test: add regress test for #27557

This commit just adds a regress test for a few of the important corner
cases that I identified in #27557, which turn out to not be tested
anywhere.

While here, annotate a few of the existing test cases where we could
improve escape analysis.

Updates #27557.

Change-Id: Ie57792a538f7899bb17915485fabc86100f469a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/197137
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -505,10 +505,7 @@ func (e *Escape) exprSkipInit(k EscHole, n *Node) {
 	case OCALLPART:
 		e.spill(k, n)
 
-		// esc.go says "Contents make it to memory, lose
-		// track."  I think we can just flow n.Left to our
-		// spilled location though.
-		// TODO(mdempsky): Try that.
+		// TODO(mdempsky): We can do better here. See #27557.
 		e.assignHeap(n.Left, "call part", n)
 
 	case OPTRLIT:

test/escape2.go

@@ -1386,6 +1386,7 @@ func (t *Tm) M() { // ERROR "t does not escape$"
 func foo141() {
 	var f func()
 
+	// BAD: new(Tm) should not escape
 	t := new(Tm) // ERROR "new\(Tm\) escapes to heap$"
 	f = t.M      // ERROR "t.M does not escape$"
 	_ = f

test/escape2n.go

@@ -1386,6 +1386,7 @@ func (t *Tm) M() { // ERROR "t does not escape$"
 func foo141() {
 	var f func()
 
+	// BAD: new(Tm) should not escape
 	t := new(Tm) // ERROR "new\(Tm\) escapes to heap$"
 	f = t.M      // ERROR "t.M does not escape$"
 	_ = f

test/fixedbugs/issue21709.go

@@ -14,6 +14,7 @@ func (s *S) Inc() {} // ERROR "s does not escape"
 var N int
 
 func F1() {
+	// BAD: s should not escape
 	var s S // ERROR "moved to heap: s"
 	for i := 0; i < N; i++ {
 		fs := []func(){ // ERROR "\[\]func\(\) literal does not escape"
@@ -26,6 +27,7 @@ func F1() {
 }
 
 func F2() {
+	// BAD: s should not escape
 	var s S // ERROR "moved to heap: s"
 	for i := 0; i < N; i++ {
 		for _, f := range []func(){ // ERROR "\[\]func\(\) literal does not escape"

test/fixedbugs/issue27557.go

+// errorcheck -0 -l -m
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var sink interface{}
+
+func _() {
+	// BAD: t should not escape
+	var t T         // ERROR "moved to heap"
+	f := t.noescape // ERROR "t.noescape does not escape"
+	f()
+}
+
+func _() {
+	var t T       // ERROR "moved to heap"
+	f := t.escape // ERROR "t.escape does not escape"
+	f()
+}
+
+func _() {
+	var t T        // ERROR "moved to heap"
+	f := t.returns // ERROR "t.returns does not escape"
+	sink = f()
+}
+
+type T struct{}
+
+func (t *T) noescape()   {}           // ERROR "t does not escape"
+func (t *T) escape()     { sink = t } // ERROR "leaking param: t$"
+func (t *T) returns() *T { return t } // ERROR "leaking param: t to result ~r0 level=0"
+
+func (t *T) recursive() { // ERROR "leaking param: t$"
+	sink = t
+
+	var t2 T          // ERROR "moved to heap"
+	f := t2.recursive // ERROR "t2.recursive does not escape"
+	f()
+}