feature: introduce a public action tracing library

doc/changes/added/13348.md

@@ -0,0 +1,4 @@
+- Introduce the `dune-action-trace` library. This public library is to be used
+  by custom actions to emit trace events while executed as part of a dune
+  build. The trace events emitted through this library will be incorporated
+  into dune's own trace (#13348, @rgrinberg)

dune-project

@@ -255,3 +255,12 @@ understood by dune language."))
  (maintenance_intent none)
  (description "Misc. Collection of File System Operations"))
 
+(package
+ (name dune-action-trace)
+ (dir otherlibs/dune-action-trace)
+ (synopsis "Dune Action Traces")
+ (depends
+  csexp
+  (ocaml (>= 4.08)))
+ (maintenance_intent none)
+ (description "Produce trace events from dune actions"))

opam/dune-action-trace.opam

@@ -0,0 +1,32 @@
+# This file is generated by dune, edit dune-project instead
+opam-version: "2.0"
+synopsis: "Dune Action Traces"
+description: "Produce trace events from dune actions"
+maintainer: ["Jane Street Group, LLC <opensource@janestreet.com>"]
+authors: ["Jane Street Group, LLC <opensource@janestreet.com>"]
+license: "MIT"
+homepage: "https://github.com/ocaml/dune"
+doc: "https://dune.readthedocs.io/"
+bug-reports: "https://github.com/ocaml/dune/issues"
+depends: [
+  "dune" {>= "3.22"}
+  "csexp"
+  "ocaml" {>= "4.08"}
+  "odoc" {with-doc}
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+]
+dev-repo: "git+https://github.com/ocaml/dune.git"
+x-maintenance-intent: ["none"]

otherlibs/dune-action-trace/dune

@@ -1,3 +1,4 @@
 (library
+ (public_name dune-action-trace)
  (name dune_action_trace)
  (libraries csexp))