cmd/gc: record line number for auto-generated wrappers as <autogenerated>:1

Before we used line 1 of the first source file.
This should be clearer.

Fixes #4388.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/92250044

src/cmd/gc/subr.c

@@ -2498,7 +2498,9 @@ genwrapper(Type *rcvr, Type *method, Sym *newnam, int iface)
 		print("genwrapper rcvrtype=%T method=%T newnam=%S\n",
 			rcvr, method, newnam);
 
-	lineno = 1;	// less confusing than end of input
+	lexlineno++;
+	lineno = lexlineno;
+	linehist("<autogenerated>", 0, 0);
 
 	dclcontext = PEXTERN;
 	markdcl();

src/liblink/obj.c

@@ -183,7 +183,7 @@ linkgetline(Link *ctxt, int32 line, LSym **f, int32 *l)
 		file = a[n].incl->name;
 		dlno = a[n].idel-1;
 	}
-	if((!ctxt->windows && file[0] == '/') || (ctxt->windows && file[1] == ':'))
+	if((!ctxt->windows && file[0] == '/') || (ctxt->windows && file[1] == ':') || file[0] == '<')
 		snprint(buf, sizeof buf, "%s", file);
 	else
 		snprint(buf, sizeof buf, "%s/%s", ctxt->pathname, file);

test/fixedbugs/issue4388.go

@@ -0,0 +1,50 @@
+// run
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"io"
+	"runtime"
+)
+
+type T struct {
+	io.Closer
+}
+
+func f1() {
+	// The 4 here and below depends on the number of internal runtime frames
+	// that sit between a deferred function called during panic and
+	// the original frame. If that changes, this test will start failing and
+	// the number here will need to be updated.
+	defer checkLine(4)
+	var t *T
+	var c io.Closer = t
+	c.Close()
+}
+
+func f2() {
+	defer checkLine(4)
+	var t T
+	var c io.Closer = t
+	c.Close()
+}
+
+func main() {
+	f1()
+	f2()
+}
+
+func checkLine(n int) {
+	if err := recover(); err == nil {
+		panic("did not panic")
+	}
+	_, file, line, _ := runtime.Caller(n)
+	if file != "<autogenerated>" || line != 1 {
+		panic(fmt.Sprintf("expected <autogenerated>:1 have %s:%d", file, line))
+	}
+}