Emit a warning if path dependency does not exist, closes #13915

Author: josevalim

lib/mix/lib/mix/dep/fetcher.ex

@@ -51,6 +51,10 @@ defmodule Mix.Dep.Fetcher do
     cond do
       # Dependencies that cannot be fetched are always compiled afterwards
       not scm.fetchable?() ->
+        if not scm.checked_out?(opts) do
+          Mix.shell().error("warning: missing dependency #{format_dep(dep)}")
+        end
+
         {dep, [app | acc], lock}
 
       # If the dependency is not available or we have a lock mismatch

lib/mix/test/mix/dep_test.exs

@@ -295,12 +295,11 @@ defmodule Mix.DepTest do
 
         Mix.Tasks.Deps.run([])
         assert_received {:mix_shell, :info, ["* deps_repo1" <> _]}
-        assert_received {:mix_shell, :info, [_]}
         assert_received {:mix_shell, :info, ["* deps_repo2" <> _]}
-        assert_received {:mix_shell, :info, [_]}
         assert_received {:mix_shell, :info, ["* git_repo" <> _]}
-        assert_received {:mix_shell, :info, [msg]}
-        assert msg =~ "different specs were given for the git_repo"
+
+        assert_received {:mix_shell, :info,
+                         ["  different specs were given for the git_repo app" <> _]}
       end)
     end)
   end
@@ -343,12 +342,11 @@ defmodule Mix.DepTest do
 
         Mix.Tasks.Deps.run([])
         assert_received {:mix_shell, :info, ["* deps_repo1" <> _]}
-        assert_received {:mix_shell, :info, [_]}
         assert_received {:mix_shell, :info, ["* deps_repo2" <> _]}
-        assert_received {:mix_shell, :info, [_]}
         assert_received {:mix_shell, :info, ["* git_repo" <> _]}
-        assert_received {:mix_shell, :info, [msg]}
-        assert msg =~ "different specs were given for the git_repo"
+
+        assert_received {:mix_shell, :info,
+                         ["  different specs were given for the git_repo app" <> _]}
       end)
     end)
   end