cmd/go/internal/modload: change mvsReqs to store roots instead of a full build list

The mvsReqs implementation has always been a bit ambivalent about
whether the root requirements return the full build list, just the
direct requirements, or some hybrid of the two.

However, a full build list always requires the Target module as the
first entry, and it's easer to remove a redundant leading element from
a slice than to add one. Changing the mvsReqs field to contain
arbitrary roots instead of a full build list eliminates the need to
add redundant elements, at the cost of needing to remove redundant
elements in more places.

For #36460

Change-Id: Idd4c2d6bc7b66f67680037dab1fb9c2d1b40ab93
Reviewed-on: https://go-review.googlesource.com/c/go/+/308811
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/cmd/go/internal/modload/mvs.go b/src/cmd/go/internal/modload/mvs.go
index 5f52017a74..87619b4ace 100644
--- a/src/cmd/go/internal/modload/mvs.go
+++ b/src/cmd/go/internal/modload/mvs.go
@@ -38,14 +38,14 @@ func cmpVersion(v1, v2 string) int {
 // mvsReqs implements mvs.Reqs for module semantic versions,
 // with any exclusions or replacements applied internally.
 type mvsReqs struct {
-	buildList []module.Version
+	roots []module.Version
 }
 
 func (r *mvsReqs) Required(mod module.Version) ([]module.Version, error) {
 	if mod == Target {
 		// Use the build list as it existed when r was constructed, not the current
 		// global build list.
-		return r.buildList[1:], nil
+		return r.roots, nil
 	}
 
 	if mod.Version == "none" {