Added feature flag to enable dune cache by default

Signed-off-by: Ambre Austen Suhamy <ambre@tarides.com>

src/dune_config_file/dune_config_file.ml

@@ -11,6 +11,7 @@ module Dune_config = struct
   module Log = Dune_util.Log
   module Config = Dune_config.Config
   module Execution_env = Dune_util.Execution_env
+  module Feature_flags = Feature_flags
 
   (* the configuration file use the same version numbers as dune-project files for
      simplicity *)
@@ -279,9 +280,14 @@ module Dune_config = struct
     ; concurrency = (if Execution_env.inside_dune then Fixed 1 else Auto)
     ; terminal_persistence = Clear_on_rebuild
     ; sandboxing_preference = []
-    ; cache_enabled = `Disabled
-    ; cache_reproducibility_check = Skip
-    ; cache_storage_mode = None
+    ; cache_enabled =
+        (if Feature_flags.cache_enabled_by_default then `Enabled else `Disabled)
+    ; cache_reproducibility_check =
+        (if Feature_flags.cache_enabled_by_default then Check else Skip)
+    ; cache_storage_mode =
+        (if Feature_flags.cache_enabled_by_default
+         then Some (Dune_cache_storage.Mode.default ())
+         else None)
     ; action_stdout_on_success = Print
     ; action_stderr_on_success = Print
     ; experimental = []

src/dune_config_file/feature_flags.ml

@@ -0,0 +1 @@
+let cache_enabled_by_default = false

src/dune_rules/format_rules.mli

@@ -8,7 +8,7 @@ open Import
 val gen_rules : Super_context.t -> output_dir:Path.Build.t -> unit Memo.t
 
 (** This must be called from the main directory, i.e. the ones containing the
-    source files and the the [formatted_dir_basename] sub-directory. *)
+    source files and the [formatted_dir_basename] sub-directory. *)
 val setup_alias : dir:Path.Build.t -> unit Memo.t
 
 val formatted_dir_basename : Filename.t

test/blackbox-tests/test-cases/default-cache.t

@@ -0,0 +1,34 @@
+The dune cache should be enabled by 'default'
+(not by default on main branch since it's a feature flag)
+  $ export DUNE_CACHE=enabled
+  $ echo "(lang dune 3.16)" > dune-project
+
+  $ cat > dune << EOF
+  > (library
+  >  (name foo))
+  > EOF
+
+  $ cat > foo.ml << EOF
+  > let f x y = x + y
+  > EOF
+
+Set up cache directory
+  $ export DUNE_CACHE_ROOT=$(pwd)/dune_test_cache
+  $ mkdir $DUNE_CACHE_ROOT
+
+  $ DUNE_CACHE=disabled dune build
+  $ ls $DUNE_CACHE_ROOT
+We have not written anything to the cache yet.
+
+Change source files to force a recompilation
+  $ cat > foo.ml << EOF
+  > let f x y = x - y
+  > EOF
+
+  $ dune build
+  $ ls $DUNE_CACHE_ROOT
+  files
+  meta
+  temp
+  values
+Cache has been written to!