cmd/gc: make forward declaration in pure Go package an error

An error during the compilation can be more precise
than an error at link time.

For 'func init', the error happens always: you can't forward
declare an init func because the name gets mangled.

For other funcs, the error happens only with the special
(and never used by hand) -= flag, which tells 6g the
package is pure go.

The go command now passes -= for pure Go packages.

Fixes #3705.

R=ken2
CC=golang-dev
https://golang.org/cl/6996054

doc/progs/error.go

@@ -20,7 +20,11 @@ import (
 
 type File struct{}
 
-func Open(name string) (file *File, err error)
+func Open(name string) (file *File, err error) {
+	// OMIT
+	panic(1)
+	// STOP OMIT
+}
 
 func openFile() { // OMIT
 	f, err := os.Open("filename.ext")

src/cmd/gc/go.h

@@ -937,6 +937,7 @@ EXTERN	int	funcdepth;
 EXTERN	int	typecheckok;
 EXTERN	int	compiling_runtime;
 EXTERN	int	compiling_wrappers;
+EXTERN	int	pure_go;
 
 EXTERN	int	nointerface;
 EXTERN	int	fieldtrack_enabled;

src/cmd/gc/lex.c

@@ -293,8 +293,9 @@ main(int argc, char *argv[])
 	if(argc < 1)
 		usage();
 
-	// special flag to detect compilation of package runtime
-	compiling_runtime = debug['+'];
+	// special flags used during build.
+	compiling_runtime = debug['+']; // detect compilation of package runtime
+	pure_go = debug['=']; // package is completely go (no C or assembly)
 
 	pathname = mal(1000);
 	if(getwd(pathname, 999) == 0)

src/cmd/gc/pgen.c

@@ -29,16 +29,19 @@ compile(Node *fn)
 		throwreturn = sysfunc("throwreturn");
 	}
 
-	if(fn->nbody == nil)
-		return;
+	lno = setlineno(fn);
+
+	if(fn->nbody == nil) {
+		if(pure_go || memcmp(fn->nname->sym->name, "init·", 6) == 0)
+			yyerror("missing function body", fn);
+		goto ret;
+	}
 
 	saveerrors();
 
 	// set up domain for labels
 	clearlabels();
 
-	lno = setlineno(fn);
-
 	curfn = fn;
 	dowidth(curfn->type);
 

src/cmd/go/build.go

@@ -1311,6 +1311,8 @@ func (gcToolchain) linker() string {
 	return tool(archChar + "l")
 }
 
+var rsc = flag.Bool("rsc", false, "rsc")
+
 func (gcToolchain) gc(b *builder, p *Package, obj string, importArgs []string, gofiles []string) (ofile string, err error) {
 	out := "_go_." + archChar
 	ofile = obj + out
@@ -1321,6 +1323,21 @@ func (gcToolchain) gc(b *builder, p *Package, obj string, importArgs []string, g
 		gcargs = append(gcargs, "-+")
 	}
 
+	// If we're giving the compiler the entire package (no C etc files), tell it that,
+	// so that it can give good error messages about forward declarations.
+	// Exceptions: a few standard packages have forward declarations for
+	// pieces supplied behind-the-scenes by package runtime.
+	extFiles := len(p.CgoFiles) + len(p.CFiles) + len(p.SFiles) + len(p.SysoFiles) + len(p.SwigFiles) + len(p.SwigCXXFiles)
+	if p.Standard {
+		switch p.ImportPath {
+		case "os", "runtime/pprof", "sync", "time":
+			extFiles++
+		}
+	}
+	if extFiles == 0 {
+		gcargs = append(gcargs, "-=")
+	}
+
 	args := stringList(tool(archChar+"g"), "-o", ofile, buildGcflags, gcargs, "-D", p.localPrefix, importArgs)
 	for _, f := range gofiles {
 		args = append(args, mkAbs(p.Dir, f))