cmd/go: smooth out upgrade paths for lazy loading

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>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/cmd/go/internal/modload/edit.go b/src/cmd/go/internal/modload/edit.go
index 2921b38157..c350b9d1b5 100644
--- a/src/cmd/go/internal/modload/edit.go
+++ b/src/cmd/go/internal/modload/edit.go
@@ -229,7 +229,7 @@ func raiseLimitsForUpgrades(ctx context.Context, maxVersion map[string]string, d
 			if err != nil {
 				return err
 			}
-			if summary.depth() == eager {
+			if summary.depth == eager {
 				// For efficiency, we'll load all of the eager upgrades as one big
 				// graph, rather than loading the (potentially-overlapping) subgraph for
 				// each upgrade individually.
@@ -522,7 +522,7 @@ func (l *versionLimiter) check(m module.Version, depth modDepth) dqState {
 		return l.disqualify(m, dqState{err: err})
 	}
 
-	if summary.depth() == eager {
+	if summary.depth == eager {
 		depth = eager
 	}
 	for _, r := range summary.require {