cmd/go: add a better error message when in a module outside workspace

When the user is trying to list or build a package in a module that's
outside of the workspace provide a more clear message hinting to the
user that they can add the module to the workspace using go work use.

Fixes #51604

Change-Id: I1202ecb2f22fd6351bfdec88ed613b8167687fb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/400014
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index e85a33dd50..7f1a88ffe7 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -605,11 +605,13 @@ func resolveLocalPackage(ctx context.Context, dir string, rs *Requirements) (str
 
 	pkg := pathInModuleCache(ctx, absDir, rs)
 	if pkg == "" {
-		scope := "main module or its selected dependencies"
 		if inWorkspaceMode() {
-			scope = "modules listed in go.work or their selected dependencies"
+			if mr := findModuleRoot(absDir); mr != "" {
+				return "", fmt.Errorf("directory %s is contained in a module that is not one of the workspace modules listed in go.work. You can add the module to the workspace using go work use %s", base.ShortPath(absDir), base.ShortPath(mr))
+			}
+			return "", fmt.Errorf("directory %s outside modules listed in go.work or their selected dependencies", base.ShortPath(absDir))
 		}
-		return "", fmt.Errorf("directory %s outside %s", base.ShortPath(absDir), scope)
+		return "", fmt.Errorf("directory %s outside main module or its selected dependencies", base.ShortPath(absDir))
 	}
 	return pkg, nil
 }