gc: descriptive panic for nil pointer -> value method call

R=ken2
CC=golang-dev
https://golang.org/cl/4646042

src/cmd/5g/reg.c

@@ -1525,6 +1525,7 @@ noreturn(Prog *p)
 		symlist[1] = pkglookup("panicslice", runtimepkg);
 		symlist[2] = pkglookup("throwinit", runtimepkg);
 		symlist[3] = pkglookup("panic", runtimepkg);
+		symlist[4] = pkglookup("panicwrap", runtimepkg);
 	}
 
 	s = p->to.sym;

src/cmd/6g/reg.c

@@ -1677,6 +1677,7 @@ noreturn(Prog *p)
 		symlist[1] = pkglookup("panicslice", runtimepkg);
 		symlist[2] = pkglookup("throwinit", runtimepkg);
 		symlist[3] = pkglookup("panic", runtimepkg);
+		symlist[4] = pkglookup("panicwrap", runtimepkg);
 	}
 
 	s = p->to.sym;

src/cmd/8g/reg.c

@@ -1533,6 +1533,7 @@ noreturn(Prog *p)
 		symlist[1] = pkglookup("panicslice", runtimepkg);
 		symlist[2] = pkglookup("throwinit", runtimepkg);
 		symlist[3] = pkglookup("panic", runtimepkg);
+		symlist[4] = pkglookup("panicwrap", runtimepkg);
 	}
 
 	s = p->to.sym;

src/cmd/gc/builtin.c.boot

@@ -6,6 +6,7 @@ char *runtimeimport =
 	"func \"\".panicslice ()\n"
 	"func \"\".throwreturn ()\n"
 	"func \"\".throwinit ()\n"
+	"func \"\".panicwrap (? string, ? string, ? string)\n"
 	"func \"\".panic (? interface { })\n"
 	"func \"\".recover (? *int32) interface { }\n"
 	"func \"\".printbool (? bool)\n"
@@ -22,6 +23,7 @@ char *runtimeimport =
 	"func \"\".printsp ()\n"
 	"func \"\".goprintf ()\n"
 	"func \"\".concatstring ()\n"
+	"func \"\".append ()\n"
 	"func \"\".appendslice (typ *uint8, x any, y []any) any\n"
 	"func \"\".cmpstring (? string, ? string) int\n"
 	"func \"\".slicestring (? string, ? int, ? int) string\n"
@@ -81,7 +83,7 @@ char *runtimeimport =
 	"func \"\".selectgo (sel *uint8)\n"
 	"func \"\".block ()\n"
 	"func \"\".makeslice (typ *uint8, nel int64, cap int64) []any\n"
-	"func \"\".growslice (typ *uint8, old []any, cap int64) []any\n"
+	"func \"\".growslice (typ *uint8, old []any, n int64) []any\n"
 	"func \"\".sliceslice1 (old []any, lb uint64, width uint64) []any\n"
 	"func \"\".sliceslice (old []any, lb uint64, hb uint64, width uint64) []any\n"
 	"func \"\".slicearray (old *any, nel uint64, lb uint64, hb uint64, width uint64) []any\n"

src/cmd/gc/runtime.go

@@ -15,6 +15,7 @@ func panicindex()
 func panicslice()
 func throwreturn()
 func throwinit()
+func panicwrap(string, string, string)
 
 func panic(interface{})
 func recover(*int32) interface{}

src/cmd/gc/subr.c

@@ -3131,8 +3131,9 @@ genwrapper(Type *rcvr, Type *method, Sym *newnam, int iface)
 	NodeList *l, *args, *in, *out;
 	Type *tpad;
 	int isddd;
+	Val v;
 
-	if(0 && debug['r'])
+	if(debug['r'])
 		print("genwrapper rcvrtype=%T method=%T newnam=%S\n",
 			rcvr, method, newnam);
 
@@ -3174,17 +3175,38 @@ genwrapper(Type *rcvr, Type *method, Sym *newnam, int iface)
 		args = list(args, l->n->left);
 		isddd = l->n->left->isddd;
 	}
+	
+	// generate nil pointer check for better error
+	if(isptr[rcvr->etype] && rcvr->type == getthisx(method->type)->type->type) {
+		// generating wrapper from *T to T.
+		n = nod(OIF, N, N);
+		n->ntest = nod(OEQ, this->left, nodnil());
+		// these strings are already in the reflect tables,
+		// so no space cost to use them here.
+		l = nil;
+		v.ctype = CTSTR;
+		v.u.sval = strlit(rcvr->type->sym->pkg->name);  // package name
+		l = list(l, nodlit(v));
+		v.u.sval = strlit(rcvr->type->sym->name);  // type name
+		l = list(l, nodlit(v));
+		v.u.sval = strlit(method->sym->name);
+		l = list(l, nodlit(v));  // method name
+		call = nod(OCALL, syslook("panicwrap", 0), N);
+		call->list = l;
+		n->nbody = list1(call);
+		fn->nbody = list(fn->nbody, n);
+	}
 
 	// generate call
 	call = nod(OCALL, adddot(nod(OXDOT, this->left, newname(method->sym))), N);
 	call->list = args;
 	call->isddd = isddd;
-	fn->nbody = list1(call);
 	if(method->type->outtuple > 0) {
 		n = nod(ORETURN, N, N);
-		n->list = fn->nbody;
-		fn->nbody = list1(n);
+		n->list = list1(call);
+		call = n;
 	}
+	fn->nbody = list(fn->nbody, call);
 
 	if(0 && debug['r'])
 		dumplist("genwrapper body", fn->nbody);

src/pkg/runtime/error.go

@@ -131,3 +131,8 @@ func printany(i interface{}) {
 		print("(", typestring(i), ") ", i)
 	}
 }
+
+// called from generated code
+func panicwrap(pkg, typ, meth string) {
+	panic("value method " + pkg + "." + typ + "." + meth + " called using nil *" + typ + " pointer")
+}