Why is dune using ocamlc when I asked to use ocamlopt? - ocaml

Question

$ dune build ./src/main.exe --profile=release
ocamlc src/.main.eobjs/byte/ast.{cmi,cmo,cmt} (exit 2)
(cd _build/default && /usr/bin/ocamlc.opt -w -40 -O3 -g -bin-annot -I src/.main.eobjs/byte -I /home/jackprograms/.opam/default/lib/cairo2 -I /home/jackprograms/.opam/default/lib/lablgtk3 -I /usr/lib/ocaml/threads -no-alias-deps -o src/.main.eobjs/byte/ast.cmo -c -impl src/ast.ml)
/usr/bin/ocamlc.opt: unknown option '-O3'.
I ran this, and dune is using ocamlc bytecode... I am using exe which means native, as I saw on the dune documentation. Why is it running ocamlc when it should be running ocamlopt?
(executable
(name main)
(libraries lablgtk3)
(modes exe))
(ocamllex
(modules lexer))
(ocamlyacc
(modules parser))
(env
(dev
(flags (:standard -w +42)))
(release
(flags (:standard -O3))))
^ In src directory

Answer 1

It will still use ocamlc to compile .cmi files.
Instead of using flags (e.g. (flags (:standard -O3))), use ocamlopt_flags.
(executable
(name main)
(libraries lablgtk3)
(modes exe))
(ocamllex
(modules lexer))
(ocamlyacc
(modules parser))
(env
(dev
(flags (:standard -w +42)))
(release
(ocamlopt_flags (:standard -O3))))