1 files changed, 143 insertions, 0 deletions
diff --git a/src/cmd/go/internal/modcmd/why.go b/src/cmd/go/internal/modcmd/why.go
new file mode 100644
index 0000000..198672d
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/why.go
@@ -0,0 +1,143 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modcmd
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modload"
+)
+
+var cmdWhy = &base.Command{
+	UsageLine: "go mod why [-m] [-vendor] packages...",
+	Short:     "explain why packages or modules are needed",
+	Long: `
+Why shows a shortest path in the import graph from the main module to
+each of the listed packages. If the -m flag is given, why treats the
+arguments as a list of modules and finds a path to any package in each
+of the modules.
+
+By default, why queries the graph of packages matched by "go list all",
+which includes tests for reachable packages. The -vendor flag causes why
+to exclude tests of dependencies.
+
+The output is a sequence of stanzas, one for each package or module
+name on the command line, separated by blank lines. Each stanza begins
+with a comment line "# package" or "# module" giving the target
+package or module. Subsequent lines give a path through the import
+graph, one package per line. If the package or module is not
+referenced from the main module, the stanza will display a single
+parenthesized note indicating that fact.
+
+For example:
+
+	$ go mod why golang.org/x/text/language golang.org/x/text/encoding
+	# golang.org/x/text/language
+	rsc.io/quote
+	rsc.io/sampler
+	golang.org/x/text/language
+
+	# golang.org/x/text/encoding
+	(main module does not need package golang.org/x/text/encoding)
+	$
+
+See https://golang.org/ref/mod#go-mod-why for more about 'go mod why'.
+	`,
+}
+
+var (
+	whyM      = cmdWhy.Flag.Bool("m", false, "")
+	whyVendor = cmdWhy.Flag.Bool("vendor", false, "")
+)
+
+func init() {
+	cmdWhy.Run = runWhy // break init cycle
+	base.AddChdirFlag(&cmdWhy.Flag)
+	base.AddModCommonFlags(&cmdWhy.Flag)
+}
+
+func runWhy(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+	modload.ForceUseModules = true
+	modload.RootMode = modload.NeedRoot
+	modload.ExplicitWriteGoMod = true // don't write go.mod in ListModules
+
+	loadOpts := modload.PackageOpts{
+		Tags:                     imports.AnyTags(),
+		VendorModulesInGOROOTSrc: true,
+		LoadTests:                !*whyVendor,
+		SilencePackageErrors:     true,
+		UseVendorAll:             *whyVendor,
+	}
+
+	if *whyM {
+		for _, arg := range args {
+			if strings.Contains(arg, "@") {
+				base.Fatalf("go: %s: 'go mod why' requires a module path, not a version query", arg)
+			}
+		}
+
+		mods, err := modload.ListModules(ctx, args, 0, "")
+		if err != nil {
+			base.Fatal(err)
+		}
+
+		byModule := make(map[string][]string)
+		_, pkgs := modload.LoadPackages(ctx, loadOpts, "all")
+		for _, path := range pkgs {
+			m := modload.PackageModule(path)
+			if m.Path != "" {
+				byModule[m.Path] = append(byModule[m.Path], path)
+			}
+		}
+		sep := ""
+		for _, m := range mods {
+			best := ""
+			bestDepth := 1000000000
+			for _, path := range byModule[m.Path] {
+				d := modload.WhyDepth(path)
+				if d > 0 && d < bestDepth {
+					best = path
+					bestDepth = d
+				}
+			}
+			why := modload.Why(best)
+			if why == "" {
+				vendoring := ""
+				if *whyVendor {
+					vendoring = " to vendor"
+				}
+				why = "(main module does not need" + vendoring + " module " + m.Path + ")\n"
+			}
+			fmt.Printf("%s# %s\n%s", sep, m.Path, why)
+			sep = "\n"
+		}
+	} else {
+		// Resolve to packages.
+		matches, _ := modload.LoadPackages(ctx, loadOpts, args...)
+
+		modload.LoadPackages(ctx, loadOpts, "all") // rebuild graph, from main module (not from named packages)
+
+		sep := ""
+		for _, m := range matches {
+			for _, path := range m.Pkgs {
+				why := modload.Why(path)
+				if why == "" {
+					vendoring := ""
+					if *whyVendor {
+						vendoring = " to vendor"
+					}
+					why = "(main module does not need" + vendoring + " package " + path + ")\n"
+				}
+				fmt.Printf("%s# %s\n%s", sep, path, why)
+				sep = "\n"
+			}
+		}
+	}
+}