This change adds two possible upgrade paths for lazy loading:
1. Run 'go mod tidy -go=1.17'.
2. Starting in a module with no existing 'go' directive,
run any 'go' command that updates the go.mod file.
In the latter case, commands other than 'go mod tidy'
may leave the go.mod file *very* untidy if it had non-trivial
dependencies. (The 'go' invocation will promote all
implicit eager dependencies to explicit lazy ones,
which preserves the original module graph — most of which is
not actually relevant.)
'go mod tidy -go=1.17' can be used to enable lazy loading without
accidentally downgrading existing transitive dependencies.
'go mod tidy -go=1.16' can be used to disable lazy loading and clear
away redundant roots in a single step (if reducing the go version), or
to prune away dependencies of tests-of-external-tests (if increasing
the go version).
'go mod tidy -go=1.15' can be used to add dependencies of
tests-of-external-tests, although there isn't much point to that.
DO NOT MERGE
This change still needs an explicit test and a release note.
Fixes #45094
For #36460
Change-Id: I68f057e39489dfd6a667cd11dc1e320c1ee1aec1
Reviewed-on: https://go-review.googlesource.com/c/go/+/315210
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Michael Matloob <matloob@golang.org>