emacsPackagesNg.racer-mode: fix default value for RUST_SRC_PATH

pkgs/applications/editors/emacs-modes/melpa-packages.nix

@@ -207,6 +207,13 @@ self:
       # upstream issue: missing file header
       qiita = markBroken super.qiita;
 
+      racer = super.racer.overrideAttrs (attrs: {
+        postPatch = attrs.postPatch or "" + ''
+          substituteInPlace racer.el \
+            --replace /usr/local/src/rust/src ${external.rustPlatform.rustcSrc}
+        '';
+      });
+
       # upstream issue: missing file footer
       seoul256-theme = markBroken super.seoul256-theme;
 

pkgs/top-level/all-packages.nix

@@ -16853,7 +16853,8 @@ in
       inherit (haskellPackages) ghc-mod structured-haskell-mode Agda hindent;
       inherit (pythonPackages) elpy;
       inherit
-        autoconf automake git libffi libpng pkgconfig poppler rtags w3m zlib;
+        autoconf automake git libffi libpng pkgconfig poppler rtags w3m zlib
+        substituteAll rustPlatform;
     };
   };