diff --git a/pkgs/development/beam-modules/build-rebar3.nix b/pkgs/development/beam-modules/build-rebar3.nix
index 77b40461f80e..f59856953af4 100644
--- a/pkgs/development/beam-modules/build-rebar3.nix
+++ b/pkgs/development/beam-modules/build-rebar3.nix
@@ -89,6 +89,14 @@ let
         ''
         + postPatch;
 
+        preConfigure = ''
+          # Copy the source so it can be used by mix projects
+          # do this before building to avoid build artifacts but after patching
+          # to include any modifications
+          mkdir -p $out/src
+          cp -r "." "$out/src"
+        '';
+
         buildPhase = ''
           runHook preBuild
           HOME=. rebar3 bare compile --paths "."