cmd/go: prohibit C sources files unless using cgo

Those C files would have been compiled with 6c.
It's close to impossible to use C correctly anymore,
and the C compilers are going away eventually.
Make them unavailable now.

go1.4.txt change in CL 145890046

LGTM=iant
R=iant
CC=golang-codereviews, r
https://codereview.appspot.com/149720043

4 files changed, 26 insertions, 0 deletions

diff --git a/src/cmd/go/pkg.go b/src/cmd/go/pkg.go
index 63875aed5..4bbcc2b97 100644
--- a/src/cmd/go/pkg.go
+++ b/src/cmd/go/pkg.go
@@ -614,6 +614,16 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 	}
 	p.Target = p.target
 
+	// Check for C code compiled with Plan 9 C compiler.
+	// No longer allowed except in runtime and runtime/cgo, for now.
+	if len(p.CFiles) > 0 && !p.usesCgo() && (!p.Standard || p.ImportPath != "runtime") {
+		p.Error = &PackageError{
+			ImportStack: stk.copy(),
+			Err:         fmt.Sprintf("C source files not allowed when not using cgo: %s", strings.Join(p.CFiles, " ")),
+		}
+		return p
+	}
+
 	// In the absence of errors lower in the dependency tree,
 	// check for case-insensitive collisions of import paths.
 	if len(p.DepsErrors) == 0 {
diff --git a/src/cmd/go/test.bash b/src/cmd/go/test.bash
index 13886e158..9ae17e105 100755
--- a/src/cmd/go/test.bash
+++ b/src/cmd/go/test.bash
@@ -157,6 +157,20 @@ fi
 rm -f ./testdata/err
 unset GOPATH
 
+export GOPATH=$(pwd)/testdata/src
+TEST disallowed C source files
+export GOPATH=$(pwd)/testdata
+if ./testgo build badc 2>testdata/err; then
+	echo 'go build badc succeeded'
+	ok=false
+elif ! grep 'C source files not allowed' testdata/err >/dev/null; then
+	echo 'go test did not say C source files not allowed:'
+	cat testdata/err
+	ok=false
+fi
+rm -f ./testdata/err
+unset GOPATH
+
 TEST error message for syntax error in test go file says FAIL
 export GOPATH=$(pwd)/testdata
 if ./testgo test syntaxerror 2>testdata/err; then
diff --git a/src/cmd/go/testdata/src/badc/x.c b/src/cmd/go/testdata/src/badc/x.c
new file mode 100644
index 000000000..f6cbf6924
--- /dev/null
+++ b/src/cmd/go/testdata/src/badc/x.c
@@ -0,0 +1 @@
+// C code!
diff --git a/src/cmd/go/testdata/src/badc/x.go b/src/cmd/go/testdata/src/badc/x.go
new file mode 100644
index 000000000..bfa1de28b
--- /dev/null
+++ b/src/cmd/go/testdata/src/badc/x.go
@@ -0,0 +1 @@
+package badc