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>

src/cmd/go/internal/modload/buildlist.go

@@ -492,7 +492,7 @@ func editRequirements(ctx context.Context, rs *Requirements, add, mustSelect []m
 		// remove roots.)
 	}
 
-	min, err := mvs.Req(Target, rootPaths, &mvsReqs{buildList: final})
+	min, err := mvs.Req(Target, rootPaths, &mvsReqs{roots: final[1:]})
 	if err != nil {
 		return nil, false, err
 	}
@@ -610,7 +610,7 @@ func updateRoots(ctx context.Context, depth modDepth, direct map[string]bool, pk
 			// dependencies, then we can't reliably compute a minimal subset of them.
 			return rs, err
 		}
-		keep = mg.BuildList()
+		keep = mg.BuildList()[1:]
 
 		for _, root := range rs.rootModules {
 			// If the selected version of the root is the same as what was already
@@ -625,7 +625,6 @@ func updateRoots(ctx context.Context, depth modDepth, direct map[string]bool, pk
 			}
 		}
 	} else {
-		keep = append(keep, Target)
 		kept := map[module.Version]bool{Target: true}
 		for _, pkg := range pkgs {
 			if pkg.mod.Path != "" && !kept[pkg.mod] {
@@ -691,7 +690,7 @@ func updateRoots(ctx context.Context, depth modDepth, direct map[string]bool, pk
 		return rs, nil
 	}
 
-	min, err := mvs.Req(Target, rootPaths, &mvsReqs{buildList: keep})
+	min, err := mvs.Req(Target, rootPaths, &mvsReqs{roots: keep})
 	if err != nil {
 		return rs, err
 	}

src/cmd/go/internal/modload/edit.go

@@ -49,7 +49,7 @@ func editBuildList(ctx context.Context, initial, tryUpgrade, mustSelect []module
 	// percieves as “downgrades” will not also result in upgrades.
 	max := make(map[string]string)
 	maxes, err := mvs.Upgrade(Target, &mvsReqs{
-		buildList: append(capVersionSlice(initial), mustSelect...),
+		roots: append(capVersionSlice(initial[1:]), mustSelect...),
 	}, tryUpgrade...)
 	if err != nil {
 		return nil, err
@@ -108,12 +108,12 @@ func editBuildList(ctx context.Context, initial, tryUpgrade, mustSelect []module
 	// another. The lower version may require extraneous dependencies that aren't
 	// actually relevant, so we need to compute the actual selected versions.
 	adjusted := make([]module.Version, 0, len(maxes))
-	for _, m := range maxes {
+	for _, m := range maxes[1:] {
 		if v, ok := limiter.selected[m.Path]; ok {
 			adjusted = append(adjusted, module.Version{Path: m.Path, Version: v})
 		}
 	}
-	consistent, err := mvs.BuildList(Target, &mvsReqs{buildList: adjusted})
+	consistent, err := mvs.BuildList(Target, &mvsReqs{roots: adjusted})
 	if err != nil {
 		return nil, err
 	}
@@ -122,7 +122,7 @@ func editBuildList(ctx context.Context, initial, tryUpgrade, mustSelect []module
 	// the actually-selected versions in order to eliminate extraneous
 	// dependencies from lower-than-selected ones.
 	compacted := consistent[:0]
-	for _, m := range consistent {
+	for _, m := range consistent[1:] {
 		if _, ok := limiter.selected[m.Path]; ok {
 			// The fact that the limiter has a version for m.Path indicates that we
 			// care about retaining that path, even if the version was upgraded for
@@ -131,7 +131,7 @@ func editBuildList(ctx context.Context, initial, tryUpgrade, mustSelect []module
 		}
 	}
 
-	return mvs.BuildList(Target, &mvsReqs{buildList: compacted})
+	return mvs.BuildList(Target, &mvsReqs{roots: compacted})
 }
 
 // A versionLimiter tracks the versions that may be selected for each module

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" {