mode = "auto"
	} else if gotoolchain == "path" {
		mode = "path"
	} else {
		min, suffix, plus := strings.Cut(gotoolchain, "+") // go1.2.3+auto
		if min != "local" {
			v := gover.FromToolchain(min)
			if v == "" {
				if plus {
					base.Fatalf("invalid GOTOOLCHAIN %q: invalid minimum toolchain %q", gotoolchain, min)
				}
				base.Fatalf("invalid GOTOOLCHAIN %q", gotoolchain)
			}

	rootPruning := orig.pruning
	for _, m := range mustSelect {
		if m.Path == "go" {
			rootPruning = pruningForGoVersion(m.Version)
			break
		} else if m.Path == "toolchain" && pruningForGoVersion(gover.FromToolchain(m.Version)) == unpruned {
			// We don't know exactly what go version we will end up at, but we know
			// that it must be a version supported by the requested toolchain, and
			// that toolchain does not support pruning.