dep: Add ineffectual constraints finder and warn

Finally fixes #302.

cmd/dep/ensure.go

@@ -197,6 +197,20 @@ func (cmd *ensureCommand) Run(ctx *dep.Ctx, args []string) error {
 			ctx.Out.Println(err)
 		}
 	}
+	if ineffs := p.FindIneffectualConstraints(sm); len(ineffs) > 0 {
+		ctx.Err.Printf("Warning: the following project(s) have [[constraint]] stanzas in %s:\n\n", dep.ManifestName)
+		for _, ineff := range ineffs {
+			ctx.Err.Println("  ✗ ", ineff)
+		}
+		// TODO(sdboyer) lazy wording, it does not mention ignores at all
+		ctx.Err.Printf("\nHowever, these projects are not direct dependencies of the current project:\n")
+		ctx.Err.Printf("they are not imported in any .go files, nor are they in the 'required' list in\n")
+		ctx.Err.Printf("%s. Dep only applies [[constraint]] rules to direct dependencies, so\n", dep.ManifestName)
+		ctx.Err.Printf("these rules will have no effect.\n\n")
+		ctx.Err.Printf("Either or import/require packages from these projects to make them into direct\n")
+		ctx.Err.Printf("dependencies, or convert the [[constraint]] to an [[override]] to enforce rules\n")
+		ctx.Err.Printf("on these projects if they are transitive dependencies,\n\n")
+	}
 
 	if cmd.add {
 		return cmd.runAdd(ctx, args, p, sm, params)

project.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"sort"
 
 	"github.com/golang/dep/gps"
 	"github.com/golang/dep/gps/paths"
@@ -218,6 +219,35 @@ func (p *Project) GetDirectDependencyNames(sm gps.SourceManager) (map[gps.Projec
 	return directDeps, nil
 }
 
+// FindIneffectualConstraints looks for constraint rules expressed in the
+// manifest that will have no effect during solving, as they are specified for
+// projects that are not direct dependencies of the Project.
+//
+// "Direct dependency" here is as implemented by GetDirectDependencyNames() -
+// after all "ignored" and "required" rules have been considered.
+func (p *Project) FindIneffectualConstraints(sm gps.SourceManager) []gps.ProjectRoot {
+	if p.Manifest == nil {
+		return nil
+	}
+
+	dd, err := p.GetDirectDependencyNames(sm)
+	if err != nil {
+		return nil
+	}
+
+	var ineff []gps.ProjectRoot
+	for pr := range p.Manifest.DependencyConstraints() {
+		if !dd[pr] {
+			ineff = append(ineff, pr)
+		}
+	}
+
+	sort.Slice(ineff, func(i, j int) bool {
+		return ineff[i] < ineff[j]
+	})
+	return ineff
+}
+
 // BackupVendor looks for existing vendor directory and if it's not empty,
 // creates a backup of it to a new directory with the provided suffix.
 func BackupVendor(vpath, suffix string) (string, error) {