diff --git a/pkgs/build-support/go/module.nix b/pkgs/build-support/go/module.nix
index 6f568c0eb4f9..dce3ddb47090 100644
--- a/pkgs/build-support/go/module.nix
+++ b/pkgs/build-support/go/module.nix
@@ -163,8 +163,10 @@ let
     inherit (go) GOOS GOARCH;
 
     GOFLAGS = GOFLAGS
-      ++ lib.optional (!proxyVendor) "-mod=vendor"
-      ++ lib.optional (!allowGoReference) "-trimpath";
+      ++ lib.warnIf (lib.any (lib.hasPrefix "-mod=") GOFLAGS) "use `proxyVendor` to control Go module/vendor behavior instead of setting `-mod=` in GOFLAGS"
+        (lib.optional (!proxyVendor) "-mod=vendor")
+      ++ lib.warnIf (builtins.elem "-trimpath" GOFLAGS) "`-trimpath` is added by default to GOFLAGS by buildGoModule when allowGoReference isn't set to true"
+        (lib.optional (!allowGoReference) "-trimpath");
     inherit CGO_ENABLED enableParallelBuilding GO111MODULE GOTOOLCHAIN;
 
     # If not set to an explicit value, set the buildid empty for reproducibility.
@@ -196,7 +198,12 @@ let
       runHook postConfigure
     '');
 
-    buildPhase = args.buildPhase or (''
+    buildPhase = args.buildPhase or (
+      lib.warnIf (buildFlags != "" || buildFlagsArray != "")
+        "Use the `ldflags` and/or `tags` attributes instead of `buildFlags`/`buildFlagsArray`"
+      lib.warnIf (builtins.elem "-buildid=" ldflags)
+        "`-buildid=` is set by default as ldflag by buildGoModule"
+    ''
       runHook preBuild
 
       exclude='\(/_\|examples\|Godeps\|testdata'
@@ -313,9 +320,4 @@ let
     } // meta;
   });
 in
-lib.warnIf (buildFlags != "" || buildFlagsArray != "")
-  "Use the `ldflags` and/or `tags` attributes instead of `buildFlags`/`buildFlagsArray`"
-lib.warnIf (builtins.elem "-buildid=" ldflags) "`-buildid=` is set by default as ldflag by buildGoModule"
-lib.warnIf (builtins.elem "-trimpath" GOFLAGS) "`-trimpath` is added by default to GOFLAGS by buildGoModule when allowGoReference isn't set to true"
-lib.warnIf (lib.any (lib.hasPrefix "-mod=") GOFLAGS) "use `proxyVendor` to control Go module/vendor behavior instead of setting `-mod=` in GOFLAGS"
-  package
+package