cmd/go: print offending -mod value in workspace mode

Workspace mode only allows -mod to be set to 'readonly'. When returning
the error for other values of -mod, specify the the value in the error
so it's easier to see what's going on when the go command is invoked
through another tool.

For #45713, #48186

Change-Id: I3ba206a71c7a20c18aeccfa48f3c9dc935a180a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/347593
Trust: Michael Matloob <matloob@golang.org>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index a1016ec5a328d583749ef94819d2f2840ce09b38..a855e6c85179f2f81f28d99adbcafd8c51530026 100644 (file)
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -976,9 +976,9 @@ func requirementsFromModFiles(ctx context.Context, modFiles []*modfile.File) *Re
 func setDefaultBuildMod() {
        if cfg.BuildModExplicit {
                if inWorkspaceMode() && cfg.BuildMod != "readonly" {
-                       base.Fatalf("go: -mod may only be set to readonly when in workspace mode." +
-                               "\n\tRemove the -mod flag to use the default readonly value," +
-                               "\n\tor set -workfile=off to disable workspace mode.")
+                       base.Fatalf("go: -mod may only be set to readonly when in workspace mode, but it is set to %q"+
+                               "\n\tRemove the -mod flag to use the default readonly value,"+
+                               "\n\tor set -workfile=off to disable workspace mode.", cfg.BuildMod)
                }
                // Don't override an explicit '-mod=' argument.
                return