1 files changed, 574 insertions, 0 deletions
diff --git a/src/go/build/constraint/expr.go b/src/go/build/constraint/expr.go
new file mode 100644
index 0000000..3b27870
--- /dev/null
+++ b/src/go/build/constraint/expr.go
@@ -0,0 +1,574 @@
+// 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 constraint implements parsing and evaluation of build constraint lines.
+// See https://golang.org/cmd/go/#hdr-Build_constraints for documentation about build constraints themselves.
+//
+// This package parses both the original “// +build” syntax and the “//go:build” syntax that will be added in Go 1.17.
+// The parser is being included in Go 1.16 to allow tools that need to process Go 1.17 source code
+// to still be built against the Go 1.16 release.
+// See https://golang.org/design/draft-gobuild for details about the “//go:build” syntax.
+package constraint
+
+import (
+	"errors"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// An Expr is a build tag constraint expression.
+// The underlying concrete type is *AndExpr, *OrExpr, *NotExpr, or *TagExpr.
+type Expr interface {
+	// String returns the string form of the expression,
+	// using the boolean syntax used in //go:build lines.
+	String() string
+
+	// Eval reports whether the expression evaluates to true.
+	// It calls ok(tag) as needed to find out whether a given build tag
+	// is satisfied by the current build configuration.
+	Eval(ok func(tag string) bool) bool
+
+	// The presence of an isExpr method explicitly marks the type as an Expr.
+	// Only implementations in this package should be used as Exprs.
+	isExpr()
+}
+
+// A TagExpr is an Expr for the single tag Tag.
+type TagExpr struct {
+	Tag string // for example, “linux” or “cgo”
+}
+
+func (x *TagExpr) isExpr() {}
+
+func (x *TagExpr) Eval(ok func(tag string) bool) bool {
+	return ok(x.Tag)
+}
+
+func (x *TagExpr) String() string {
+	return x.Tag
+}
+
+func tag(tag string) Expr { return &TagExpr{tag} }
+
+// A NotExpr represents the expression !X (the negation of X).
+type NotExpr struct {
+	X Expr
+}
+
+func (x *NotExpr) isExpr() {}
+
+func (x *NotExpr) Eval(ok func(tag string) bool) bool {
+	return !x.X.Eval(ok)
+}
+
+func (x *NotExpr) String() string {
+	s := x.X.String()
+	switch x.X.(type) {
+	case *AndExpr, *OrExpr:
+		s = "(" + s + ")"
+	}
+	return "!" + s
+}
+
+func not(x Expr) Expr { return &NotExpr{x} }
+
+// An AndExpr represents the expression X && Y.
+type AndExpr struct {
+	X, Y Expr
+}
+
+func (x *AndExpr) isExpr() {}
+
+func (x *AndExpr) Eval(ok func(tag string) bool) bool {
+	// Note: Eval both, to make sure ok func observes all tags.
+	xok := x.X.Eval(ok)
+	yok := x.Y.Eval(ok)
+	return xok && yok
+}
+
+func (x *AndExpr) String() string {
+	return andArg(x.X) + " && " + andArg(x.Y)
+}
+
+func andArg(x Expr) string {
+	s := x.String()
+	if _, ok := x.(*OrExpr); ok {
+		s = "(" + s + ")"
+	}
+	return s
+}
+
+func and(x, y Expr) Expr {
+	return &AndExpr{x, y}
+}
+
+// An OrExpr represents the expression X || Y.
+type OrExpr struct {
+	X, Y Expr
+}
+
+func (x *OrExpr) isExpr() {}
+
+func (x *OrExpr) Eval(ok func(tag string) bool) bool {
+	// Note: Eval both, to make sure ok func observes all tags.
+	xok := x.X.Eval(ok)
+	yok := x.Y.Eval(ok)
+	return xok || yok
+}
+
+func (x *OrExpr) String() string {
+	return orArg(x.X) + " || " + orArg(x.Y)
+}
+
+func orArg(x Expr) string {
+	s := x.String()
+	if _, ok := x.(*AndExpr); ok {
+		s = "(" + s + ")"
+	}
+	return s
+}
+
+func or(x, y Expr) Expr {
+	return &OrExpr{x, y}
+}
+
+// A SyntaxError reports a syntax error in a parsed build expression.
+type SyntaxError struct {
+	Offset int    // byte offset in input where error was detected
+	Err    string // description of error
+}
+
+func (e *SyntaxError) Error() string {
+	return e.Err
+}
+
+var errNotConstraint = errors.New("not a build constraint")
+
+// Parse parses a single build constraint line of the form “//go:build ...” or “// +build ...”
+// and returns the corresponding boolean expression.
+func Parse(line string) (Expr, error) {
+	if text, ok := splitGoBuild(line); ok {
+		return parseExpr(text)
+	}
+	if text, ok := splitPlusBuild(line); ok {
+		return parsePlusBuildExpr(text), nil
+	}
+	return nil, errNotConstraint
+}
+
+// IsGoBuild reports whether the line of text is a “//go:build” constraint.
+// It only checks the prefix of the text, not that the expression itself parses.
+func IsGoBuild(line string) bool {
+	_, ok := splitGoBuild(line)
+	return ok
+}
+
+// splitGoBuild splits apart the leading //go:build prefix in line from the build expression itself.
+// It returns "", false if the input is not a //go:build line or if the input contains multiple lines.
+func splitGoBuild(line string) (expr string, ok bool) {
+	// A single trailing newline is OK; otherwise multiple lines are not.
+	if len(line) > 0 && line[len(line)-1] == '\n' {
+		line = line[:len(line)-1]
+	}
+	if strings.Contains(line, "\n") {
+		return "", false
+	}
+
+	if !strings.HasPrefix(line, "//go:build") {
+		return "", false
+	}
+
+	line = strings.TrimSpace(line)
+	line = line[len("//go:build"):]
+
+	// If strings.TrimSpace finds more to trim after removing the //go:build prefix,
+	// it means that the prefix was followed by a space, making this a //go:build line
+	// (as opposed to a //go:buildsomethingelse line).
+	// If line is empty, we had "//go:build" by itself, which also counts.
+	trim := strings.TrimSpace(line)
+	if len(line) == len(trim) && line != "" {
+		return "", false
+	}
+
+	return trim, true
+}
+
+// An exprParser holds state for parsing a build expression.
+type exprParser struct {
+	s string // input string
+	i int    // next read location in s
+
+	tok   string // last token read
+	isTag bool
+	pos   int // position (start) of last token
+}
+
+// parseExpr parses a boolean build tag expression.
+func parseExpr(text string) (x Expr, err error) {
+	defer func() {
+		if e := recover(); e != nil {
+			if e, ok := e.(*SyntaxError); ok {
+				err = e
+				return
+			}
+			panic(e) // unreachable unless parser has a bug
+		}
+	}()
+
+	p := &exprParser{s: text}
+	x = p.or()
+	if p.tok != "" {
+		panic(&SyntaxError{Offset: p.pos, Err: "unexpected token " + p.tok})
+	}
+	return x, nil
+}
+
+// or parses a sequence of || expressions.
+// On entry, the next input token has not yet been lexed.
+// On exit, the next input token has been lexed and is in p.tok.
+func (p *exprParser) or() Expr {
+	x := p.and()
+	for p.tok == "||" {
+		x = or(x, p.and())
+	}
+	return x
+}
+
+// and parses a sequence of && expressions.
+// On entry, the next input token has not yet been lexed.
+// On exit, the next input token has been lexed and is in p.tok.
+func (p *exprParser) and() Expr {
+	x := p.not()
+	for p.tok == "&&" {
+		x = and(x, p.not())
+	}
+	return x
+}
+
+// not parses a ! expression.
+// On entry, the next input token has not yet been lexed.
+// On exit, the next input token has been lexed and is in p.tok.
+func (p *exprParser) not() Expr {
+	p.lex()
+	if p.tok == "!" {
+		p.lex()
+		if p.tok == "!" {
+			panic(&SyntaxError{Offset: p.pos, Err: "double negation not allowed"})
+		}
+		return not(p.atom())
+	}
+	return p.atom()
+}
+
+// atom parses a tag or a parenthesized expression.
+// On entry, the next input token HAS been lexed.
+// On exit, the next input token has been lexed and is in p.tok.
+func (p *exprParser) atom() Expr {
+	// first token already in p.tok
+	if p.tok == "(" {
+		pos := p.pos
+		defer func() {
+			if e := recover(); e != nil {
+				if e, ok := e.(*SyntaxError); ok && e.Err == "unexpected end of expression" {
+					e.Err = "missing close paren"
+				}
+				panic(e)
+			}
+		}()
+		x := p.or()
+		if p.tok != ")" {
+			panic(&SyntaxError{Offset: pos, Err: "missing close paren"})
+		}
+		p.lex()
+		return x
+	}
+
+	if !p.isTag {
+		if p.tok == "" {
+			panic(&SyntaxError{Offset: p.pos, Err: "unexpected end of expression"})
+		}
+		panic(&SyntaxError{Offset: p.pos, Err: "unexpected token " + p.tok})
+	}
+	tok := p.tok
+	p.lex()
+	return tag(tok)
+}
+
+// lex finds and consumes the next token in the input stream.
+// On return, p.tok is set to the token text,
+// p.isTag reports whether the token was a tag,
+// and p.pos records the byte offset of the start of the token in the input stream.
+// If lex reaches the end of the input, p.tok is set to the empty string.
+// For any other syntax error, lex panics with a SyntaxError.
+func (p *exprParser) lex() {
+	p.isTag = false
+	for p.i < len(p.s) && (p.s[p.i] == ' ' || p.s[p.i] == '\t') {
+		p.i++
+	}
+	if p.i >= len(p.s) {
+		p.tok = ""
+		p.pos = p.i
+		return
+	}
+	switch p.s[p.i] {
+	case '(', ')', '!':
+		p.pos = p.i
+		p.i++
+		p.tok = p.s[p.pos:p.i]
+		return
+
+	case '&', '|':
+		if p.i+1 >= len(p.s) || p.s[p.i+1] != p.s[p.i] {
+			panic(&SyntaxError{Offset: p.i, Err: "invalid syntax at " + string(rune(p.s[p.i]))})
+		}
+		p.pos = p.i
+		p.i += 2
+		p.tok = p.s[p.pos:p.i]
+		return
+	}
+
+	tag := p.s[p.i:]
+	for i, c := range tag {
+		if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
+			tag = tag[:i]
+			break
+		}
+	}
+	if tag == "" {
+		c, _ := utf8.DecodeRuneInString(p.s[p.i:])
+		panic(&SyntaxError{Offset: p.i, Err: "invalid syntax at " + string(c)})
+	}
+
+	p.pos = p.i
+	p.i += len(tag)
+	p.tok = p.s[p.pos:p.i]
+	p.isTag = true
+	return
+}
+
+// IsPlusBuild reports whether the line of text is a “// +build” constraint.
+// It only checks the prefix of the text, not that the expression itself parses.
+func IsPlusBuild(line string) bool {
+	_, ok := splitPlusBuild(line)
+	return ok
+}
+
+// splitGoBuild splits apart the leading //go:build prefix in line from the build expression itself.
+// It returns "", false if the input is not a //go:build line or if the input contains multiple lines.
+func splitPlusBuild(line string) (expr string, ok bool) {
+	// A single trailing newline is OK; otherwise multiple lines are not.
+	if len(line) > 0 && line[len(line)-1] == '\n' {
+		line = line[:len(line)-1]
+	}
+	if strings.Contains(line, "\n") {
+		return "", false
+	}
+
+	if !strings.HasPrefix(line, "//") {
+		return "", false
+	}
+	line = line[len("//"):]
+	// Note the space is optional; "//+build" is recognized too.
+	line = strings.TrimSpace(line)
+
+	if !strings.HasPrefix(line, "+build") {
+		return "", false
+	}
+	line = line[len("+build"):]
+
+	// If strings.TrimSpace finds more to trim after removing the +build prefix,
+	// it means that the prefix was followed by a space, making this a +build line
+	// (as opposed to a +buildsomethingelse line).
+	// If line is empty, we had "// +build" by itself, which also counts.
+	trim := strings.TrimSpace(line)
+	if len(line) == len(trim) && line != "" {
+		return "", false
+	}
+
+	return trim, true
+}
+
+// parsePlusBuildExpr parses a legacy build tag expression (as used with “// +build”).
+func parsePlusBuildExpr(text string) Expr {
+	var x Expr
+	for _, clause := range strings.Fields(text) {
+		var y Expr
+		for _, lit := range strings.Split(clause, ",") {
+			var z Expr
+			var neg bool
+			if strings.HasPrefix(lit, "!!") || lit == "!" {
+				z = tag("ignore")
+			} else {
+				if strings.HasPrefix(lit, "!") {
+					neg = true
+					lit = lit[len("!"):]
+				}
+				if isValidTag(lit) {
+					z = tag(lit)
+				} else {
+					z = tag("ignore")
+				}
+				if neg {
+					z = not(z)
+				}
+			}
+			if y == nil {
+				y = z
+			} else {
+				y = and(y, z)
+			}
+		}
+		if x == nil {
+			x = y
+		} else {
+			x = or(x, y)
+		}
+	}
+	return x
+}
+
+// isValidTag reports whether the word is a valid build tag.
+// Tags must be letters, digits, underscores or dots.
+// Unlike in Go identifiers, all digits are fine (e.g., "386").
+func isValidTag(word string) bool {
+	if word == "" {
+		return false
+	}
+	for _, c := range word {
+		if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
+			return false
+		}
+	}
+	return true
+}
+
+var errComplex = errors.New("expression too complex for // +build lines")
+
+// PlusBuildLines returns a sequence of “// +build” lines that evaluate to the build expression x.
+// If the expression is too complex to convert directly to “// +build” lines, PlusBuildLines returns an error.
+func PlusBuildLines(x Expr) ([]string, error) {
+	// Push all NOTs to the expression leaves, so that //go:build !(x && y) can be treated as !x || !y.
+	// This rewrite is both efficient and commonly needed, so it's worth doing.
+	// Essentially all other possible rewrites are too expensive and too rarely needed.
+	x = pushNot(x, false)
+
+	// Split into AND of ORs of ANDs of literals (tag or NOT tag).
+	var split [][][]Expr
+	for _, or := range appendSplitAnd(nil, x) {
+		var ands [][]Expr
+		for _, and := range appendSplitOr(nil, or) {
+			var lits []Expr
+			for _, lit := range appendSplitAnd(nil, and) {
+				switch lit.(type) {
+				case *TagExpr, *NotExpr:
+					lits = append(lits, lit)
+				default:
+					return nil, errComplex
+				}
+			}
+			ands = append(ands, lits)
+		}
+		split = append(split, ands)
+	}
+
+	// If all the ORs have length 1 (no actual OR'ing going on),
+	// push the top-level ANDs to the bottom level, so that we get
+	// one // +build line instead of many.
+	maxOr := 0
+	for _, or := range split {
+		if maxOr < len(or) {
+			maxOr = len(or)
+		}
+	}
+	if maxOr == 1 {
+		var lits []Expr
+		for _, or := range split {
+			lits = append(lits, or[0]...)
+		}
+		split = [][][]Expr{{lits}}
+	}
+
+	// Prepare the +build lines.
+	var lines []string
+	for _, or := range split {
+		line := "// +build"
+		for _, and := range or {
+			clause := ""
+			for i, lit := range and {
+				if i > 0 {
+					clause += ","
+				}
+				clause += lit.String()
+			}
+			line += " " + clause
+		}
+		lines = append(lines, line)
+	}
+
+	return lines, nil
+}
+
+// pushNot applies DeMorgan's law to push negations down the expression,
+// so that only tags are negated in the result.
+// (It applies the rewrites !(X && Y) => (!X || !Y) and !(X || Y) => (!X && !Y).)
+func pushNot(x Expr, not bool) Expr {
+	switch x := x.(type) {
+	default:
+		// unreachable
+		return x
+	case *NotExpr:
+		if _, ok := x.X.(*TagExpr); ok && !not {
+			return x
+		}
+		return pushNot(x.X, !not)
+	case *TagExpr:
+		if not {
+			return &NotExpr{X: x}
+		}
+		return x
+	case *AndExpr:
+		x1 := pushNot(x.X, not)
+		y1 := pushNot(x.Y, not)
+		if not {
+			return or(x1, y1)
+		}
+		if x1 == x.X && y1 == x.Y {
+			return x
+		}
+		return and(x1, y1)
+	case *OrExpr:
+		x1 := pushNot(x.X, not)
+		y1 := pushNot(x.Y, not)
+		if not {
+			return and(x1, y1)
+		}
+		if x1 == x.X && y1 == x.Y {
+			return x
+		}
+		return or(x1, y1)
+	}
+}
+
+// appendSplitAnd appends x to list while splitting apart any top-level && expressions.
+// For example, appendSplitAnd({W}, X && Y && Z) = {W, X, Y, Z}.
+func appendSplitAnd(list []Expr, x Expr) []Expr {
+	if x, ok := x.(*AndExpr); ok {
+		list = appendSplitAnd(list, x.X)
+		list = appendSplitAnd(list, x.Y)
+		return list
+	}
+	return append(list, x)
+}
+
+// appendSplitOr appends x to list while splitting apart any top-level || expressions.
+// For example, appendSplitOr({W}, X || Y || Z) = {W, X, Y, Z}.
+func appendSplitOr(list []Expr, x Expr) []Expr {
+	if x, ok := x.(*OrExpr); ok {
+		list = appendSplitOr(list, x.X)
+		list = appendSplitOr(list, x.Y)
+		return list
+	}
+	return append(list, x)
+}