cmd/go: add a -compat flag to 'go mod tidy'

Fixes #46141

Change-Id: I9d4032e75252ade9eaa937389ea97ef3fb287499
Reviewed-on: https://go-review.googlesource.com/c/go/+/321071
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
index fcc7f36335..bad2b7f16e 100644
--- a/src/cmd/go/alldocs.go
+++ b/src/cmd/go/alldocs.go
@@ -1221,7 +1221,7 @@
 //
 // Usage:
 //
-// 	go mod tidy [-e] [-v] [-go=version]
+// 	go mod tidy [-e] [-v] [-go=version] [-compat=version]
 //
 // Tidy makes sure go.mod matches the source code in the module.
 // It adds any missing modules necessary to build the current module's
@@ -1241,6 +1241,14 @@
 // (Go versions 1.17 and higher retain more requirements in order to
 // support lazy module loading.)
 //
+// The -compat flag preserves any additional checksums needed for the
+// 'go' command from the indicated major Go release to successfully load
+// the module graph, and causes tidy to error out if that version of the
+// 'go' command would load any imported package from a different module
+// version. By default, tidy acts as if the -compat flag were set to the
+// version prior to the one indicated by the 'go' directive in the go.mod
+// file.
+//
 // See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
 //
 //