cmd/go: print deprecation messages for 'go get' installing executables

For #40276

Change-Id: I5e631a4c9ce07f23640fb56eb455457bc55072c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/266360
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

doc/go1.16.html

@@ -71,6 +71,16 @@ Do not send CLs removing the interior tags from such phrases.
   TODO: write and link to blog post
 </p>
 
+<p><!-- golang.org/issue/40276 -->
+  <code>go</code> <code>install</code>, with or without a version suffix (as
+  described above), is now the recommended way to build and install packages in
+  module mode. <code>go</code> <code>get</code> should be used with the
+  <code>-d</code> flag to adjust the current module's dependencies without
+  building packages, and use of <code>go</code> <code>get</code> to build and
+  install packages is deprecated. In a future release, the <code>-d</code> flag
+  will always be enabled.
+</p>
+
 <p><!-- golang.org/issue/24031 -->
   <code>retract</code> directives may now be used in a <code>go.mod</code> file
   to indicate that certain published versions of the module should not be used

src/cmd/go/alldocs.go

@@ -673,6 +673,17 @@
 // The second step is to download (if needed), build, and install
 // the named packages.
 //
+// The -d flag instructs get to skip this step, downloading source code
+// needed to build the named packages and their dependencies, but not
+// building or installing.
+//
+// Building and installing packages with get is deprecated. In a future release,
+// the -d flag will be enabled by default, and 'go get' will be only be used to
+// adjust dependencies of the current module. To install a package using
+// dependencies from the current module, use 'go install'. To install a package
+// ignoring the current module, use 'go install' with an @version suffix like
+// "@latest" after each argument.
+//
 // If an argument names a module but not a package (because there is no
 // Go source code in the module's root directory), then the install step
 // is skipped for that argument, instead of causing a build failure.
@@ -684,10 +695,6 @@
 // adds the latest golang.org/x/perf and then installs the commands in that
 // latest version.
 //
-// The -d flag instructs get to download the source code needed to build
-// the named packages, including downloading necessary dependencies,
-// but not to build and install them.
-//
 // With no package arguments, 'go get' applies to Go package in the
 // current directory, if any. In particular, 'go get -u' and
 // 'go get -u=patch' update all the dependencies of that package.

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

@@ -144,6 +144,17 @@ GONOSUMDB. See 'go help environment' for details.
 The second step is to download (if needed), build, and install
 the named packages.
 
+The -d flag instructs get to skip this step, downloading source code
+needed to build the named packages and their dependencies, but not
+building or installing.
+
+Building and installing packages with get is deprecated. In a future release,
+the -d flag will be enabled by default, and 'go get' will be only be used to
+adjust dependencies of the current module. To install a package using
+dependencies from the current module, use 'go install'. To install a package
+ignoring the current module, use 'go install' with an @version suffix like
+"@latest" after each argument.
+
 If an argument names a module but not a package (because there is no
 Go source code in the module's root directory), then the install step
 is skipped for that argument, instead of causing a build failure.
@@ -155,10 +166,6 @@ the module versions. For example, 'go get golang.org/x/perf/cmd/...'
 adds the latest golang.org/x/perf and then installs the commands in that
 latest version.
 
-The -d flag instructs get to download the source code needed to build
-the named packages, including downloading necessary dependencies,
-but not to build and install them.
-
 With no package arguments, 'go get' applies to Go package in the
 current directory, if any. In particular, 'go get -u' and
 'go get -u=patch' update all the dependencies of that package.
@@ -436,12 +443,36 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 	// Note that 'go get -u' without arguments is equivalent to
 	// 'go get -u .', so we'll typically build the package in the current
 	// directory.
-	if !*getD {
-		if len(pkgPatterns) > 0 {
-			work.BuildInit()
-			pkgs := load.PackagesForBuild(ctx, pkgPatterns)
-			work.InstallPackages(ctx, pkgPatterns, pkgs)
+	if !*getD && len(pkgPatterns) > 0 {
+		work.BuildInit()
+		pkgs := load.PackagesForBuild(ctx, pkgPatterns)
+		work.InstallPackages(ctx, pkgPatterns, pkgs)
+
+		haveExe := false
+		for _, pkg := range pkgs {
+			if pkg.Name == "main" {
+				haveExe = true
+				break
+			}
 		}
+		if haveExe {
+			fmt.Fprint(os.Stderr, "go get: installing executables with 'go get' in module mode is deprecated.")
+			var altMsg string
+			if modload.HasModRoot() {
+				altMsg = `
+	To adjust dependencies of the current module, use 'go get -d'.
+	To install using requirements of the current module, use 'go install'.
+	To install ignoring the current module, use 'go install' with a version,
+	like 'go install example.com/cmd@latest'.
+`
+			} else {
+				altMsg = "\n\tUse 'go install pkg@version' instead.\n"
+			}
+			fmt.Fprint(os.Stderr, altMsg)
+			fmt.Fprint(os.Stderr, "\tSee 'go help get' and 'go help install' for more information.\n")
+		}
+		// TODO(golang.org/issue/40276): link to HTML documentation explaining
+		// what's changing and gives more examples.
 	}
 
 	// Everything succeeded. Update go.mod.

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

@@ -0,0 +1,22 @@
+[short] skip
+
+env GO111MODULE=on
+
+# 'go get' outside a module with an executable prints a deprecation message.
+go get example.com/cmd/a
+stderr '^go get: installing executables with ''go get'' in module mode is deprecated.$'
+stderr 'Use ''go install pkg@version'' instead.'
+
+
+go mod init m
+
+# 'go get' inside a module with a non-main package does not print a message.
+# This will stop building in the future, but it's the command we want to use.
+go get rsc.io/quote
+! stderr deprecated
+
+# 'go get' inside a module with an executable prints a different
+# deprecation message.
+go get example.com/cmd/a
+stderr '^go get: installing executables with ''go get'' in module mode is deprecated.$'
+stderr 'To adjust dependencies of the current module, use ''go get -d'''