cmt files as input and output proposal

Signed-off-by: Rudi Grinberg <[email protected]>

rfcs/cmt_input.md

@@ -0,0 +1,52 @@
+# Cmt files - both as input and output
+
+This RFC asks for two separate additions to the compiler front end. They are
+technically independent, but mostly useful together. So I decided to bundled
+them.
+
+The first request is to type check compilation units and produce `.cmt` files:
+
+```
+$ ocamlc -c foo.ml -o foo.cmt
+```
+
+The second request is to take `.cmt` files as input to produce object files:
+
+```
+$ ocamlc -c foo.cmt -o foo.cmo
+$ ocamlopt -c foo.cmt -o foo.cmx
+```
+
+How does this help? The motivation is mainly to simplify some rough edges of
+build tools and improve compilation speed in some use cases. Some concrete
+examples where this is useful:
+
+## Fast "check" mode
+
+Dune's check mode (`$ dune build @check`) attempts to verify the user's code
+type checks in the fastest way possible. Currently, this is done by building all
+`.cmi`, `.cmt`, `.cmti` targets. Targets such as `.cma`, `.cmx`, `.cmxa`, files
+are omitted as an optimization. Obviously producing `.cmt` files requires
+building bytecode, which slows things down somewhat.
+
+## Duplicate warnings
+
+When a project is built in both bytecode and native mode concurrently,
+compilation warnings are displayed twice (once per mode). This is bad user
+experience. Build systems such as dune need to add hacks to filter duplicate
+errors to make things more bearable. A separate type checking phase would now be
+responsible for these errors and such warnings would only appear once.
+
+## Speed up compilation when type checking is a bottle neck
+
+This likely doesn't occur very often, but there are cases where type checking is
+slow. This proposal should improve compilation speed in such projects, because
+type checking will be done only once per compilation unit.
+
+## More flexible build rules for users
+
+This one is kind of dune specific, but I'm sure it would help other build
+systems as well. Since dune uses bytecode targets to produce `.cmt` files, users
+cannot have full editor integration (since it requires `.cmt` files) and disable
+bytecode builds. It could be argued that dune can be improved to produce `.cmt`
+files via the native rules, but this a needless complication.