gopls/internal/golang/extract: preserve comments in extracted block

Use printer.CommentedNode to preserve comments in function and method
extraction.

Fixes golang/go#50851

Change-Id: I7d8aa2683c980e613592f64646f8077952ea61be
Reviewed-on: https://go-review.googlesource.com/c/tools/+/629376
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/golang/extract.go

@@ -10,6 +10,7 @@ import (
 	"go/ast"
 	"go/format"
 	"go/parser"
+	"go/printer"
 	"go/token"
 	"go/types"
 	"slices"
@@ -449,7 +450,8 @@ func extractFunctionMethod(fset *token.FileSet, start, end token.Pos, src []byte
 		return nil, nil, err
 	}
 	selection := src[startOffset:endOffset]
-	extractedBlock, err := parseBlockStmt(fset, selection)
+
+	extractedBlock, extractedComments, err := parseStmts(fset, selection)
 	if err != nil {
 		return nil, nil, err
 	}
@@ -570,40 +572,16 @@ func extractFunctionMethod(fset *token.FileSet, start, end token.Pos, src []byte
 	if canDefine {
 		sym = token.DEFINE
 	}
-	var name, funName string
+	var funName string
 	if isMethod {
-		name = "newMethod"
 		// TODO(suzmue): generate a name that does not conflict for "newMethod".
-		funName = name
+		funName = "newMethod"
 	} else {
-		name = "newFunction"
+		funName, _ = generateAvailableName(start, path, pkg, info, "newFunction", 0)
-		funName, _ = generateAvailableName(start, path, pkg, info, name, 0)
 	}
 	extractedFunCall := generateFuncCall(hasNonNestedReturn, hasReturnValues, params,
 		append(returns, getNames(retVars)...), funName, sym, receiverName)
 
-	// Build the extracted function.
-	newFunc := &ast.FuncDecl{
-		Name: ast.NewIdent(funName),
-		Type: &ast.FuncType{
-			Params:  &ast.FieldList{List: paramTypes},
-			Results: &ast.FieldList{List: append(returnTypes, getDecls(retVars)...)},
-		},
-		Body: extractedBlock,
-	}
-	if isMethod {
-		var names []*ast.Ident
-		if receiverUsed {
-			names = append(names, ast.NewIdent(receiverName))
-		}
-		newFunc.Recv = &ast.FieldList{
-			List: []*ast.Field{{
-				Names: names,
-				Type:  receiver.Type,
-			}},
-		}
-	}
-
 	// Create variable declarations for any identifiers that need to be initialized prior to
 	// calling the extracted function. We do not manually initialize variables if every return
 	// value is uninitialized. We can use := to initialize the variables in this situation.
@@ -624,17 +602,49 @@ func extractFunctionMethod(fset *token.FileSet, start, end token.Pos, src []byte
 			return nil, nil, err
 		}
 	}
+
+	// Build the extracted function. We format the function declaration and body
+	// separately, so that comments are printed relative to the extracted
+	// BlockStmt.
+	//
+	// In other words, extractedBlock and extractedComments were parsed from a
+	// synthetic function declaration of the form func _() { ... }. If we now
+	// print the real function declaration, the length of the signature will have
+	// grown, causing some comment positions to be computed as inside the
+	// signature itself.
+	newFunc := &ast.FuncDecl{
+		Name: ast.NewIdent(funName),
+		Type: &ast.FuncType{
+			Params:  &ast.FieldList{List: paramTypes},
+			Results: &ast.FieldList{List: append(returnTypes, getDecls(retVars)...)},
+		},
+		// Body handled separately -- see above.
+	}
+	if isMethod {
+		var names []*ast.Ident
+		if receiverUsed {
+			names = append(names, ast.NewIdent(receiverName))
+		}
+		newFunc.Recv = &ast.FieldList{
+			List: []*ast.Field{{
+				Names: names,
+				Type:  receiver.Type,
+			}},
+		}
+	}
 	if err := format.Node(&newFuncBuf, fset, newFunc); err != nil {
 		return nil, nil, err
 	}
-	// Find all the comments within the range and print them to be put somewhere.
+	// Write a space between the end of the function signature and opening '{'.
-	// TODO(suzmue): print these in the extracted function at the correct place.
+	if err := newFuncBuf.WriteByte(' '); err != nil {
-	for _, cg := range file.Comments {
+		return nil, nil, err
-		if cg.Pos().IsValid() && cg.Pos() < end && cg.Pos() >= start {
+	}
-			for _, c := range cg.List {
+	commentedNode := &printer.CommentedNode{
-				fmt.Fprintln(&commentBuf, c.Text)
+		Node:     extractedBlock,
-			}
+		Comments: extractedComments,
-		}
+	}
+	if err := format.Node(&newFuncBuf, fset, commentedNode); err != nil {
+		return nil, nil, err
 	}
 
 	// We're going to replace the whole enclosing function,
@@ -1187,25 +1197,25 @@ func varOverridden(info *types.Info, firstUse *ast.Ident, obj types.Object, isFr
 	return isOverriden
 }
 
-// parseBlockStmt generates an AST file from the given text. We then return the portion of the
+// parseStmts parses the specified source (a list of statements) and
-// file that represents the text.
+// returns them as a BlockStmt along with any associated comments.
-func parseBlockStmt(fset *token.FileSet, src []byte) (*ast.BlockStmt, error) {
+func parseStmts(fset *token.FileSet, src []byte) (*ast.BlockStmt, []*ast.CommentGroup, error) {
 	text := "package main\nfunc _() { " + string(src) + " }"
-	extract, err := parser.ParseFile(fset, "", text, parser.SkipObjectResolution)
+	file, err := parser.ParseFile(fset, "", text, parser.ParseComments|parser.SkipObjectResolution)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
-	if len(extract.Decls) == 0 {
+	if len(file.Decls) != 1 {
-		return nil, fmt.Errorf("parsed file does not contain any declarations")
+		return nil, nil, fmt.Errorf("got %d declarations, want 1", len(file.Decls))
 	}
-	decl, ok := extract.Decls[0].(*ast.FuncDecl)
+	decl, ok := file.Decls[0].(*ast.FuncDecl)
 	if !ok {
-		return nil, fmt.Errorf("parsed file does not contain expected function declaration")
+		return nil, nil, bug.Errorf("parsed file does not contain expected function declaration")
 	}
 	if decl.Body == nil {
-		return nil, fmt.Errorf("extracted function has no body")
+		return nil, nil, bug.Errorf("extracted function has no body")
 	}
-	return decl.Body, nil
+	return decl.Body, file.Comments, nil
 }
 
 // generateReturnInfo generates the information we need to adjust the return statements and

gopls/internal/test/marker/testdata/codeaction/extract_method.txt

@@ -237,20 +237,14 @@ func (b *B) LongListWithT(ctx context.Context, t *testing.T) (int, error) {
 +}
 +
 -- @contextFuncB/context.go --
-@@ -33 +33,6 @@
+@@ -33 +33,4 @@
--	sum := b.x + b.y //@loc(B_AddPWithB, re`(?s:^.*?Err\(\))`)
-+	//@loc(B_AddPWithB, re`(?s:^.*?Err\(\))`)
 +	return newFunction(ctx, tB, b)
 +}
 +
 +func newFunction(ctx context.Context, tB *testing.B, b *B) (int, error) {
-+	sum := b.x + b.y
 -- @contextFuncT/context.go --
-@@ -42 +42,6 @@
+@@ -42 +42,4 @@
--	p4 := p1 + p2 //@loc(B_LongListWithT, re`(?s:^.*?Err\(\))`)
-+	//@loc(B_LongListWithT, re`(?s:^.*?Err\(\))`)
 +	return newFunction(ctx, t, p1, p2, p3)
 +}
 +
 +func newFunction(ctx context.Context, t *testing.T, p1 int, p2 int, p3 int) (int, error) {
-+	p4 := p1 + p2

gopls/internal/test/marker/testdata/codeaction/functionextraction.txt

@@ -17,12 +17,11 @@ func _() { //@codeaction("{", "refactor.extract.function", end=closeBracket, res
 package extract
 
 func _() { //@codeaction("{", "refactor.extract.function", end=closeBracket, result=outer)
-	//@codeaction("a", "refactor.extract.function", end=end, result=inner)
 	newFunction() //@loc(end, "4")
 }
 
 func newFunction() {
-	a := 1
+	a := 1 //@codeaction("a", "refactor.extract.function", end=end, result=inner)
 	_ = a + 4
 } //@loc(closeBracket, "}")
 
@@ -30,12 +29,11 @@ func newFunction() {
 package extract
 
 func _() { //@codeaction("{", "refactor.extract.function", end=closeBracket, result=outer)
-	//@codeaction("a", "refactor.extract.function", end=end, result=inner)
 	newFunction() //@loc(end, "4")
 }
 
 func newFunction() {
-	a := 1
+	a := 1 //@codeaction("a", "refactor.extract.function", end=end, result=inner)
 	_ = a + 4
 } //@loc(closeBracket, "}")
 
@@ -55,7 +53,6 @@ package extract
 
 func _() bool {
 	x := 1
-	//@codeaction("if", "refactor.extract.function", end=ifend, result=return)
 	shouldReturn, b := newFunction(x)
 	if shouldReturn {
 		return b
@@ -64,7 +61,7 @@ func _() bool {
 }
 
 func newFunction(x int) (bool, bool) {
-	if x == 0 {
+	if x == 0 { //@codeaction("if", "refactor.extract.function", end=ifend, result=return)
 		return true, true
 	}
 	return false, false
@@ -85,12 +82,11 @@ func _() bool {
 package extract
 
 func _() bool {
-	//@codeaction("x", "refactor.extract.function", end=rnnEnd, result=rnn)
 	return newFunction() //@loc(rnnEnd, "false")
 }
 
 func newFunction() bool {
-	x := 1
+	x := 1 //@codeaction("x", "refactor.extract.function", end=rnnEnd, result=rnn)
 	if x == 0 {
 		return true
 	}
@@ -123,7 +119,6 @@ import "fmt"
 func _() (int, string, error) {
 	x := 1
 	y := "hello"
-	//@codeaction("z", "refactor.extract.function", end=rcEnd, result=rc)
 	z, shouldReturn, i, s, err := newFunction(y, x)
 	if shouldReturn {
 		return i, s, err
@@ -132,7 +127,7 @@ func _() (int, string, error) {
 }
 
 func newFunction(y string, x int) (string, bool, int, string, error) {
-	z := "bye"
+	z := "bye" //@codeaction("z", "refactor.extract.function", end=rcEnd, result=rc)
 	if y == z {
 		return "", true, x, y, fmt.Errorf("same")
 	} else if false {
@@ -168,12 +163,11 @@ import "fmt"
 func _() (int, string, error) {
 	x := 1
 	y := "hello"
-	//@codeaction("z", "refactor.extract.function", end=rcnnEnd, result=rcnn)
 	return newFunction(y, x) //@loc(rcnnEnd, "nil")
 }
 
 func newFunction(y string, x int) (int, string, error) {
-	z := "bye"
+	z := "bye" //@codeaction("z", "refactor.extract.function", end=rcnnEnd, result=rcnn)
 	if y == z {
 		return x, y, fmt.Errorf("same")
 	} else if false {
@@ -204,7 +198,6 @@ import "go/ast"
 
 func _() {
 	ast.Inspect(ast.NewIdent("a"), func(n ast.Node) bool {
-		//@codeaction("if", "refactor.extract.function", end=rflEnd, result=rfl)
 		shouldReturn, b := newFunction(n)
 		if shouldReturn {
 			return b
@@ -214,7 +207,7 @@ func _() {
 }
 
 func newFunction(n ast.Node) (bool, bool) {
-	if n == nil {
+	if n == nil { //@codeaction("if", "refactor.extract.function", end=rflEnd, result=rfl)
 		return true, true
 	}
 	return false, false
@@ -241,13 +234,12 @@ import "go/ast"
 
 func _() {
 	ast.Inspect(ast.NewIdent("a"), func(n ast.Node) bool {
-		//@codeaction("if", "refactor.extract.function", end=rflnnEnd, result=rflnn)
 		return newFunction(n) //@loc(rflnnEnd, "false")
 	})
 }
 
 func newFunction(n ast.Node) bool {
-	if n == nil {
+	if n == nil { //@codeaction("if", "refactor.extract.function", end=rflnnEnd, result=rflnn)
 		return true
 	}
 	return false
@@ -271,7 +263,6 @@ package extract
 
 func _() string {
 	x := 1
-	//@codeaction("if", "refactor.extract.function", end=riEnd, result=ri)
 	shouldReturn, s := newFunction(x)
 	if shouldReturn {
 		return s
@@ -281,7 +272,7 @@ func _() string {
 }
 
 func newFunction(x int) (bool, string) {
-	if x == 0 {
+	if x == 0 { //@codeaction("if", "refactor.extract.function", end=riEnd, result=ri)
 		x = 3
 		return true, "a"
 	}
@@ -306,12 +297,11 @@ package extract
 
 func _() string {
 	x := 1
-	//@codeaction("if", "refactor.extract.function", end=rinnEnd, result=rinn)
 	return newFunction(x) //@loc(rinnEnd, "\"b\"")
 }
 
 func newFunction(x int) string {
-	if x == 0 {
+	if x == 0 { //@codeaction("if", "refactor.extract.function", end=rinnEnd, result=rinn)
 		x = 3
 		return "a"
 	}
@@ -336,7 +326,6 @@ package extract
 
 func _() {
 	a := 1
-	//@codeaction("a", "refactor.extract.function", end=araend, result=ara)
 	a = newFunction(a) //@loc(araend, "2")
 
 	b := a * 2 //@codeaction("b", "refactor.extract.function", end=arbend, result=arb)
@@ -344,7 +333,7 @@ func _() {
 }
 
 func newFunction(a int) int {
-	a = 5
+	a = 5 //@codeaction("a", "refactor.extract.function", end=araend, result=ara)
 	a = a + 2
 	return a
 }
@@ -357,12 +346,11 @@ func _() {
 	a = 5     //@codeaction("a", "refactor.extract.function", end=araend, result=ara)
 	a = a + 2 //@loc(araend, "2")
 
-	//@codeaction("b", "refactor.extract.function", end=arbend, result=arb)
 	newFunction(a)  //@loc(arbend, "4")
 }
 
 func newFunction(a int) {
-	b := a * 2
+	b := a * 2 //@codeaction("b", "refactor.extract.function", end=arbend, result=arb)
 	_ = b + 4
 }
 
@@ -412,13 +400,12 @@ package extract
 
 func _() {
 	var a []int
-	//@codeaction("a", "refactor.extract.function", end=siEnd, result=si)
 	a, b := newFunction(a)           //@loc(siEnd, "4")
 	a = append(a, b)
 }
 
 func newFunction(a []int) ([]int, int) {
-	a = append(a, 2)
+	a = append(a, 2) //@codeaction("a", "refactor.extract.function", end=siEnd, result=si)
 	b := 4
 	return a, b
 }
@@ -441,13 +428,12 @@ package extract
 func _() {
 	var b []int
 	var a int
-	//@codeaction("a", "refactor.extract.function", end=srEnd, result=sr)
 	b = newFunction(a, b) //@loc(srEnd, ")")
 	b[0] = 1
 }
 
 func newFunction(a int, b []int) []int {
-	a = 2
+	a = 2 //@codeaction("a", "refactor.extract.function", end=srEnd, result=sr)
 	b = []int{}
 	b = append(b, a)
 	return b
@@ -472,7 +458,6 @@ package extract
 
 func _() {
 	var b []int
-	//@codeaction("a", "refactor.extract.function", end=upEnd, result=up)
 	a, b := newFunction(b) //@loc(upEnd, ")")
 	b[0] = 1
 	if a == 2 {
@@ -481,7 +466,7 @@ func _() {
 }
 
 func newFunction(b []int) (int, []int) {
-	a := 2
+	a := 2 //@codeaction("a", "refactor.extract.function", end=upEnd, result=up)
 	b = []int{}
 	b = append(b, a)
 	return a, b
@@ -503,9 +488,6 @@ func _() {
 package extract
 
 func _() {
-	/* comment in the middle of a line */
-	//@codeaction("a", "refactor.extract.function", end=commentEnd, result=comment1)
-	// Comment on its own line  //@codeaction("Comment", "refactor.extract.function", end=commentEnd, result=comment2)
 	newFunction() //@loc(commentEnd, "4"),codeaction("_", "refactor.extract.function", end=lastComment, result=comment3)
 	// Comment right after 3 + 4
 
@@ -513,8 +495,8 @@ func _() {
 }
 
 func newFunction() {
-	a := 1
+	a := /* comment in the middle of a line */ 1 //@codeaction("a", "refactor.extract.function", end=commentEnd, result=comment1)
-
+	// Comment on its own line  //@codeaction("Comment", "refactor.extract.function", end=commentEnd, result=comment2)
 	_ = a + 4
 }
 
@@ -602,12 +584,11 @@ import "slices"
 
 // issue go#64821
 func _() {
-	//@codeaction("var", "refactor.extract.function", end=anonEnd, result=anon1)
 	newFunction() //@loc(anonEnd, ")")
 }
 
 func newFunction() {
-	var s []string
+	var s []string //@codeaction("var", "refactor.extract.function", end=anonEnd, result=anon1)
 	slices.SortFunc(s, func(a, b string) int {
 		return cmp.Compare(a, b)
 	})

gopls/internal/test/marker/testdata/codeaction/functionextraction_issue44813.txt

@@ -26,13 +26,12 @@ package extract
 import "fmt"
 
 func main() {
-	//@codeaction("x", "refactor.extract.function", end=end, result=ext)
 	x := newFunction() //@loc(end, "}")
 	fmt.Printf("%x\n", x)
 }
 
 func newFunction() []rune {
-	x := []rune{}
+	x := []rune{} //@codeaction("x", "refactor.extract.function", end=end, result=ext)
 	s := "HELLO"
 	for _, c := range s {
 		x = append(x, c)

gopls/internal/test/marker/testdata/codeaction/functionextraction_issue50851.txt

@@ -0,0 +1,35 @@
+This test checks that function extraction moves comments along with the
+extracted code.
+
+-- main.go --
+package main
+
+type F struct{}
+
+func (f *F) func1() {
+	println("a")
+
+	println("b") //@ codeaction("print", "refactor.extract.function", end=end, result=result)
+	// This line prints the third letter of the alphabet.
+	println("c") //@loc(end, ")")
+
+	println("d")
+}
+-- @result/main.go --
+package main
+
+type F struct{}
+
+func (f *F) func1() {
+	println("a")
+
+	newFunction() //@loc(end, ")")
+
+	println("d")
+}
+
+func newFunction() {
+	println("b") //@ codeaction("print", "refactor.extract.function", end=end, result=result)
+	// This line prints the third letter of the alphabet.
+	println("c")
+}

gopls/internal/test/marker/testdata/codeaction/functionextraction_issue66289.txt

@@ -29,7 +29,6 @@ import (
 )
 
 func F() error {
-	//@codeaction("a", "refactor.extract.function", end=endF, result=F)
 	a, b, shouldReturn, err := newFunction()
 	if shouldReturn {
 		return err
@@ -39,7 +38,7 @@ func F() error {
 }
 
 func newFunction() ([]byte, []byte, bool, error) {
-	a, err := json.Marshal(0)
+	a, err := json.Marshal(0) //@codeaction("a", "refactor.extract.function", end=endF, result=F)
 	if err != nil {
 		return nil, nil, true, fmt.Errorf("1: %w", err)
 	}
@@ -78,7 +77,6 @@ import (
 )
 
 func G() (x, y int) {
-	//@codeaction("v", "refactor.extract.function", end=endG, result=G)
 	v, shouldReturn, x1, y1 := newFunction()
 	if shouldReturn {
 		return x1, y1
@@ -88,7 +86,7 @@ func G() (x, y int) {
 }
 
 func newFunction() (int, bool, int, int) {
-	v := rand.Int()
+	v := rand.Int() //@codeaction("v", "refactor.extract.function", end=endG, result=G)
 	if v < 0 {
 		return 0, true, 1, 2
 	}