cmd/gofmt: fix computation of function header size

Function sizes are computed to determine whether a function
can be kept on one line or should be split to several lines. Part of the
computation is the function header from the FUNC token and until the
opening { token.

Prior to this change, the function header size used distance from the
original source position of the current token, which led to issues when
the source between FUNC and the original source position was rewritten
(such as whitespace being collapsed). Now we take the current output
position into account, so that header size represents the reformatted
source rather than the original source.

The following files in the Go repository are reformatted with this
change:

* strings/strings_test.go
* cmd/compile/internal/gc/fmt.go

In both cases the reformatting is minor and seems to be correct given
the heuristic to single-line functions longer than 100 columns to
multiple lines.

Fixes #28082

Change-Id: Ib737f6933e09b79e83715211421d5262b366ec93
Reviewed-on: https://go-review.googlesource.com/c/go/+/188818
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -277,7 +277,9 @@ func (o Op) Format(s fmt.State, verb rune)              { o.format(s, verb, FErr
 
 func (t *fmtTypeErr) Format(s fmt.State, verb rune) { typeFormat((*types.Type)(t), s, verb, FErr) }
 func (t *fmtTypeDbg) Format(s fmt.State, verb rune) { typeFormat((*types.Type)(t), s, verb, FDbg) }
-func (t *fmtTypeTypeId) Format(s fmt.State, verb rune) { typeFormat((*types.Type)(t), s, verb, FTypeId) }
+func (t *fmtTypeTypeId) Format(s fmt.State, verb rune) {
+	typeFormat((*types.Type)(t), s, verb, FTypeId)
+}
 func (t *fmtTypeTypeIdName) Format(s fmt.State, verb rune) {
 	typeFormat((*types.Type)(t), s, verb, FTypeIdName)
 }

src/cmd/gofmt/testdata/issue28082.golden

+// 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
+
+// testcase for issue #28082
+
+func foo() {}
+
+func main() {}
+
+func bar() {}

src/cmd/gofmt/testdata/issue28082.input

+// 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
+
+// testcase for issue #28082
+
+func foo(                                                                                                ) {}
+
+func main(                                                                                                      ) {}
+
+func bar() {}

src/go/printer/nodes.go

@@ -794,8 +794,11 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
 		p.print(x)
 
 	case *ast.FuncLit:
-		p.expr(x.Type)
-		p.funcBody(p.distanceFrom(x.Type.Pos()), blank, x.Body)
+		p.print(x.Type.Pos(), token.FUNC)
+		// See the comment in funcDecl about how the header size is computed.
+		startCol := p.out.Column - len("func")
+		p.signature(x.Type.Params, x.Type.Results)
+		p.funcBody(p.distanceFrom(x.Type.Pos(), startCol), blank, x.Body)
 
 	case *ast.ParenExpr:
 		if _, hasParens := x.X.(*ast.ParenExpr); hasParens {
@@ -1689,14 +1692,12 @@ func (p *printer) funcBody(headerSize int, sep whiteSpace, b *ast.BlockStmt) {
 	p.block(b, 1)
 }
 
-// distanceFrom returns the column difference between from and p.pos (the current
-// estimated position) if both are on the same line; if they are on different lines
-// (or unknown) the result is infinity.
-func (p *printer) distanceFrom(from token.Pos) int {
-	if from.IsValid() && p.pos.IsValid() {
-		if f := p.posFor(from); f.Line == p.pos.Line {
-			return p.pos.Column - f.Column
-		}
+// distanceFrom returns the column difference between p.out (the current output
+// position) and startOutCol. If the start position is on a different line from
+// the current position (or either is unknown), the result is infinity.
+func (p *printer) distanceFrom(startPos token.Pos, startOutCol int) int {
+	if startPos.IsValid() && p.pos.IsValid() && p.posFor(startPos).Line == p.pos.Line {
+		return p.out.Column - startOutCol
 	}
 	return infinity
 }
@@ -1704,13 +1705,17 @@ func (p *printer) distanceFrom(from token.Pos) int {
 func (p *printer) funcDecl(d *ast.FuncDecl) {
 	p.setComment(d.Doc)
 	p.print(d.Pos(), token.FUNC, blank)
+	// We have to save startCol only after emitting FUNC; otherwise it can be on a
+	// different line (all whitespace preceding the FUNC is emitted only when the
+	// FUNC is emitted).
+	startCol := p.out.Column - len("func ")
 	if d.Recv != nil {
 		p.parameters(d.Recv) // method: print receiver
 		p.print(blank)
 	}
 	p.expr(d.Name)
 	p.signature(d.Type.Params, d.Type.Results)
-	p.funcBody(p.distanceFrom(d.Pos()), vtab, d.Body)
+	p.funcBody(p.distanceFrom(d.Pos(), startCol), vtab, d.Body)
 }
 
 func (p *printer) decl(decl ast.Decl) {

src/strings/strings_test.go

@@ -197,7 +197,9 @@ func runIndexTests(t *testing.T, f func(s, sep string) int, funcName string, tes
 func TestIndex(t *testing.T)     { runIndexTests(t, Index, "Index", indexTests) }
 func TestLastIndex(t *testing.T) { runIndexTests(t, LastIndex, "LastIndex", lastIndexTests) }
 func TestIndexAny(t *testing.T)  { runIndexTests(t, IndexAny, "IndexAny", indexAnyTests) }
-func TestLastIndexAny(t *testing.T) { runIndexTests(t, LastIndexAny, "LastIndexAny", lastIndexAnyTests) }
+func TestLastIndexAny(t *testing.T) {
+	runIndexTests(t, LastIndexAny, "LastIndexAny", lastIndexAnyTests)
+}
 
 func TestIndexByte(t *testing.T) {
 	for _, tt := range indexTests {