go/analysis/passes/fieldalignment: support fields without name

The flattening didn't account for fields without name resulting in a
panic when calculating suggested fixes for such struct.

They are now added while flattening and there is also a test to ensure
that it works.

Change-Id: I8f8740c8d9132d10fb09053a322633eb29e8bcf8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/278372
Trust: Pontus Leitzler <leitzler@gmail.com>
Run-TryBot: Pontus Leitzler <leitzler@gmail.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

go/analysis/passes/fieldalignment/fieldalignment.go

@@ -76,7 +76,7 @@ func fieldalignment(pass *analysis.Pass, node *ast.StructType, typ *types.Struct
 	// TODO: Preserve multi-named fields instead of flattening.
 	var flat []*ast.Field
 	for _, f := range node.Fields.List {
-		if len(f.Names) == 1 {
+		if len(f.Names) <= 1 {
 			flat = append(flat, f)
 			continue
 		}

go/analysis/passes/fieldalignment/testdata/src/a/a.go

@@ -21,3 +21,17 @@ type ZeroBad struct { // want "struct of size 8 could be 4"
 	a uint32
 	b [0]byte
 }
+
+type NoNameGood struct {
+	Good
+	y int32
+	x byte
+	z byte
+}
+
+type NoNameBad struct { // want "struct of size 20 could be 16"
+	Good
+	x byte
+	y int32
+	z byte
+}

go/analysis/passes/fieldalignment/testdata/src/a/a.go.golden

@@ -21,3 +21,17 @@ type ZeroBad struct {
 	b [0]byte
 	a uint32
 }
+
+type NoNameGood struct {
+	Good
+	y int32
+	x byte
+	z byte
+}
+
+type NoNameBad struct {
+	Good
+	y int32
+	x byte
+	z byte
+}