cmd/go: add support for go get -tool

Running `go get -tool example.com/m1` will add a tool line to your mod
file and add any missing dependencies.

Running `go get -tool example.com/m1@none` will drop the tool line from
your mod file.

For #48429

Change-Id: I07b4776f1f55eff588d08cb6649d94cc42a729d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/563175
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

src/cmd/go/internal/modget/get.go

@@ -54,7 +54,7 @@ import (
 var CmdGet = &base.Command{
 	// Note: flags below are listed explicitly because they're the most common.
 	// Do not send CLs removing them because they're covered by [get flags].
-	UsageLine: "go get [-t] [-u] [-v] [build flags] [packages]",
+	UsageLine: "go get [-t] [-u] [-v] [-tool] [build flags] [packages]",
 	Short:     "add dependencies to current module and install them",
 	Long: `
 Get resolves its command-line arguments to packages at specific module versions,
@@ -109,6 +109,9 @@ but changes the default to select patch releases.
 When the -t and -u flags are used together, get will update
 test dependencies as well.
 
+The -tool flag instructs go to add a matching tool line to go.mod for each
+listed package. If -tool is used with @none, the line will be removed.
+
 The -x flag prints commands as they are executed. This is useful for
 debugging version control commands when a module is downloaded directly
 from a repository.
@@ -217,6 +220,7 @@ var (
 	getM        = CmdGet.Flag.Bool("m", false, "")
 	getT        = CmdGet.Flag.Bool("t", false, "")
 	getU        upgradeFlag
+	getTool     = CmdGet.Flag.Bool("tool", false, "")
 	getInsecure = CmdGet.Flag.Bool("insecure", false, "")
 	// -v is cfg.BuildV
 )
@@ -402,6 +406,10 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 	}
 	r.checkPackageProblems(ctx, pkgPatterns)
 
+	if *getTool {
+		updateTools(ctx, queries, &opts)
+	}
+
 	// Everything succeeded. Update go.mod.
 	oldReqs := reqsFromGoMod(modload.ModFile())
 
@@ -425,6 +433,32 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 	}
 }
 
+func updateTools(ctx context.Context, queries []*query, opts *modload.WriteOpts) {
+	pkgOpts := modload.PackageOpts{
+		VendorModulesInGOROOTSrc: true,
+		LoadTests:                *getT,
+		ResolveMissingImports:    false,
+		AllowErrors:              true,
+		SilenceNoGoErrors:        true,
+	}
+	patterns := []string{}
+	for _, q := range queries {
+		if search.IsMetaPackage(q.pattern) || q.pattern == "toolchain" {
+			base.Fatalf("go: go get -tool does not work with \"%s\".", q.pattern)
+		}
+		patterns = append(patterns, q.pattern)
+	}
+
+	matches, _ := modload.LoadPackages(ctx, pkgOpts, patterns...)
+	for i, m := range matches {
+		if queries[i].version == "none" {
+			opts.DropTools = append(opts.DropTools, m.Pkgs...)
+		} else {
+			opts.AddTools = append(opts.DropTools, m.Pkgs...)
+		}
+	}
+}
+
 // parseArgs parses command-line arguments and reports errors.
 //
 // The command-line arguments are of the form path@version or simply path, with

src/cmd/go/internal/modload/init.go

@@ -1767,6 +1767,9 @@ type WriteOpts struct {
 	DropToolchain     bool // go get toolchain@none
 	ExplicitToolchain bool // go get has set explicit toolchain version
 
+	AddTools  []string // go get -tool example.com/m1
+	DropTools []string // go get -tool example.com/m1@none
+
 	// TODO(bcmills): Make 'go mod tidy' update the go version in the Requirements
 	// instead of writing directly to the modfile.File
 	TidyWroteGo bool // Go.Version field already updated by 'go mod tidy'
@@ -1866,6 +1869,14 @@ func UpdateGoModFromReqs(ctx context.Context, opts WriteOpts) (before, after []b
 		modFile.AddToolchainStmt(toolchain)
 	}
 
+	for _, path := range opts.AddTools {
+		modFile.AddTool(path)
+	}
+
+	for _, path := range opts.DropTools {
+		modFile.DropTool(path)
+	}
+
 	// Update require blocks.
 	if gover.Compare(goVersion, gover.SeparateIndirectVersion) < 0 {
 		modFile.SetRequire(list)
@@ -1904,7 +1915,7 @@ func commitRequirements(ctx context.Context, opts WriteOpts) (err error) {
 	}
 
 	index := MainModules.GetSingleIndexOrNil()
-	dirty := index.modFileIsDirty(modFile)
+	dirty := index.modFileIsDirty(modFile) || len(opts.DropTools) > 0 || len(opts.AddTools) > 0
 	if dirty && cfg.BuildMod != "mod" {
 		// If we're about to fail due to -mod=readonly,
 		// prefer to report a dirty go.mod over a dirty go.sum

src/cmd/go/testdata/script/mod_get_tool.txt

@@ -0,0 +1,66 @@
+# test go get -tool
+go get -tool example.com/tools/cmd/hello
+cmp go.mod go.mod.want
+
+# test -tool with @none
+go get -tool example.com/tools/cmd/hello@none
+cmp go.mod go.mod.gone
+
+go mod tidy
+cmp go.mod go.mod.empty
+
+# test -tool with wildcards
+go get -tool ./cmd/...
+cmp go.mod go.mod.wildcard
+! go get -tool ./cmd/...@none
+stderr 'can''t request explicit version "none" of path "./cmd/..." in main module'
+
+# test -tool with all
+! go get -tool all
+stderr 'go get -tool does not work with "all"'
+
+-- main.go --
+package main
+
+func main() {}
+
+-- go.mod --
+module example.com/foo
+go 1.24
+
+-- go.mod.want --
+module example.com/foo
+
+go 1.24
+
+tool example.com/tools/cmd/hello
+
+require example.com/tools v1.0.0 // indirect
+-- go.mod.gone --
+module example.com/foo
+
+go 1.24
+
+require example.com/tools v1.0.0 // indirect
+-- go.mod.empty --
+module example.com/foo
+
+go 1.24
+-- go.mod.wildcard --
+module example.com/foo
+
+go 1.24
+
+tool (
+	example.com/foo/cmd/a
+	example.com/foo/cmd/b
+)
+-- cmd/a/a.go --
+package a
+
+func main() {}
+
+-- cmd/b/b.go --
+package b
+
+func main() {}