cmd/gc: Specify which package import caused an redeclaration error.

Fixes #4012.

R=dave, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/6490082

src/cmd/gc/dcl.c

@@ -150,11 +150,14 @@ testdclstack(void)
 void
 redeclare(Sym *s, char *where)
 {
-	if(s->lastlineno == 0)
+	Strlit *pkgstr;
+
+	if(s->lastlineno == 0) {
+		pkgstr = s->origpkg ? s->origpkg->path : s->pkg->path;
 		yyerror("%S redeclared %s\n"
-			"\tprevious declaration during import",
-			s, where);
-	else
+			"\tprevious declaration during import \"%Z\"",
+			s, where, pkgstr);
+	} else
 		yyerror("%S redeclared %s\n"
 			"\tprevious declaration at %L",
 			s, where, s->lastlineno);

src/cmd/gc/export.c

@@ -349,8 +349,12 @@ dumpexport(void)
 Sym*
 importsym(Sym *s, int op)
 {
-	if(s->def != N && s->def->op != op)
-		redeclare(s, "during import");
+	char *pkgstr;
+
+	if(s->def != N && s->def->op != op) {
+		pkgstr = smprint("during import \"%Z\"", importpkg->path);
+		redeclare(s, pkgstr);
+	}
 
 	// mark the symbol so it is not reexported
 	if(s->def == N) {

src/cmd/gc/go.h

@@ -365,6 +365,7 @@ struct	Sym
 	Label*	label;	// corresponding label (ephemeral)
 	int32	block;		// blocknumber to catch redeclaration
 	int32	lastlineno;	// last declaration for diagnostic
+	Pkg*	origpkg;	// original package for . import
 };
 #define	S	((Sym*)0)
 

src/cmd/gc/subr.c

@@ -382,6 +382,7 @@ importdot(Pkg *opkg, Node *pack)
 	Sym *s, *s1;
 	uint32 h;
 	int n;
+	char *pkgerror;
 
 	n = 0;
 	for(h=0; h<NHASH; h++) {
@@ -394,12 +395,14 @@ importdot(Pkg *opkg, Node *pack)
 				continue;
 			s1 = lookup(s->name);
 			if(s1->def != N) {
-				redeclare(s1, "during import");
+				pkgerror = smprint("during import \"%Z\"", opkg->path);
+				redeclare(s1, pkgerror);
 				continue;
 			}
 			s1->def = s->def;
 			s1->block = s->block;
 			s1->def->pack = pack;
+			s1->origpkg = opkg;
 			n++;
 		}
 	}