cmd/compile: disallow embed of var inside func

Allowing embedding into []byte inside a func creates an
unfortunate problem: either all calls start with the same
underlying data and can see each other's changes to the
underlying data (surprising and racy!) or all calls start
by making their own copy of the underlying data
(surprising and expensive!).

After discussion on #43216, the consensus was to remove
support for all vars embedded inside functions.

Fixes #43216.


Change-Id: I01e62b5f0dcd9e8566c6d2286218e97803f54704
Reviewed-on: https://go-review.googlesource.com/c/go/+/282714
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

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

@@ -133,13 +133,8 @@ func varEmbed(p *noder, names []*Node, typ *Node, exprs []*Node, embeds []Pragma
 
 	v := names[0]
 	if dclcontext != PEXTERN {
-		numLocalEmbed++
-		v = newnamel(v.Pos, lookupN("embed.", numLocalEmbed))
-		v.Sym.Def = asTypesNode(v)
-		v.Name.Param.Ntype = typ
-		v.SetClass(PEXTERN)
-		externdcl = append(externdcl, v)
-		exprs = []*Node{v}
+		p.yyerrorpos(pos, "go:embed cannot apply to var inside func")
+		return exprs
 	}
 
 	v.Name.Param.SetEmbedFiles(list)

src/embed/internal/embedtest/embed_test.go

@@ -73,24 +73,11 @@ func TestGlobal(t *testing.T) {
 	testString(t, string(glass), "glass", "I can eat glass and it doesn't hurt me.\n")
 }
 
-func TestLocal(t *testing.T) {
-	//go:embed testdata/k*.txt
-	var local embed.FS
-	testFiles(t, local, "testdata/ken.txt", "If a program is too slow, it must have a loop.\n")
-
-	//go:embed testdata/k*.txt
-	var s string
-	testString(t, s, "local variable s", "If a program is too slow, it must have a loop.\n")
-
-	//go:embed testdata/h*.txt
-	var b []byte
-	testString(t, string(b), "local variable b", "hello, world\n")
-}
+//go:embed testdata
+var testDirAll embed.FS
 
 func TestDir(t *testing.T) {
-	//go:embed testdata
-	var all embed.FS
-
+	all := testDirAll
 	testFiles(t, all, "testdata/hello.txt", "hello, world\n")
 	testFiles(t, all, "testdata/i/i18n.txt", "internationalization\n")
 	testFiles(t, all, "testdata/i/j/k/k8s.txt", "kubernetes\n")
@@ -102,12 +89,15 @@ func TestDir(t *testing.T) {
 	testDir(t, all, "testdata/i/j/k", "k8s.txt")
 }
 
-func TestHidden(t *testing.T) {
-	//go:embed testdata
-	var dir embed.FS
+//go:embed testdata
+var testHiddenDir embed.FS
 
-	//go:embed testdata/*
-	var star embed.FS
+//go:embed testdata/*
+var testHiddenStar embed.FS
+
+func TestHidden(t *testing.T) {
+	dir := testHiddenDir
+	star := testHiddenStar
 
 	t.Logf("//go:embed testdata")
 

src/embed/internal/embedtest/embedx_test.go

@@ -90,17 +90,3 @@ func TestXGlobal(t *testing.T) {
 	}
 	bbig[0] = old
 }
-
-func TestXLocal(t *testing.T) {
-	//go:embed testdata/*o.txt
-	var local embed.FS
-	testFiles(t, local, "testdata/hello.txt", "hello, world\n")
-
-	//go:embed testdata/k*.txt
-	var s string
-	testString(t, s, "local variable s", "If a program is too slow, it must have a loop.\n")
-
-	//go:embed testdata/h*.txt
-	var b []byte
-	testString(t, string(b), "local variable b", "hello, world\n")
-}