%     0/178    <- entirely unexercised
    wasm.ml         0.24%     1/423
    types.ml        9.95%    19/191
    lean_codegen   10.48%    13/124
    value.ml       13.13%    26/198
    c_codegen.ml   14.60%    67/459
    lsp_server.ml  15.47%    43/278

wasm_gc.ml at 0/178 is the standout: 178 points, none reached by any of the 537
tests.

One change only: lib/dune.
---
 lib/dune | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/lib/dune b/lib/dune
index e85af42f..df737673 100644
--- a/lib/dune
+++ b/lib/dune
@@ -114,7 +114,15 @@
  (flags
   (:standard -w -8-9))
  (preprocess
-  (pps ppx_deriving.show ppx_deriving.eq ppx_deriving.ord sedlex.ppx)))
+  (pps ppx_deriving.show ppx_deriving.eq ppx_deriving.ord sedlex.ppx))
+ ; Without this stanza `dune runtest --instrument-with bisect_ppx` has nothing
+ ; to attach to: dune instruments nothing, the tests run uninstrumented, and no
+ ; .coverage files are written. `bisect-ppx-report summary` then fails with
+ ; "no *.coverage files found" — and because the CI step that calls it carries
+ ; `continue-on-error: true`, that failure was swallowed and the coverage job
+ ; went green while measuring nothing.
+ (instrumentation
+  (backend bisect_ppx)))
 
 (menhir
  (modules parser)