cmd/gc: fix escape analysis for slice of array

Fixes #7931.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/100390044

src/cmd/gc/esc.c

@@ -767,8 +767,8 @@ escassign(EscState *e, Node *dst, Node *src)
 	case ODOTTYPE:
 	case ODOTTYPE2:
 	case OSLICE:
-	case OSLICEARR:
 	case OSLICE3:
+	case OSLICEARR:
 	case OSLICE3ARR:
 		// Conversions, field access, slice all preserve the input value.
 		escassign(e, dst, src->left);
@@ -1155,6 +1155,10 @@ escwalk(EscState *e, int level, Node *dst, Node *src)
 		break;
 
 	case ODOT:
+	case OSLICE:
+	case OSLICEARR:
+	case OSLICE3:
+	case OSLICE3ARR:
 		escwalk(e, level, dst, src->left);
 		break;
 
@@ -1164,7 +1168,6 @@ escwalk(EscState *e, int level, Node *dst, Node *src)
 			break;
 		}
 		// fall through
-	case OSLICE:
 	case ODOTPTR:
 	case OINDEXMAP:
 	case OIND:

test/escape2.go

@@ -1411,3 +1411,35 @@ func foo150(x ...byte) { // ERROR "leaking param: x"
 func bar150() {
 	foo150(1, 2, 3) // ERROR "[.][.][.] argument escapes to heap"
 }
+
+// issue 7931: bad handling of slice of array
+
+var save151 *int
+
+func foo151(x *int) { // ERROR "leaking param: x"
+	save151 = x
+}
+
+func bar151() {
+	var a [64]int // ERROR "moved to heap: a"
+	a[4] = 101
+	foo151(&(&a)[4:8][0]) // ERROR "&\(&a\)\[4:8\]\[0\] escapes to heap" "&a escapes to heap"
+}
+
+func bar151b() {
+	var a [10]int      // ERROR "moved to heap: a"
+	b := a[:]          // ERROR "a escapes to heap"
+	foo151(&b[4:8][0]) // ERROR "&b\[4:8\]\[0\] escapes to heap"
+}
+
+func bar151c() {
+	var a [64]int // ERROR "moved to heap: a"
+	a[4] = 101
+	foo151(&(&a)[4:8:8][0]) // ERROR "&\(&a\)\[4:8:8\]\[0\] escapes to heap" "&a escapes to heap"
+}
+
+func bar151d() {
+	var a [10]int        // ERROR "moved to heap: a"
+	b := a[:]            // ERROR "a escapes to heap"
+	foo151(&b[4:8:8][0]) // ERROR "&b\[4:8:8\]\[0\] escapes to heap"
+}