cmd/go: print deprecation notice for 'go get cmd'

The notice is shown when 'go get' is invoked with the -d flag, and
the arguments match at least one main package.

This reverts CL 274552.

For #43684

Change-Id: I42e6731455f22988bf72dde1d5a76d197e9e3954
Reviewed-on: https://go-review.googlesource.com/c/go/+/305670
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

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

@@ -385,10 +385,32 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 			pkgs = append(pkgs, pkg)
 		}
 		load.CheckPackageErrors(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 and download 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.Fprintf(os.Stderr, "\tFor more information, see https://golang.org/doc/go-get-install-deprecation\n\tor run 'go help get' or 'go help install'.\n")
+		}
+
 		work.InstallPackages(ctx, pkgPatterns, pkgs)
-		// TODO(#40276): After Go 1.16, print a deprecation notice when building and
-		// installing main packages. 'go install pkg' or 'go install pkg@version'
-		// should be used instead. Give the specific argument to use if possible.
 	}
 
 	if !modload.HasModRoot() {

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 and download dependencies of the current module, use ''go get -d'''