go/analysis/passes/fieldalignment: suggest fix for multi-named fields

Add support for providing a suggested fix when the struct have field
lines with multiple names, such as:

type s struct {
    b bool
    i1, i2 int
    a3 [3]bool
    _ [0]func()
}

This was a leftover from CL 270217.

Change-Id: Icccec5b8b4b82823cee33b7a84d0cccab38b6f45
Reviewed-on: https://go-review.googlesource.com/c/tools/+/273346
Reviewed-by: Michael Matloob <matloob@golang.org>
Trust: Pontus Leitzler <leitzler@gmail.com>

go/analysis/passes/fieldalignment/fieldalignment.go

@@ -71,22 +71,27 @@ func fieldalignment(pass *analysis.Pass, node *ast.StructType, typ *types.Struct
 		return
 	}
 
-	// TODO: The ast node could contain multiple field names per list item while "indexes"
-	// based on *types.struct contain one item per field. Until this case is handled we
-	// do not provide any suggested fix for it.
-	if len(indexes) != len(node.Fields.List) {
-		pass.Report(analysis.Diagnostic{
-			Pos:     node.Pos(),
-			End:     node.Pos() + token.Pos(len("struct")),
-			Message: message,
-		})
-		return
+	// Flatten the ast node since it could have multiple field names per list item while
+	// *types.Struct only have one item per field.
+	// TODO: Preserve multi-named fields instead of flattening.
+	var flat []*ast.Field
+	for _, f := range node.Fields.List {
+		if len(f.Names) == 1 {
+			flat = append(flat, f)
+			continue
+		}
+		for _, name := range f.Names {
+			flat = append(flat, &ast.Field{
+				Names: []*ast.Ident{name},
+				Type:  f.Type,
+			})
+		}
 	}
 
 	// Sort fields according to the optimal order.
 	var reordered []*ast.Field
 	for _, index := range indexes {
-		reordered = append(reordered, node.Fields.List[index])
+		reordered = append(reordered, flat[index])
 	}
 
 	newStr := &ast.StructType{

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

@@ -11,3 +11,13 @@ type Bad struct { // want "struct of size 12 could be 8"
 	y int32
 	z byte
 }
+
+type ZeroGood struct {
+	a [0]byte
+	b uint32
+}
+
+type ZeroBad struct { // want "struct of size 8 could be 4"
+	a uint32
+	b [0]byte
+}

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

@@ -11,3 +11,13 @@ type Bad struct {
 	x byte
 	z byte
 }
+
+type ZeroGood struct {
+	a [0]byte
+	b uint32
+}
+
+type ZeroBad struct {
+	b [0]byte
+	a uint32
+}

go/analysis/passes/fieldalignment/testdata/src/a/a_386.go

@@ -31,3 +31,10 @@ type PointerSortaBad struct { // want "struct with 16 pointer bytes could be 12"
 		q uintptr
 	}
 }
+
+type MultiField struct { // want "struct of size 20 could be 12"
+	b      bool
+	i1, i2 int
+	a3     [3]bool
+	_      [0]func()
+}

go/analysis/passes/fieldalignment/testdata/src/a/a_386.go.golden

@@ -31,3 +31,11 @@ type PointerSortaBad struct {
 		q [2]uintptr
 	}
 }
+
+type MultiField struct {
+	_  [0]func()
+	i1 int
+	i2 int
+	a3 [3]bool
+	b  bool
+}

go/analysis/passes/fieldalignment/testdata/src/a/a_amd64.go

@@ -31,3 +31,10 @@ type PointerSortaBad struct { // want "struct with 32 pointer bytes could be 24"
 		q uintptr
 	}
 }
+
+type MultiField struct { // want "struct of size 40 could be 24"
+	b      bool
+	i1, i2 int
+	a3     [3]bool
+	_      [0]func()
+}

go/analysis/passes/fieldalignment/testdata/src/a/a_amd64.go.golden

@@ -31,3 +31,11 @@ type PointerSortaBad struct {
 		q [2]uintptr
 	}
 }
+
+type MultiField struct {
+	_  [0]func()
+	i1 int
+	i2 int
+	a3 [3]bool
+	b  bool
+}

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

@@ -1,11 +0,0 @@
-package a
-
-type ZeroGood struct {
-	a [0]byte
-	b uint32
-}
-
-type ZeroBad struct { // want "struct of size 8 could be 4"
-	a uint32
-	b [0]byte
-}

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

@@ -1,11 +0,0 @@
-package a
-
-type ZeroGood struct {
-	a [0]byte
-	b uint32
-}
-
-type ZeroBad struct {
-	b [0]byte
-	a uint32
-}