1 files changed, 151 insertions, 65 deletions

diff --git a/src/cmd/go/internal/imports/build.go b/src/cmd/go/internal/imports/build.go
index 50aeabc578..ff6bea6777 100644
--- a/src/cmd/go/internal/imports/build.go
+++ b/src/cmd/go/internal/imports/build.go
@@ -2,17 +2,51 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Copied from Go distribution src/go/build/build.go, syslist.go
+// Copied from Go distribution src/go/build/build.go, syslist.go.
+// That package does not export the ability to process raw file data,
+// although we could fake it with an appropriate build.Context
+// and a lot of unwrapping.
+// More importantly, that package does not implement the tags["*"]
+// special case, in which both tag and !tag are considered to be true
+// for essentially all tags (except "ignore").
+//
+// If we added this API to go/build directly, we wouldn't need this
+// file anymore, but this API is not terribly general-purpose and we
+// don't really want to commit to any public form of it, nor do we
+// want to move the core parts of go/build into a top-level internal package.
+// These details change very infrequently, so the copy is fine.
 
 package imports
 
 import (
 	"bytes"
+	"errors"
+	"fmt"
+	"go/build/constraint"
 	"strings"
 	"unicode"
 )
 
-var slashslash = []byte("//")
+var (
+	bSlashSlash = []byte("//")
+	bStarSlash  = []byte("*/")
+	bSlashStar  = []byte("/*")
+	bPlusBuild  = []byte("+build")
+
+	goBuildComment = []byte("//go:build")
+
+	errGoBuildWithoutBuild = errors.New("//go:build comment without // +build comment")
+	errMultipleGoBuild     = errors.New("multiple //go:build comments")
+)
+
+func isGoBuildComment(line []byte) bool {
+	if !bytes.HasPrefix(line, goBuildComment) {
+		return false
+	}
+	line = bytes.TrimSpace(line)
+	rest := line[len(goBuildComment):]
+	return len(rest) == 0 || len(bytes.TrimSpace(rest)) < len(rest)
+}
 
 // ShouldBuild reports whether it is okay to use this file,
 // The rule is that in the file's leading run of // comments
@@ -34,10 +68,61 @@ var slashslash = []byte("//")
 // in any build.
 //
 func ShouldBuild(content []byte, tags map[string]bool) bool {
-	// Pass 1. Identify leading run of // comments and blank lines,
+	// Identify leading run of // comments and blank lines,
 	// which must be followed by a blank line.
+	// Also identify any //go:build comments.
+	content, goBuild, _, err := parseFileHeader(content)
+	if err != nil {
+		return false
+	}
+
+	// If //go:build line is present, it controls.
+	// Otherwise fall back to +build processing.
+	var shouldBuild bool
+	switch {
+	case goBuild != nil:
+		x, err := constraint.Parse(string(goBuild))
+		if err != nil {
+			return false
+		}
+		shouldBuild = eval(x, tags, true)
+
+	default:
+		shouldBuild = true
+		p := content
+		for len(p) > 0 {
+			line := p
+			if i := bytes.IndexByte(line, '\n'); i >= 0 {
+				line, p = line[:i], p[i+1:]
+			} else {
+				p = p[len(p):]
+			}
+			line = bytes.TrimSpace(line)
+			if !bytes.HasPrefix(line, bSlashSlash) || !bytes.Contains(line, bPlusBuild) {
+				continue
+			}
+			text := string(line)
+			if !constraint.IsPlusBuild(text) {
+				continue
+			}
+			if x, err := constraint.Parse(text); err == nil {
+				if !eval(x, tags, true) {
+					shouldBuild = false
+				}
+			}
+		}
+	}
+
+	return shouldBuild
+}
+
+func parseFileHeader(content []byte) (trimmed, goBuild []byte, sawBinaryOnly bool, err error) {
 	end := 0
 	p := content
+	ended := false       // found non-blank, non-// line, so stopped accepting // +build lines
+	inSlashStar := false // in /* */ comment
+
+Lines:
 	for len(p) > 0 {
 		line := p
 		if i := bytes.IndexByte(line, '\n'); i >= 0 {
@@ -46,78 +131,61 @@ func ShouldBuild(content []byte, tags map[string]bool) bool {
 			p = p[len(p):]
 		}
 		line = bytes.TrimSpace(line)
-		if len(line) == 0 { // Blank line
+		if len(line) == 0 && !ended { // Blank line
+			// Remember position of most recent blank line.
+			// When we find the first non-blank, non-// line,
+			// this "end" position marks the latest file position
+			// where a // +build line can appear.
+			// (It must appear _before_ a blank line before the non-blank, non-// line.
+			// Yes, that's confusing, which is part of why we moved to //go:build lines.)
+			// Note that ended==false here means that inSlashStar==false,
+			// since seeing a /* would have set ended==true.
 			end = len(content) - len(p)
-			continue
+			continue Lines
 		}
-		if !bytes.HasPrefix(line, slashslash) { // Not comment line
-			break
+		if !bytes.HasPrefix(line, bSlashSlash) { // Not comment line
+			ended = true
 		}
-	}
-	content = content[:end]
 
-	// Pass 2.  Process each line in the run.
-	p = content
-	allok := true
-	for len(p) > 0 {
-		line := p
-		if i := bytes.IndexByte(line, '\n'); i >= 0 {
-			line, p = line[:i], p[i+1:]
-		} else {
-			p = p[len(p):]
-		}
-		line = bytes.TrimSpace(line)
-		if !bytes.HasPrefix(line, slashslash) {
-			continue
+		if !inSlashStar && isGoBuildComment(line) {
+			if goBuild != nil {
+				return nil, nil, false, errMultipleGoBuild
+			}
+			goBuild = line
 		}
-		line = bytes.TrimSpace(line[len(slashslash):])
-		if len(line) > 0 && line[0] == '+' {
-			// Looks like a comment +line.
-			f := strings.Fields(string(line))
-			if f[0] == "+build" {
-				ok := false
-				for _, tok := range f[1:] {
-					if matchTags(tok, tags) {
-						ok = true
-					}
-				}
-				if !ok {
-					allok = false
+
+	Comments:
+		for len(line) > 0 {
+			if inSlashStar {
+				if i := bytes.Index(line, bStarSlash); i >= 0 {
+					inSlashStar = false
+					line = bytes.TrimSpace(line[i+len(bStarSlash):])
+					continue Comments
 				}
+				continue Lines
 			}
+			if bytes.HasPrefix(line, bSlashSlash) {
+				continue Lines
+			}
+			if bytes.HasPrefix(line, bSlashStar) {
+				inSlashStar = true
+				line = bytes.TrimSpace(line[len(bSlashStar):])
+				continue Comments
+			}
+			// Found non-comment text.
+			break Lines
 		}
 	}
 
-	return allok
-}
-
-// matchTags reports whether the name is one of:
-//
-//	tag (if tags[tag] is true)
-//	!tag (if tags[tag] is false)
-//	a comma-separated list of any of these
-//
-func matchTags(name string, tags map[string]bool) bool {
-	if name == "" {
-		return false
-	}
-	if i := strings.Index(name, ","); i >= 0 {
-		// comma-separated list
-		ok1 := matchTags(name[:i], tags)
-		ok2 := matchTags(name[i+1:], tags)
-		return ok1 && ok2
-	}
-	if strings.HasPrefix(name, "!!") { // bad syntax, reject always
-		return false
-	}
-	if strings.HasPrefix(name, "!") { // negation
-		return len(name) > 1 && matchTag(name[1:], tags, false)
-	}
-	return matchTag(name, tags, true)
+	return content[:end], goBuild, sawBinaryOnly, nil
 }
 
-// matchTag reports whether the tag name is valid and satisfied by tags[name]==want.
-func matchTag(name string, tags map[string]bool, want bool) bool {
+// matchTag reports whether the tag name is valid and tags[name] is true.
+// As a special case, if tags["*"] is true and name is not empty or ignore,
+// then matchTag will return prefer instead of the actual answer,
+// which allows the caller to pretend in that case that most tags are
+// both true and false.
+func matchTag(name string, tags map[string]bool, prefer bool) bool {
 	// Tags must be letters, digits, underscores or dots.
 	// Unlike in Go identifiers, all digits are fine (e.g., "386").
 	for _, c := range name {
@@ -131,7 +199,7 @@ func matchTag(name string, tags map[string]bool, want bool) bool {
 		// if we put * in the tags map then all tags
 		// except "ignore" are considered both present and not
 		// (so we return true no matter how 'want' is set).
-		return true
+		return prefer
 	}
 
 	have := tags[name]
@@ -144,7 +212,25 @@ func matchTag(name string, tags map[string]bool, want bool) bool {
 	if name == "darwin" {
 		have = have || tags["ios"]
 	}
-	return have == want
+	return have
+}
+
+// eval is like
+//	x.Eval(func(tag string) bool { return matchTag(tag, tags) })
+// except that it implements the special case for tags["*"] meaning
+// all tags are both true and false at the same time.
+func eval(x constraint.Expr, tags map[string]bool, prefer bool) bool {
+	switch x := x.(type) {
+	case *constraint.TagExpr:
+		return matchTag(x.Tag, tags, prefer)
+	case *constraint.NotExpr:
+		return !eval(x.X, tags, !prefer)
+	case *constraint.AndExpr:
+		return eval(x.X, tags, prefer) && eval(x.Y, tags, prefer)
+	case *constraint.OrExpr:
+		return eval(x.X, tags, prefer) || eval(x.Y, tags, prefer)
+	}
+	panic(fmt.Sprintf("unexpected constraint expression %T", x))
 }
 
 // MatchFile returns false if the name contains a $GOOS or $GOARCH