cmd/gc: for loop init statement misanalyzed by escape analysis

Logically, the init statement is in the enclosing scopes loopdepth, not inside the for loop.

Fixes golang#7313.

LGTM=rsc
R=golang-codereviews, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/62430043

src/cmd/gc/esc.c

@@ -423,14 +423,16 @@ esc(EscState *e, Node *n)
 
 	lno = setlineno(n);
 
+	// ninit logically runs at a different loopdepth than the rest of the for loop.
+	esclist(e, n->ninit);
+
 	if(n->op == OFOR || n->op == ORANGE)
 		e->loopdepth++;
 
 	esc(e, n->left);
 	esc(e, n->right);
 	esc(e, n->ntest);
 	esc(e, n->nincr);
-	esclist(e, n->ninit);
 	esclist(e, n->nbody);
 	esclist(e, n->nelse);
 	esclist(e, n->list);

test/escape2.go

@@ -1357,3 +1357,35 @@ func foo144() {
 //go:noescape
 
 func foo144b(*int)
+
+// issue 7313: for loop init should not be treated as "in loop"
+
+type List struct {
+	Next *List
+}
+
+func foo145(l List) { // ERROR "l does not escape"
+	var p *List
+	for p = &l; p.Next != nil; p = p.Next { // ERROR "&l does not escape"
+	}
+}
+
+func foo146(l List) { // ERROR "l does not escape"
+	var p *List
+	p = &l // ERROR "&l does not escape"
+	for ; p.Next != nil; p = p.Next {
+	}
+}
+
+func foo147(l List) { // ERROR "l does not escape"
+	var p *List
+	p = &l // ERROR "&l does not escape"
+	for p.Next != nil {
+		p = p.Next
+	}
+}
+
+func foo148(l List) { // ERROR " l does not escape"
+	for p := &l; p.Next != nil; p = p.Next { // ERROR "&l does not escape"
+	}
+}