add comment on why plugins list uses checkouts_dir not checkouts_out_dir

rebar.config.sample

@@ -122,6 +122,8 @@
 {root_dir, "."}.
 %% where checkout dependencies are to be located
 {checkouts_dir, "_checkouts"}.
+%% where, under <base_dir>/<profile> checkout dependencies are to be built
+{checkouts_out_dir, "checkouts"}.
 %% directory in '<base_dir>/<profile>/' where plugins go
 {plugins_dir, "plugins"}.
 %% directories where OTP applications for the project can be located

src/rebar_env.erl

@@ -12,6 +12,7 @@
 %% REBAR_BUILD_DIR         = rebar_dir:base_dir/1
 %% REBAR_ROOT_DIR          = rebar_dir:root_dir/1
 %% REBAR_CHECKOUTS_DIR     = rebar_dir:checkouts_dir/1
+%% REBAR_CHECKOUTS_OUT_DIR = rebar_dir:checkouts_out_dir/1
 %% REBAR_PLUGINS_DIR       = rebar_dir:plugins_dir/1
 %% REBAR_GLOBAL_CONFIG_DIR = rebar_dir:global_config_dir/1
 %% REBAR_GLOBAL_CACHE_DIR  = rebar_dir:global_cache_dir/1
@@ -42,6 +43,7 @@ create_env(State, Opts) ->
         {"REBAR_BUILD_DIR",         filename:absname(rebar_dir:base_dir(State))},
         {"REBAR_ROOT_DIR",          filename:absname(rebar_dir:root_dir(State))},
         {"REBAR_CHECKOUTS_DIR",     filename:absname(rebar_dir:checkouts_dir(State))},
+        {"REBAR_CHECKOUTS_OUT_DIR", filename:absname(rebar_dir:checkouts_out_dir(State))},
         {"REBAR_PLUGINS_DIR",       filename:absname(rebar_dir:plugins_dir(State))},
         {"REBAR_GLOBAL_CONFIG_DIR", filename:absname(rebar_dir:global_config_dir(State))},
         {"REBAR_GLOBAL_CACHE_DIR",  filename:absname(rebar_dir:global_cache_dir(Opts))},

src/rebar_prv_plugins.erl

@@ -44,6 +44,10 @@ do(State) ->
     Plugins = rebar_state:get(State, plugins, []),
     ProjectPlugins = rebar_state:get(State, project_plugins, []),
     PluginsDirs = filelib:wildcard(filename:join(rebar_dir:plugins_dir(State), "*")),
+
+    %% use `checkouts_dir' and not `checkouts_out_dir'. Since we use `all' in `find_apps'
+    %% so it doesn't need to be built and the apps in `checkouts_dir' could be old
+    %% because the user removing from `_checkouts/' doesn't cause removal of the output
     CheckoutsDirs = filelib:wildcard(filename:join(rebar_dir:checkouts_dir(State), "*")),
     Apps = rebar_app_discover:find_apps(CheckoutsDirs++PluginsDirs, SrcDirs, all, State),
     display_plugins("Local plugins", Apps, Plugins ++ ProjectPlugins),