cmd/compile/internal/gc: fix invalid positions for sink nodes in esc.go

Make OAS2 and OAS2FUNC sink locations point to the assignment position,
not the nth LHS position.

Fixes #26987

Change-Id: Ibeb9df2da754da8b6638fe1e49e813f37515c13c
Reviewed-on: https://go-review.googlesource.com/129315
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -886,8 +886,9 @@ opSwitch:
 	case OAS2: // x,y = a,b
 		if n.List.Len() == n.Rlist.Len() {
 			rs := n.Rlist.Slice()
+			where := n
 			for i, n := range n.List.Slice() {
-				e.escassignWhyWhere(n, rs[i], "assign-pair", n)
+				e.escassignWhyWhere(n, rs[i], "assign-pair", where)
 			}
 		}
 
@@ -928,11 +929,12 @@ opSwitch:
 		// esccall already done on n.Rlist.First(). tie it's Retval to n.List
 	case OAS2FUNC: // x,y = f()
 		rs := e.nodeEscState(n.Rlist.First()).Retval.Slice()
+		where := n
 		for i, n := range n.List.Slice() {
 			if i >= len(rs) {
 				break
 			}
-			e.escassignWhyWhere(n, rs[i], "assign-pair-func-call", n)
+			e.escassignWhyWhere(n, rs[i], "assign-pair-func-call", where)
 		}
 		if n.List.Len() != len(rs) {
 			Fatalf("esc oas2func")

test/escape_because.go

@@ -125,6 +125,24 @@ func f14() {
 	_, _ = s1, s2
 }
 
+func leakParams(p1, p2 *int) (*int, *int) { // ERROR "leaking param: p1 to result ~r2 level=0$" "from ~r2 \(return\) at escape_because.go:129$" "leaking param: p2 to result ~r3 level=0$" "from ~r3 \(return\) at escape_because.go:129$"
+	return p1, p2
+}
+
+func leakThroughOAS2() {
+	// See #26987.
+	i := 0              // ERROR "moved to heap: i$"
+	j := 0              // ERROR "moved to heap: j$"
+	sink, sink = &i, &j // ERROR "&i escapes to heap$" "from sink \(assign-pair\) at escape_because.go:136$" "from &i \(interface-converted\) at escape_because.go:136$" "&j escapes to heap$" "from &j \(interface-converted\) at escape_because.go:136"
+}
+
+func leakThroughOAS2FUNC() {
+	// See #26987.
+	i := 0 // ERROR "moved to heap: i$"
+	j := 0
+	sink, _ = leakParams(&i, &j) // ERROR "&i escapes to heap$" "&j does not escape$" "from .out0 \(passed-to-and-returned-from-call\) at escape_because.go:143$" "from sink \(assign-pair-func-call\) at escape_because.go:143$"
+}
+
 // The list below is all of the why-escapes messages seen building the escape analysis tests.
 /*
    for i in escape*go ; do echo compile $i; go build -gcflags '-l -m -m' $i >& `basename $i .go`.log ; done