go/printer: canonicalize //go:build and // +build lines while formatting

Part of //go:build change (#41184).
See https://golang.org/design/draft-gobuild

Gofmt and any other go/printer-using program will now:

 - move //go:build and //+build lines to the appropriate file location
 - if there's no //go:build line, add one derived from the // +build lines
 - if there is a //go:build line, recompute and replace any // +build lines
   to match what the //go:build line says

For Go 1.17.

Change-Id: Ide5cc3b4a07507ba9ed6f8b0de846e840876f49f
Reviewed-on: https://go-review.googlesource.com/c/go/+/240608
Trust: Russ Cox <rsc@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/go/build/deps_test.go

@@ -279,7 +279,10 @@ var depsRules = `
 	< go/ast
 	< go/parser;
 
-	go/parser, text/tabwriter
+	FMT
+	< go/build/constraint;
+
+	go/build/constraint, go/parser, text/tabwriter
 	< go/printer
 	< go/format;
 
@@ -292,9 +295,6 @@ var depsRules = `
 	container/heap, go/constant, go/parser, regexp
 	< go/types;
 
-	FMT
-	< go/build/constraint;
-
 	go/build/constraint, go/doc, go/parser, internal/goroot, internal/goversion
 	< go/build;
 

src/go/format/format_test.go

@@ -151,6 +151,10 @@ var tests = []string{
 	// erroneous programs
 	"ERROR1 + 2 +",
 	"ERRORx :=  0",
+
+	// build comments
+	"// copyright\n\n//go:build x\n\npackage p\n",
+	"// copyright\n\n//go:build x\n// +build x\n\npackage p\n",
 }
 
 func String(s string) (string, error) {

src/go/printer/gobuild.go

@@ -0,0 +1,170 @@
+// Copyright 2020 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 printer
+
+import (
+	"go/build/constraint"
+	"sort"
+	"text/tabwriter"
+)
+
+func (p *printer) fixGoBuildLines() {
+	if len(p.goBuild)+len(p.plusBuild) == 0 {
+		return
+	}
+
+	// Find latest possible placement of //go:build and // +build comments.
+	// That's just after the last blank line before we find a non-comment.
+	// (We'll add another blank line after our comment block.)
+	// When we start dropping // +build comments, we can skip over /* */ comments too.
+	// Note that we are processing tabwriter input, so every comment
+	// begins and ends with a tabwriter.Escape byte.
+	// And some newlines have turned into \f bytes.
+	insert := 0
+	for pos := 0; ; {
+		// Skip leading space at beginning of line.
+		blank := true
+		for pos < len(p.output) && (p.output[pos] == ' ' || p.output[pos] == '\t') {
+			pos++
+		}
+		// Skip over // comment if any.
+		if pos+3 < len(p.output) && p.output[pos] == tabwriter.Escape && p.output[pos+1] == '/' && p.output[pos+2] == '/' {
+			blank = false
+			for pos < len(p.output) && !isNL(p.output[pos]) {
+				pos++
+			}
+		}
+		// Skip over \n at end of line.
+		if pos >= len(p.output) || !isNL(p.output[pos]) {
+			break
+		}
+		pos++
+
+		if blank {
+			insert = pos
+		}
+	}
+
+	// If there is a //go:build comment before the place we identified,
+	// use that point instead. (Earlier in the file is always fine.)
+	if len(p.goBuild) > 0 && p.goBuild[0] < insert {
+		insert = p.goBuild[0]
+	} else if len(p.plusBuild) > 0 && p.plusBuild[0] < insert {
+		insert = p.plusBuild[0]
+	}
+
+	var x constraint.Expr
+	switch len(p.goBuild) {
+	case 0:
+		// Synthesize //go:build expression from // +build lines.
+		for _, pos := range p.plusBuild {
+			y, err := constraint.Parse(p.commentTextAt(pos))
+			if err != nil {
+				x = nil
+				break
+			}
+			if x == nil {
+				x = y
+			} else {
+				x = &constraint.AndExpr{X: x, Y: y}
+			}
+		}
+	case 1:
+		// Parse //go:build expression.
+		x, _ = constraint.Parse(p.commentTextAt(p.goBuild[0]))
+	}
+
+	var block []byte
+	if x == nil {
+		// Don't have a valid //go:build expression to treat as truth.
+		// Bring all the lines together but leave them alone.
+		// Note that these are already tabwriter-escaped.
+		for _, pos := range p.goBuild {
+			block = append(block, p.lineAt(pos)...)
+		}
+		for _, pos := range p.plusBuild {
+			block = append(block, p.lineAt(pos)...)
+		}
+	} else {
+		block = append(block, tabwriter.Escape)
+		block = append(block, "//go:build "...)
+		block = append(block, x.String()...)
+		block = append(block, tabwriter.Escape, '\n')
+		if len(p.plusBuild) > 0 {
+			lines, err := constraint.PlusBuildLines(x)
+			if err != nil {
+				lines = []string{"// +build error: " + err.Error()}
+			}
+			for _, line := range lines {
+				block = append(block, tabwriter.Escape)
+				block = append(block, line...)
+				block = append(block, tabwriter.Escape, '\n')
+			}
+		}
+	}
+	block = append(block, '\n')
+
+	// Build sorted list of lines to delete from remainder of output.
+	toDelete := append(p.goBuild, p.plusBuild...)
+	sort.Ints(toDelete)
+
+	// Collect output after insertion point, with lines deleted, into after.
+	var after []byte
+	start := insert
+	for _, end := range toDelete {
+		if end < start {
+			continue
+		}
+		after = appendLines(after, p.output[start:end])
+		start = end + len(p.lineAt(end))
+	}
+	after = appendLines(after, p.output[start:])
+	if n := len(after); n >= 2 && isNL(after[n-1]) && isNL(after[n-2]) {
+		after = after[:n-1]
+	}
+
+	p.output = p.output[:insert]
+	p.output = append(p.output, block...)
+	p.output = append(p.output, after...)
+}
+
+// appendLines is like append(x, y...)
+// but it avoids creating doubled blank lines,
+// which would not be gofmt-standard output.
+// It assumes that only whole blocks of lines are being appended,
+// not line fragments.
+func appendLines(x, y []byte) []byte {
+	if len(y) > 0 && isNL(y[0]) && // y starts in blank line
+		(len(x) == 0 || len(x) >= 2 && isNL(x[len(x)-1]) && isNL(x[len(x)-2])) { // x is empty or ends in blank line
+		y = y[1:] // delete y's leading blank line
+	}
+	return append(x, y...)
+}
+
+func (p *printer) lineAt(start int) []byte {
+	pos := start
+	for pos < len(p.output) && !isNL(p.output[pos]) {
+		pos++
+	}
+	if pos < len(p.output) {
+		pos++
+	}
+	return p.output[start:pos]
+}
+
+func (p *printer) commentTextAt(start int) string {
+	if start < len(p.output) && p.output[start] == tabwriter.Escape {
+		start++
+	}
+	pos := start
+	for pos < len(p.output) && p.output[pos] != tabwriter.Escape && !isNL(p.output[pos]) {
+		pos++
+	}
+	return string(p.output[start:pos])
+}
+
+func isNL(b byte) bool {
+	return b == '\n' || b == '\f'
+}

src/go/printer/printer.go

@@ -8,6 +8,7 @@ package printer
 import (
 	"fmt"
 	"go/ast"
+	"go/build/constraint"
 	"go/token"
 	"io"
 	"os"
@@ -64,6 +65,8 @@ type printer struct {
 	lastTok      token.Token  // last token printed (token.ILLEGAL if it's whitespace)
 	prevOpen     token.Token  // previous non-brace "open" token (, [, or token.ILLEGAL
 	wsbuf        []whiteSpace // delayed white space
+	goBuild      []int        // start index of all //go:build comments in output
+	plusBuild    []int        // start index of all // +build comments in output
 
 	// Positions
 	// The out position differs from the pos position when the result
@@ -649,6 +652,11 @@ func (p *printer) writeComment(comment *ast.Comment) {
 
 	// shortcut common case of //-style comments
 	if text[1] == '/' {
+		if constraint.IsGoBuild(text) {
+			p.goBuild = append(p.goBuild, len(p.output))
+		} else if constraint.IsPlusBuild(text) {
+			p.plusBuild = append(p.plusBuild, len(p.output))
+		}
 		p.writeString(pos, trimRight(text), true)
 		return
 	}
@@ -1122,6 +1130,8 @@ func (p *printer) printNode(node interface{}) error {
 	// get comments ready for use
 	p.nextComment()
 
+	p.print(pmode(0))
+
 	// format node
 	switch n := node.(type) {
 	case ast.Expr:
@@ -1313,6 +1323,10 @@ func (cfg *Config) fprint(output io.Writer, fset *token.FileSet, node interface{
 	p.impliedSemi = false // EOF acts like a newline
 	p.flush(token.Position{Offset: infinity, Line: infinity}, token.EOF)
 
+	// output is buffered in p.output now.
+	// fix //go:build and // +build comments if needed.
+	p.fixGoBuildLines()
+
 	// redirect output through a trimmer to eliminate trailing whitespace
 	// (Input to a tabwriter must be untrimmed since trailing tabs provide
 	// formatting information. The tabwriter could provide trimming

src/go/printer/printer_test.go

@@ -88,8 +88,11 @@ func lineAt(text []byte, offs int) []byte {
 
 // diff compares a and b.
 func diff(aname, bname string, a, b []byte) error {
-	var buf bytes.Buffer // holding long error message
+	if bytes.Equal(a, b) {
+		return nil
+	}
 
+	var buf bytes.Buffer // holding long error message
 	// compare lengths
 	if len(a) != len(b) {
 		fmt.Fprintf(&buf, "\nlength changed: len(%s) = %d, len(%s) = %d", aname, len(a), bname, len(b))
@@ -97,7 +100,7 @@ func diff(aname, bname string, a, b []byte) error {
 
 	// compare contents
 	line := 1
-	offs := 1
+	offs := 0
 	for i := 0; i < len(a) && i < len(b); i++ {
 		ch := a[i]
 		if ch != b[i] {
@@ -112,11 +115,9 @@ func diff(aname, bname string, a, b []byte) error {
 		}
 	}
 
-	if buf.Len() > 0 {
+	fmt.Fprintf(&buf, "\n%s:\n%s\n%s:\n%s", aname, a, bname, b)
 	return errors.New(buf.String())
 }
-	return nil
-}
 
 func runcheck(t *testing.T, source, golden string, mode checkMode) {
 	src, err := os.ReadFile(source)
@@ -207,6 +208,13 @@ var data = []entry{
 	{"go2numbers.input", "go2numbers.golden", idempotent},
 	{"go2numbers.input", "go2numbers.norm", normNumber | idempotent},
 	{"generics.input", "generics.golden", idempotent},
+	{"gobuild1.input", "gobuild1.golden", idempotent},
+	{"gobuild2.input", "gobuild2.golden", idempotent},
+	{"gobuild3.input", "gobuild3.golden", idempotent},
+	{"gobuild4.input", "gobuild4.golden", idempotent},
+	{"gobuild5.input", "gobuild5.golden", idempotent},
+	{"gobuild6.input", "gobuild6.golden", idempotent},
+	{"gobuild7.input", "gobuild7.golden", idempotent},
 }
 
 func TestFiles(t *testing.T) {

src/go/printer/testdata/gobuild1.golden

@@ -0,0 +1,6 @@
+//go:build x
+// +build x
+
+package p
+
+func f()

src/go/printer/testdata/gobuild1.input

@@ -0,0 +1,7 @@
+package p
+
+//go:build x
+
+func f()
+
+// +build y

src/go/printer/testdata/gobuild2.golden

@@ -0,0 +1,8 @@
+//go:build x
+// +build x
+
+// other comment
+
+package p
+
+func f()

src/go/printer/testdata/gobuild2.input

@@ -0,0 +1,9 @@
+// +build y
+
+// other comment
+
+package p
+
+func f()
+
+//go:build x

src/go/printer/testdata/gobuild3.golden

@@ -0,0 +1,10 @@
+// other comment
+
+//go:build x
+// +build x
+
+// yet another comment
+
+package p
+
+func f()

src/go/printer/testdata/gobuild3.input

@@ -0,0 +1,11 @@
+// other comment
+
+// +build y
+
+// yet another comment
+
+package p
+
+//go:build x
+
+func f()

src/go/printer/testdata/gobuild4.golden

@@ -0,0 +1,6 @@
+//go:build (x || y) && z
+// +build x y
+// +build z
+
+// doc comment
+package p

src/go/printer/testdata/gobuild4.input

@@ -0,0 +1,5 @@
+// doc comment
+package p
+
+// +build x y
+// +build z

src/go/printer/testdata/gobuild5.golden

@@ -0,0 +1,4 @@
+//go:build !(x || y) && z
+// +build !x,!y,z
+
+package p

src/go/printer/testdata/gobuild5.input

@@ -0,0 +1,4 @@
+//go:build !(x || y) && z
+// +build something else
+
+package p

src/go/printer/testdata/gobuild6.golden

@@ -0,0 +1,5 @@
+//go:build !(x || y) && z
+
+// no +build line
+
+package p

src/go/printer/testdata/gobuild6.input

@@ -0,0 +1,4 @@
+//go:build !(x || y) && z
+// no +build line
+
+package p

src/go/printer/testdata/gobuild7.golden

@@ -0,0 +1,11 @@
+// Copyright 2013 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.
+
+// TODO(rsc): Delete this file once Go 1.17 comes out and we can retire Go 1.15 support.
+
+//go:build !go1.16
+// +build !go1.16
+
+// Package buildtag defines an Analyzer that checks build tags.
+package buildtag

src/go/printer/testdata/gobuild7.input

@@ -0,0 +1,11 @@
+// Copyright 2013 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.
+
+// TODO(rsc): Delete this file once Go 1.17 comes out and we can retire Go 1.15 support.
+
+//go:build !go1.16
+// +build !go1.16
+
+// Package buildtag defines an Analyzer that checks build tags.
+package buildtag