2 files changed, 40 insertions, 7 deletions
diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
index b70f25cec3..7265e62a2f 100644
--- a/src/cmd/go/internal/modcmd/vendor.go
+++ b/src/cmd/go/internal/modcmd/vendor.go
@@ -15,6 +15,7 @@ import (
 
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/imports"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
 )
@@ -100,7 +101,7 @@ func vendorPkg(vdir, pkg string) {
 	if src == "" {
 		fmt.Fprintf(os.Stderr, "internal error: no pkg for %s -> %s\n", pkg, realPath)
 	}
-	copyDir(dst, src, matchNonTest)
+	copyDir(dst, src, matchPotentialSourceFile)
 	if m := modload.PackageModule(realPath); m.Path != "" {
 		copyMetadata(m.Path, realPath, dst, src)
 	}
@@ -153,7 +154,7 @@ var metaPrefixes = []string{
 }
 
 // matchMetadata reports whether info is a metadata file.
-func matchMetadata(info os.FileInfo) bool {
+func matchMetadata(dir string, info os.FileInfo) bool {
 	name := info.Name()
 	for _, p := range metaPrefixes {
 		if strings.HasPrefix(name, p) {
@@ -163,13 +164,36 @@ func matchMetadata(info os.FileInfo) bool {
 	return false
 }
 
-// matchNonTest reports whether info is any non-test file (including non-Go files).
-func matchNonTest(info os.FileInfo) bool {
-	return !strings.HasSuffix(info.Name(), "_test.go")
+var anyTagsExceptIgnore = map[string]bool{"*": true}
+
+// matchPotentialSourceFile reports whether info may be relevant to a build operation.
+func matchPotentialSourceFile(dir string, info os.FileInfo) bool {
+	if strings.HasSuffix(info.Name(), "_test.go") {
+		return false
+	}
+	if strings.HasSuffix(info.Name(), ".go") {
+		f, err := os.Open(filepath.Join(dir, info.Name()))
+		if err != nil {
+			base.Fatalf("go mod vendor: %v", err)
+		}
+		defer f.Close()
+
+		content, err := imports.ReadImports(f, false, nil)
+		if err == nil && !imports.ShouldBuild(content, anyTagsExceptIgnore) {
+			// The file is explicitly tagged "ignore", so it can't affect the build.
+			// Leave it out.
+			return false
+		}
+		return true
+	}
+
+	// We don't know anything about this file, so optimistically assume that it is
+	// needed.
+	return true
 }
 
 // copyDir copies all regular files satisfying match(info) from src to dst.
-func copyDir(dst, src string, match func(os.FileInfo) bool) {
+func copyDir(dst, src string, match func(dir string, info os.FileInfo) bool) {
 	files, err := ioutil.ReadDir(src)
 	if err != nil {
 		base.Fatalf("go mod vendor: %v", err)
@@ -178,7 +202,7 @@ func copyDir(dst, src string, match func(os.FileInfo) bool) {
 		base.Fatalf("go mod vendor: %v", err)
 	}
 	for _, file := range files {
-		if file.IsDir() || !file.Mode().IsRegular() || !match(file) {
+		if file.IsDir() || !file.Mode().IsRegular() || !match(src, file) {
 			continue
 		}
 		r, err := os.Open(filepath.Join(src, file.Name()))
diff --git a/src/cmd/go/testdata/script/mod_vendor.txt b/src/cmd/go/testdata/script/mod_vendor.txt
index 203183be88..25a77a3670 100644
--- a/src/cmd/go/testdata/script/mod_vendor.txt
+++ b/src/cmd/go/testdata/script/mod_vendor.txt
@@ -38,6 +38,7 @@ stdout 'src[\\/]w'
 stderr 'src[\\/]vendor[\\/]w'
 
 ! exists vendor/x/testdata
+! exists vendor/a/foo/bar/b/ignored.go
 ! exists vendor/a/foo/bar/b/main_test.go
 
 exists vendor/a/foo/AUTHORS.txt
@@ -102,6 +103,14 @@ replace (
 
 -- a/foo/bar/b/main.go --
 package b
+-- a/foo/bar/b/ignored.go --
+// This file is intended for use with "go run"; it isn't really part of the package.
+
+// +build ignore
+
+package main
+
+func main() {}
 -- a/foo/bar/b/main_test.go --
 package b