Fix the build system when using dune 3.11 and above

HACKING.md

@@ -96,10 +96,7 @@ To rebuild after making changes, you can just type `make`. You need to
 have a working OCaml 4.14 or 4.14.1 compiler on your PATH before doing so,
 e.g. installed via OPAM. You also need to have dune and menhir.
 
-<!-- CR someone: investigate this -->
-The build currently fails when using the latest version of `dune` (3.11.1).
-To install a known-good dune, run `opam pin add dune 3.8.1`. `menhir` should be pinned to a specific
-version as well: `opam pin add menhir 20210419`.
+`menhir` should be pinned to a specific version: `opam pin add menhir 20210419`.
 
 There is a special target `make hacking` which starts Dune in polling mode.  The rebuild
 performed here is equivalent to `make ocamlopt` in the upstream distribution: it rebuilds the

ocaml/HACKING.jst.adoc

@@ -123,4 +123,4 @@ Then build the compiler — the following command will build the compiler us
 We can now benchmark our compiler against `typecore.ml`. The following `_bootinstall` is built using the opam switch and has FP enabled. 
 
     $ cd _build/main
-    $ perf stat -r 5 ../_bootinstall/bin/ocamlc.opt -strict-sequence -principal -w +a-4-9-40-41-42-44-45-48-66-70 -warn-error A -bin-annot -safe-string -strict-formats -w -67 -g -bin-annot -I .ocamlcommon.objs/byte -I ../install/runtime_stdlib/lib/ocaml_runtime_stdlib/  -intf-suffix .ml -no-alias-deps -o .ocamlcommon.objs/byte/typecore.cmo -c -impl typecore.ml
+    $ perf stat -r 5 ../_bootinstall/bin/ocamlc.opt -strict-sequence -principal -w +a-4-9-40-41-42-44-45-48-66-70 -warn-error A -bin-annot -safe-string -strict-formats -w -67 -g -bin-annot -I .ocamlcommon.objs/byte -I ../runtime_stdlib_install/lib/ocaml_runtime_stdlib/  -intf-suffix .ml -no-alias-deps -o .ocamlcommon.objs/byte/typecore.cmo -c -impl typecore.ml

ocaml/Makefile.common-jst

@@ -45,7 +45,7 @@ define dune_main_context
   (profile $(main_build_profile))
   (paths
     (PATH ("$(CURDIR)/_build/_bootinstall/bin" :standard))
-    (OCAMLLIB ("$(CURDIR)/_build/install/runtime_stdlib/lib/ocaml_runtime_stdlib")))
+    (OCAMLLIB ("$(CURDIR)/_build/runtime_stdlib_install/lib/ocaml_runtime_stdlib")))
   (env (_
     (flags (:standard -warn-error +A))
     (env-vars ("OCAMLPARAM" "$(BUILD_OCAMLPARAM)"))))))
@@ -70,8 +70,12 @@ boot-runtest: boot-compiler
 
 runtime-stdlib: boot-compiler
 	RUNTIME_DIR=$(RUNTIME_DIR) $(dune) build $(ws_runstd) --only-package=ocaml_runtime_stdlib @install
-#	dune does not believe the compiler can make .cmxs unless the following file exists
-	@touch _build/install/runtime_stdlib/lib/ocaml_runtime_stdlib/dynlink.cmxa
+	rm -rf _build/runtime_stdlib_install
+# Dune 3.11+ clears `_build/install` before building, move the runtime stdlib
+# to a separate directory.
+	$(cpl) -R _build/install/runtime_stdlib _build/runtime_stdlib_install
+# Dune does not believe the compiler can make .cmxs unless the following file exists.
+	@touch _build/runtime_stdlib_install/lib/ocaml_runtime_stdlib/dynlink.cmxa
 
 compiler: runtime-stdlib
 	RUNTIME_DIR=$(RUNTIME_DIR) $(dune) build $(ws_main) --only-package=ocaml @install \
@@ -164,9 +168,10 @@ cpl=$(if $(filter linux,$(SYSTEM)),cp -l,cp -L)
 _install: compiler
 	rm -rf _install
 	mkdir -p _install/{bin,lib/ocaml}
-	$(cpl) _build/install/{runtime_stdlib,main}/bin/* _install/bin/
+	$(cpl) _build/runtime_stdlib_install/bin/* _install/bin/
+	$(cpl) _build/install/main/bin/* _install/bin/
 	( cd _install/bin; for i in *.opt; do ln -s $$i $${i%.opt}; done )
-	$(cpl) -R _build/install/runtime_stdlib/lib/ocaml_runtime_stdlib/* _install/lib/ocaml/
+	$(cpl) -R _build/runtime_stdlib_install/lib/ocaml_runtime_stdlib/* _install/lib/ocaml/
 	rm -f _install/lib/ocaml/{META,dune-package,Makefile.config,dynlink.cmxa}
 	$(cpl) -R _build/install/main/lib/ocaml/* _install/lib/ocaml/
 	if [ "x$(legacy_layout)" == "xyes" ] ; \