cmd/go: link to go.dev/doc/godebug for removed GODEBUG settings

This makes the user experience better, before users would receive
an unknown godebug error message, now we explicitly mention that
it was removed and link to go.dev/doc/godebug where users can find
more information about the removal.

Additionally we keep all the removed GODEBUGs in the source, making
sure we do not reuse such GODEBUG after it is removed.

Updates #72111
Updates #75316

Change-Id: I6a6a6964cce1c100108fdba4bfba7d13cd9a893a
Reviewed-on: https://go-review.googlesource.com/c/go/+/701875
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>

src/cmd/go/internal/modload/init.go

@@ -2242,10 +2242,13 @@ func CheckGodebug(verb, k, v string) error {
 		}
 		return nil
 	}
-	for _, info := range godebugs.All {
+	if godebugs.Lookup(k) != nil {
-		if k == info.Name {
 		return nil
 	}
+	for _, info := range godebugs.Removed {
+		if info.Name == k {
+			return fmt.Errorf("use of removed %s %q, see https://go.dev/doc/godebug#go-1%v", verb, k, info.Removed)
+		}
 	}
 	return fmt.Errorf("unknown %s %q", verb, k)
 }

src/cmd/go/testdata/script/mod_removed_godebug.txt

@@ -0,0 +1,11 @@
+# Test case that makes sure we print a nice error message
+# instead of the generic "unknown godebug" error message
+# for removed GODEBUGs.
+
+! go list
+stderr '^go.mod:3: use of removed godebug "x509sha1", see https://go.dev/doc/godebug#go-124$'
+
+-- go.mod --
+module example.com/bar
+
+godebug x509sha1=1

src/internal/godebugs/godebugs_test.go

@@ -93,3 +93,11 @@ func incNonDefaults(t *testing.T) map[string]bool {
 	}
 	return seen
 }
+
+func TestRemoved(t *testing.T) {
+	for _, info := range godebugs.Removed {
+		if godebugs.Lookup(info.Name) != nil {
+			t.Fatalf("GODEBUG: %v exists in both Removed and All", info.Name)
+		}
+	}
+}

src/internal/godebugs/table.go

@@ -78,6 +78,16 @@ var All = []Info{
 	{Name: "zipinsecurepath", Package: "archive/zip"},
 }
 
+type RemovedInfo struct {
+	Name    string // name of the removed GODEBUG setting.
+	Removed int    // minor version of Go, when the removal happened
+}
+
+// Removed contains all GODEBUGs that we have removed.
+var Removed = []RemovedInfo{
+	{Name: "x509sha1", Removed: 24},
+}
+
 // Lookup returns the Info with the given name.
 func Lookup(name string) *Info {
 	// binary search, avoiding import of sort.