cmd/go: hoist C++, Objective-C, and Fortran checks earlier

Today they happen during the build phase; they should happen
during the load phase instead, along with the C check.

Change-Id: I6074a995b8e29275549aafa574511b735642d85b
Reviewed-on: https://go-review.googlesource.com/69051
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/go/internal/load/pkg.go

@@ -1086,12 +1086,31 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 		// code; see issue #16050).
 	}
 
-	// The gc toolchain only permits C source files with cgo.
-	if len(p.CFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() && cfg.BuildContext.Compiler == "gc" {
+	setError := func(msg string) {
 		p.Error = &PackageError{
 			ImportStack: stk.Copy(),
-			Err:         fmt.Sprintf("C source files not allowed when not using cgo or SWIG: %s", strings.Join(p.CFiles, " ")),
+			Err:         msg,
+		}
+	}
+
+	// The gc toolchain only permits C source files with cgo or SWIG.
+	if len(p.CFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() && cfg.BuildContext.Compiler == "gc" {
+		setError(fmt.Sprintf("C source files not allowed when not using cgo or SWIG: %s", strings.Join(p.CFiles, " ")))
+		return
+	}
+
+	// C++, Objective-C, and Fortran source files are permitted only with cgo or SWIG,
+	// regardless of toolchain.
+	if len(p.CXXFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() {
+		setError(fmt.Sprintf("C++ source files not allowed when not using cgo or SWIG: %s", strings.Join(p.CXXFiles, " ")))
+		return
+	}
+	if len(p.MFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() {
+		setError(fmt.Sprintf("Objective-C source files not allowed when not using cgo or SWIG: %s", strings.Join(p.MFiles, " ")))
+		return
 	}
+	if len(p.FFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() {
+		setError(fmt.Sprintf("Fortran source files not allowed when not using cgo or SWIG: %s", strings.Join(p.FFiles, " ")))
 		return
 	}
 
@@ -1100,10 +1119,7 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 	if other := foldPath[fold]; other == "" {
 		foldPath[fold] = p.ImportPath
 	} else if other != p.ImportPath {
-		p.Error = &PackageError{
-			ImportStack: stk.Copy(),
-			Err:         fmt.Sprintf("case-insensitive import collision: %q and %q", p.ImportPath, other),
-		}
+		setError(fmt.Sprintf("case-insensitive import collision: %q and %q", p.ImportPath, other))
 		return
 	}
 

src/cmd/go/internal/work/build.go

@@ -1438,24 +1438,6 @@ func (b *Builder) build(a *Action) (err error) {
 		return fmt.Errorf("missing or invalid package binary for binary-only package %s", a.Package.ImportPath)
 	}
 
-	// Return an error if the package has CXX files but it's not using
-	// cgo nor SWIG, since the CXX files can only be processed by cgo
-	// and SWIG.
-	if len(a.Package.CXXFiles) > 0 && !a.Package.UsesCgo() && !a.Package.UsesSwig() {
-		return fmt.Errorf("can't build package %s because it contains C++ files (%s) but it's not using cgo nor SWIG",
-			a.Package.ImportPath, strings.Join(a.Package.CXXFiles, ","))
-	}
-	// Same as above for Objective-C files
-	if len(a.Package.MFiles) > 0 && !a.Package.UsesCgo() && !a.Package.UsesSwig() {
-		return fmt.Errorf("can't build package %s because it contains Objective-C files (%s) but it's not using cgo nor SWIG",
-			a.Package.ImportPath, strings.Join(a.Package.MFiles, ","))
-	}
-	// Same as above for Fortran files
-	if len(a.Package.FFiles) > 0 && !a.Package.UsesCgo() && !a.Package.UsesSwig() {
-		return fmt.Errorf("can't build package %s because it contains Fortran files (%s) but it's not using cgo nor SWIG",
-			a.Package.ImportPath, strings.Join(a.Package.FFiles, ","))
-	}
-
 	defer func() {
 		if err != nil && err != errPrintedOutput {
 			err = fmt.Errorf("go build %s: %v", a.Package.ImportPath, err)
@@ -1558,6 +1540,7 @@ func (b *Builder) build(a *Action) (err error) {
 		gofiles = append(gofiles, outGo...)
 	}
 
+	// Sanity check only, since Package.load already checked as well.
 	if len(gofiles) == 0 {
 		return &load.NoGoError{Package: a.Package}
 	}