cmd/compile: make sure to initialize static entries of slices

If a slice's entries are sparse, we decide to initialize it dynamically
instead of statically. That's CL 151319.

But if we do initialize it dynamically, we still need to initialize
the static entries. Typically we do that, but the bug fixed here is
that we don't if the entry's value is itself an array or struct.

To fix, use initKindLocalCode to ensure that both static and
dynamic entries are initialized via code.

Fixes #31987

Change-Id: I1192ffdbfb5cd50445c1206c4a3d8253295201dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/176904
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -561,6 +561,13 @@ const (
 	inNonInitFunction
 )
 
+func (c initContext) String() string {
+	if c == inInitFunction {
+		return "inInitFunction"
+	}
+	return "inNonInitFunction"
+}
+
 // from here down is the walk analysis
 // of composite literals.
 // most of the work is to generate
@@ -920,7 +927,13 @@ func slicelit(ctxt initContext, n *Node, var_ *Node, init *Nodes) {
 			break
 
 		case OARRAYLIT, OSTRUCTLIT:
-			fixedlit(ctxt, initKindDynamic, value, a, init)
+			k := initKindDynamic
+			if vstat == nil {
+				// Generate both static and dynamic initializations.
+				// See issue #31987.
+				k = initKindLocalCode
+			}
+			fixedlit(ctxt, k, value, a, init)
 			continue
 		}
 

test/fixedbugs/issue31987.go

+// run
+
+// 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 main
+
+import "fmt"
+
+type container struct {
+	Value string
+}
+
+func main() {
+	s := []container{
+		7: {Value: "string value"},
+	}
+	if s[7].Value != "string value" {
+		panic(fmt.Errorf("wanted \"string value\", got \"%s\"", s[7].Value))
+	}
+}