diff --git a/src/cmd/go/build.go b/src/cmd/go/build.go
index 5977828a3a1..3f9537834c9 100644
--- a/src/cmd/go/build.go
+++ b/src/cmd/go/build.go
@@ -1419,6 +1419,8 @@ func (b *builder) build(a *action) (err error) {
 		// cgo and non-cgo worlds, so it necessarily has files in both.
 		// In that case gcc only gets the gcc_* files.
 		var gccfiles []string
+		gccfiles = append(gccfiles, cfiles...)
+		cfiles = nil
 		if a.p.Standard && a.p.ImportPath == "runtime/cgo" {
 			filter := func(files, nongcc, gcc []string) ([]string, []string) {
 				for _, f := range files {
@@ -1430,11 +1432,9 @@ func (b *builder) build(a *action) (err error) {
 				}
 				return nongcc, gcc
 			}
-			cfiles, gccfiles = filter(cfiles, cfiles[:0], gccfiles)
 			sfiles, gccfiles = filter(sfiles, sfiles[:0], gccfiles)
 		} else {
-			gccfiles = append(cfiles, sfiles...)
-			cfiles = nil
+			gccfiles = append(gccfiles, sfiles...)
 			sfiles = nil
 		}
 
diff --git a/src/cmd/go/go_test.go b/src/cmd/go/go_test.go
index 2af715a3a46..5526aec8f93 100644
--- a/src/cmd/go/go_test.go
+++ b/src/cmd/go/go_test.go
@@ -1226,14 +1226,6 @@ func TestGetGitDefaultBranch(t *testing.T) {
 	tg.grepStdout(`\* another-branch`, "not on correct default branch")
 }
 
-func TestDisallowedCSourceFiles(t *testing.T) {
-	tg := testgo(t)
-	defer tg.cleanup()
-	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
-	tg.runFail("build", "badc")
-	tg.grepStderr("C source files not allowed", "go test did not say C source files not allowed")
-}
-
 func TestErrorMessageForSyntaxErrorInTestGoFileSaysFAIL(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()
diff --git a/src/cmd/go/pkg.go b/src/cmd/go/pkg.go
index 8b0508894fe..927d68d1c60 100644
--- a/src/cmd/go/pkg.go
+++ b/src/cmd/go/pkg.go
@@ -1012,6 +1012,19 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 	}
 	p.Target = p.target
 
+	// If cgo is not enabled, ignore cgo supporting sources
+	// just as we ignore go files containing import "C".
+	if !buildContext.CgoEnabled {
+		p.CFiles = nil
+		p.CXXFiles = nil
+		p.MFiles = nil
+		p.SwigFiles = nil
+		p.SwigCXXFiles = nil
+		p.SysoFiles = nil
+		// Note that SFiles are okay (they go to the Go assembler)
+		// and HFiles are okay (they might be used by the SFiles).
+	}
+
 	// The gc toolchain only permits C source files with cgo.
 	if len(p.CFiles) > 0 && !p.usesCgo() && !p.usesSwig() && buildContext.Compiler == "gc" {
 		p.Error = &PackageError{
diff --git a/src/runtime/cgo/gcc_android.c b/src/runtime/cgo/gcc_android.c
index b500b29c5ed..b756edefa90 100644
--- a/src/runtime/cgo/gcc_android.c
+++ b/src/runtime/cgo/gcc_android.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <stdarg.h>
 #include <android/log.h>
 #include "libcgo.h"
diff --git a/src/runtime/cgo/gcc_android_386.c b/src/runtime/cgo/gcc_android_386.c
index 92c77900d21..23a15f1c87e 100644
--- a/src/runtime/cgo/gcc_android_386.c
+++ b/src/runtime/cgo/gcc_android_386.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <string.h> /* for strerror */
 #include <pthread.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_android_amd64.c b/src/runtime/cgo/gcc_android_amd64.c
index fce7d56a4b9..e006c49bcf5 100644
--- a/src/runtime/cgo/gcc_android_amd64.c
+++ b/src/runtime/cgo/gcc_android_amd64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <string.h> /* for strerror */
 #include <pthread.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_android_arm.c b/src/runtime/cgo/gcc_android_arm.c
index 06f4217d22b..c7b13f9a7f4 100644
--- a/src/runtime/cgo/gcc_android_arm.c
+++ b/src/runtime/cgo/gcc_android_arm.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <pthread.h>
 #include <signal.h>
 #include <stdio.h>
diff --git a/src/runtime/cgo/gcc_android_arm64.c b/src/runtime/cgo/gcc_android_arm64.c
index 9aaf1ba73fd..f8ad684de34 100644
--- a/src/runtime/cgo/gcc_android_arm64.c
+++ b/src/runtime/cgo/gcc_android_arm64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <pthread.h>
 #include <signal.h>
 #include <stdio.h>
diff --git a/src/runtime/cgo/gcc_darwin_386.c b/src/runtime/cgo/gcc_darwin_386.c
index a633728a453..effbcdfd4b4 100644
--- a/src/runtime/cgo/gcc_darwin_386.c
+++ b/src/runtime/cgo/gcc_darwin_386.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <string.h> /* for strerror */
 #include <pthread.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_darwin_amd64.c b/src/runtime/cgo/gcc_darwin_amd64.c
index 58e99e5549b..15396b0d258 100644
--- a/src/runtime/cgo/gcc_darwin_amd64.c
+++ b/src/runtime/cgo/gcc_darwin_amd64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <string.h> /* for strerror */
 #include <pthread.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_darwin_arm.c b/src/runtime/cgo/gcc_darwin_arm.c
index 967f44385d1..dbf88c34ac5 100644
--- a/src/runtime/cgo/gcc_darwin_arm.c
+++ b/src/runtime/cgo/gcc_darwin_arm.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <limits.h>
 #include <pthread.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_darwin_arm64.c b/src/runtime/cgo/gcc_darwin_arm64.c
index 21912298cfe..a9eb4f2cd2d 100644
--- a/src/runtime/cgo/gcc_darwin_arm64.c
+++ b/src/runtime/cgo/gcc_darwin_arm64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <limits.h>
 #include <pthread.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_dragonfly_amd64.c b/src/runtime/cgo/gcc_dragonfly_amd64.c
index 85c53ca7076..b534dccf797 100644
--- a/src/runtime/cgo/gcc_dragonfly_amd64.c
+++ b/src/runtime/cgo/gcc_dragonfly_amd64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <sys/types.h>
 #include <sys/signalvar.h>
 #include <pthread.h>
diff --git a/src/runtime/cgo/gcc_fatalf.c b/src/runtime/cgo/gcc_fatalf.c
index 08aebaa806d..5ac419b4120 100644
--- a/src/runtime/cgo/gcc_fatalf.c
+++ b/src/runtime/cgo/gcc_fatalf.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 // +build !android,linux
 
 #include <stdarg.h>
diff --git a/src/runtime/cgo/gcc_freebsd_386.c b/src/runtime/cgo/gcc_freebsd_386.c
index 522f95b2ddc..d288666a3db 100644
--- a/src/runtime/cgo/gcc_freebsd_386.c
+++ b/src/runtime/cgo/gcc_freebsd_386.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <sys/types.h>
 #include <sys/signalvar.h>
 #include <pthread.h>
diff --git a/src/runtime/cgo/gcc_freebsd_amd64.c b/src/runtime/cgo/gcc_freebsd_amd64.c
index 31ab136998e..e532ad69d69 100644
--- a/src/runtime/cgo/gcc_freebsd_amd64.c
+++ b/src/runtime/cgo/gcc_freebsd_amd64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <sys/types.h>
 #include <sys/signalvar.h>
 #include <pthread.h>
diff --git a/src/runtime/cgo/gcc_freebsd_arm.c b/src/runtime/cgo/gcc_freebsd_arm.c
index 73f32792c55..c4e75743261 100644
--- a/src/runtime/cgo/gcc_freebsd_arm.c
+++ b/src/runtime/cgo/gcc_freebsd_arm.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <sys/types.h>
 #include <machine/sysarch.h>
 #include <sys/signalvar.h>
diff --git a/src/runtime/cgo/gcc_libinit_linux_ppc64x.c b/src/runtime/cgo/gcc_libinit_linux_ppc64x.c
index 147872a7bf2..c133142f938 100644
--- a/src/runtime/cgo/gcc_libinit_linux_ppc64x.c
+++ b/src/runtime/cgo/gcc_libinit_linux_ppc64x.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 // TODO: see issue #10410
 // +build linux
 // +build ppc64 ppc64le
diff --git a/src/runtime/cgo/gcc_libinit_openbsd.c b/src/runtime/cgo/gcc_libinit_openbsd.c
index 5fa84c4de6e..eb798ce5e80 100644
--- a/src/runtime/cgo/gcc_libinit_openbsd.c
+++ b/src/runtime/cgo/gcc_libinit_openbsd.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <stdio.h>
 #include <stdlib.h>
 
diff --git a/src/runtime/cgo/gcc_libinit_windows.c b/src/runtime/cgo/gcc_libinit_windows.c
index 5fa84c4de6e..eb798ce5e80 100644
--- a/src/runtime/cgo/gcc_libinit_windows.c
+++ b/src/runtime/cgo/gcc_libinit_windows.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <stdio.h>
 #include <stdlib.h>
 
diff --git a/src/runtime/cgo/gcc_linux_386.c b/src/runtime/cgo/gcc_linux_386.c
index 15e0a8a3025..30fe92bfea9 100644
--- a/src/runtime/cgo/gcc_linux_386.c
+++ b/src/runtime/cgo/gcc_linux_386.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <pthread.h>
 #include <string.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_linux_amd64.c b/src/runtime/cgo/gcc_linux_amd64.c
index 0ab49122389..50a7e6e078e 100644
--- a/src/runtime/cgo/gcc_linux_amd64.c
+++ b/src/runtime/cgo/gcc_linux_amd64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <pthread.h>
 #include <errno.h>
 #include <string.h> // strerror
diff --git a/src/runtime/cgo/gcc_linux_arm.c b/src/runtime/cgo/gcc_linux_arm.c
index f5524219570..945c3f19e49 100644
--- a/src/runtime/cgo/gcc_linux_arm.c
+++ b/src/runtime/cgo/gcc_linux_arm.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <pthread.h>
 #include <string.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_linux_arm64.c b/src/runtime/cgo/gcc_linux_arm64.c
index 84bd7c0b499..ca9ba0ba6e7 100644
--- a/src/runtime/cgo/gcc_linux_arm64.c
+++ b/src/runtime/cgo/gcc_linux_arm64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <pthread.h>
 #include <string.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_linux_ppc64x.c b/src/runtime/cgo/gcc_linux_ppc64x.c
index 2721384e6be..fb19805bdae 100644
--- a/src/runtime/cgo/gcc_linux_ppc64x.c
+++ b/src/runtime/cgo/gcc_linux_ppc64x.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 // +build ppc64 ppc64le
 
 #include <pthread.h>
diff --git a/src/runtime/cgo/gcc_mmap.c b/src/runtime/cgo/gcc_mmap.c
index 45786f94ea2..14efa5489da 100644
--- a/src/runtime/cgo/gcc_mmap.c
+++ b/src/runtime/cgo/gcc_mmap.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 // +build linux,amd64
 
 #include <errno.h>
diff --git a/src/runtime/cgo/gcc_netbsd_386.c b/src/runtime/cgo/gcc_netbsd_386.c
index 32f2e156789..99558ea1401 100644
--- a/src/runtime/cgo/gcc_netbsd_386.c
+++ b/src/runtime/cgo/gcc_netbsd_386.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <sys/types.h>
 #include <pthread.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_netbsd_amd64.c b/src/runtime/cgo/gcc_netbsd_amd64.c
index aa357459c72..f5c8b1e74f0 100644
--- a/src/runtime/cgo/gcc_netbsd_amd64.c
+++ b/src/runtime/cgo/gcc_netbsd_amd64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <sys/types.h>
 #include <pthread.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_netbsd_arm.c b/src/runtime/cgo/gcc_netbsd_arm.c
index 9589780ac84..97ce908485e 100644
--- a/src/runtime/cgo/gcc_netbsd_arm.c
+++ b/src/runtime/cgo/gcc_netbsd_arm.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <sys/types.h>
 #include <pthread.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_openbsd_386.c b/src/runtime/cgo/gcc_openbsd_386.c
index bf56c8db264..22941a4c6d4 100644
--- a/src/runtime/cgo/gcc_openbsd_386.c
+++ b/src/runtime/cgo/gcc_openbsd_386.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <sys/types.h>
 #include <dlfcn.h>
 #include <errno.h>
diff --git a/src/runtime/cgo/gcc_openbsd_amd64.c b/src/runtime/cgo/gcc_openbsd_amd64.c
index b9f2a9d5a99..e84fe6c18b0 100644
--- a/src/runtime/cgo/gcc_openbsd_amd64.c
+++ b/src/runtime/cgo/gcc_openbsd_amd64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <sys/types.h>
 #include <dlfcn.h>
 #include <errno.h>
diff --git a/src/runtime/cgo/gcc_signal_darwin_armx.c b/src/runtime/cgo/gcc_signal_darwin_armx.c
index ac3c2571c0d..e77c507e935 100644
--- a/src/runtime/cgo/gcc_signal_darwin_armx.c
+++ b/src/runtime/cgo/gcc_signal_darwin_armx.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 // Emulation of the Unix signal SIGSEGV.
 //
 // On iOS, Go tests and apps under development are run by lldb.
diff --git a/src/runtime/cgo/gcc_signal_darwin_lldb.c b/src/runtime/cgo/gcc_signal_darwin_lldb.c
index 598482d350d..12cc388400f 100644
--- a/src/runtime/cgo/gcc_signal_darwin_lldb.c
+++ b/src/runtime/cgo/gcc_signal_darwin_lldb.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 // +build !lldb
 // +build darwin
 // +build arm arm64
diff --git a/src/runtime/cgo/gcc_solaris_amd64.c b/src/runtime/cgo/gcc_solaris_amd64.c
index 5a01e0826e3..98a1a8be532 100644
--- a/src/runtime/cgo/gcc_solaris_amd64.c
+++ b/src/runtime/cgo/gcc_solaris_amd64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include <pthread.h>
 #include <string.h>
 #include <signal.h>
diff --git a/src/runtime/cgo/gcc_util.c b/src/runtime/cgo/gcc_util.c
index a15c2d23bbc..e20d206be6d 100644
--- a/src/runtime/cgo/gcc_util.c
+++ b/src/runtime/cgo/gcc_util.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #include "libcgo.h"
 
 /* Stub for calling malloc from Go */
diff --git a/src/runtime/cgo/gcc_windows_386.c b/src/runtime/cgo/gcc_windows_386.c
index 536a024c564..fa0c69bc138 100644
--- a/src/runtime/cgo/gcc_windows_386.c
+++ b/src/runtime/cgo/gcc_windows_386.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 #include <process.h>
diff --git a/src/runtime/cgo/gcc_windows_amd64.c b/src/runtime/cgo/gcc_windows_amd64.c
index 75d87f0cf8b..a3c3896edfc 100644
--- a/src/runtime/cgo/gcc_windows_amd64.c
+++ b/src/runtime/cgo/gcc_windows_amd64.c
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build cgo
-
 #define WIN64_LEAN_AND_MEAN
 #include <windows.h>
 #include <process.h>