cmd/gc: fix escape analysis bug with variable capture in loops.

Fixes #3975.

R=rsc, lvd
CC=golang-dev, remy
https://golang.org/cl/6475061

src/cmd/gc/esc.c

@@ -543,6 +543,7 @@ esc(EscState *e, Node *n)
 				continue;
 			a = nod(OADDR, ll->n->closure, N);
 			a->lineno = ll->n->lineno;
+			a->escloopdepth = e->loopdepth;
 			typecheck(&a, Erv);
 			escassign(e, n, a);
 		}

test/escape.go

@@ -18,15 +18,15 @@ var allptr = make([]*int, 0, 100)
 
 func noalias(p, q *int, s string) {
 	n := len(allptr)
-	*p = -(n+1)
-	*q = -(n+2)
-	allptr = allptr[0:n+2]
+	*p = -(n + 1)
+	*q = -(n + 2)
+	allptr = allptr[0 : n+2]
 	allptr[n] = p
 	allptr[n+1] = q
 	n += 2
 	for i := 0; i < n; i++ {
 		if allptr[i] != nil && *allptr[i] != -(i+1) {
-			println("aliased pointers", -(i+1), *allptr[i], "after", s)
+			println("aliased pointers", -(i + 1), *allptr[i], "after", s)
 			allptr[i] = nil
 			bad = true
 		}
@@ -141,6 +141,18 @@ func for_escapes2(x int, y int) (*int, *int) {
 	return p[0], p[1]
 }
 
+func for_escapes3(x int, y int) (*int, *int) {
+	var f [2]func() *int
+	n := 0
+	for i := x; n < 2; i = y {
+		p := new(int)
+		*p = i
+		f[n] = func() *int { return p }
+		n++
+	}
+	return f[0](), f[1]()
+}
+
 func out_escapes(i int) (x int, p *int) {
 	x = i
 	p = &x // ERROR "address of out parameter"
@@ -187,6 +199,9 @@ func main() {
 	p, q = for_escapes2(103, 104)
 	chkalias(p, q, 103, "for_escapes2")
 
+	p, q = for_escapes3(105, 106)
+	chk(p, q, 105, "for_escapes3")
+
 	_, p = out_escapes(15)
 	_, q = out_escapes(16)
 	chk(p, q, 15, "out_escapes")

test/escape2.go

@@ -540,6 +540,19 @@ func foo74() {
 	}
 }
 
+// issue 3975
+func foo74b() {
+	var array [3]func()
+	s := []int{3, 2, 1} // ERROR "\[\]int literal does not escape"
+	for i, v := range s {
+		vv := v // ERROR "moved to heap: vv"
+		// actually just escapes its scope
+		array[i] = func() { // ERROR "func literal escapes to heap"
+			println(vv) // ERROR "&vv escapes to heap"
+		}
+	}
+}
+
 func myprint(y *int, x ...interface{}) *int { // ERROR "x does not escape" "leaking param: y"
 	return y
 }