cmd/compile: require 'go 1.16' go.mod line for //go:embed

This will produce better errors when earlier versions of
Go compile code using //go:embed. (The import will cause
a compilation error but then the go command will add to
the output that the Go toolchain in use looks too old
and maybe that's the problem.)

This CL also adds a test for disallowing embed of a var inside a func.
It's a bit too difficult to rebase down into that CL.

The build system configuration check is delayed in order to
make it possible to use errorcheck for these tests.

Change-Id: I12ece4ff2d8d53380b63f54866e8f3497657d54c
Reviewed-on: https://go-review.googlesource.com/c/go/+/282718
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/compile/internal/gc/embed.go

@@ -67,10 +67,6 @@ func varEmbed(p *noder, names []*Node, typ *Node, exprs []*Node, embeds []Pragma
 		p.yyerrorpos(pos, "invalid go:embed: missing import \"embed\"")
 		return
 	}
-	if embedCfg.Patterns == nil {
-		p.yyerrorpos(pos, "invalid go:embed: build system did not supply embed configuration")
-		return
-	}
 	if len(names) > 1 {
 		p.yyerrorpos(pos, "go:embed cannot apply to multiple vars")
 		return
@@ -186,6 +182,18 @@ func dumpembeds() {
 // initEmbed emits the init data for a //go:embed variable,
 // which is either a string, a []byte, or an embed.FS.
 func initEmbed(v *Node) {
+	commentPos := v.Name.Param.EmbedList()[0].Pos
+	if !langSupported(1, 16, localpkg) {
+		lno := lineno
+		lineno = commentPos
+		yyerrorv("go1.16", "go:embed")
+		lineno = lno
+		return
+	}
+	if embedCfg.Patterns == nil {
+		yyerrorl(commentPos, "invalid go:embed: build system did not supply embed configuration")
+		return
+	}
 	kind := embedKind(v.Type)
 	if kind == embedUnknown {
 		yyerrorl(v.Pos, "go:embed cannot apply to var of type %v", v.Type)

src/go/types/stdlib_test.go

@@ -155,6 +155,8 @@ func TestStdTest(t *testing.T) {
 	testTestDir(t, filepath.Join(runtime.GOROOT(), "test"),
 		"cmplxdivide.go", // also needs file cmplxdivide1.go - ignore
 		"directive.go",   // tests compiler rejection of bad directive placement - ignore
+		"embedfunc.go",   // tests //go:embed
+		"embedvers.go",   // tests //go:embed
 	)
 }
 

test/embedfunc.go

@@ -0,0 +1,15 @@
+// errorcheck
+
+// Copyright 2021 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 p
+
+import _ "embed"
+
+func f() {
+	//go:embed x.txt // ERROR "go:embed cannot apply to var inside func"
+	var x string
+	_ = x
+}

test/embedvers.go

@@ -0,0 +1,12 @@
+// errorcheck -lang=go1.15
+
+// Copyright 2021 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 p
+
+import _ "embed"
+
+//go:embed x.txt // ERROR "go:embed requires go1.16 or later"
+var x string