cmd/go: re-enable 'go list -m' with -mod=vendor for limited patterns

I had prohibited 'go list -m' with -mod=vendor because the module
graph is incomplete, but I've realized that many queries do not
actually require the full graph — and may, in fact, be driven using
modules previously reported by 'go list' for specific, vendored
packages. Queries for those modules should succeed.

Updates #33848

Change-Id: I1000b4cf586a830bb78faf620ebf62d73a3cb300
Reviewed-on: https://go-review.googlesource.com/c/go/+/203138
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 15 insertions, 4 deletions

diff --git a/src/cmd/go/internal/modload/list.go b/src/cmd/go/internal/modload/list.go
index 6c0b3945cb..cd162f8875 100644
--- a/src/cmd/go/internal/modload/list.go
+++ b/src/cmd/go/internal/modload/list.go
@@ -11,6 +11,7 @@ import (
 	"strings"
 
 	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
 	"cmd/go/internal/modinfo"
 	"cmd/go/internal/module"
 	"cmd/go/internal/par"
@@ -124,10 +125,20 @@ func listModules(args []string, listVersions bool) []*modinfo.ModulePublic {
 					}
 					continue
 				}
-				mods = append(mods, &modinfo.ModulePublic{
-					Path:  arg,
-					Error: modinfoError(arg, "", errors.New("not a known dependency")),
-				})
+				if cfg.BuildMod == "vendor" {
+					// In vendor mode, we can't determine whether a missing module is “a
+					// known dependency” because the module graph is incomplete.
+					// Give a more explicit error message.
+					mods = append(mods, &modinfo.ModulePublic{
+						Path:  arg,
+						Error: modinfoError(arg, "", errors.New("can't resolve module using the vendor directory\n\t(Use -mod=mod or -mod=readonly to bypass.)")),
+					})
+				} else {
+					mods = append(mods, &modinfo.ModulePublic{
+						Path:  arg,
+						Error: modinfoError(arg, "", errors.New("not a known dependency")),
+					})
+				}
 			} else {
 				fmt.Fprintf(os.Stderr, "warning: pattern %q matched no module dependencies\n", arg)
 			}