cmd/compile: make a better bogus line for empty infinite loops

The old recipe for making an infinite loop not be infinite
in the debugger could create an instruction (Prog) with a
line number not tied to any file (index == 0).  This caused
downstream failures in DWARF processing.

So don't do that.  Also adds a test, also adds a check+panic
to ensure that the next time this happens the error is less
mystifying.

Fixes #35652

Change-Id: I04f30bc94fdc4aef20dd9130561303ff84fd945e
Reviewed-on: https://go-review.googlesource.com/c/go/+/207613
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/gc/ssa.go

@@ -6041,6 +6041,9 @@ func genssa(f *ssa.Func, pp *Progs) {
 		if s.bstart[b.ID] == s.pp.next && len(b.Succs) == 1 && b.Succs[0].Block() == b {
 			p := thearch.Ginsnop(s.pp)
 			p.Pos = p.Pos.WithIsStmt()
+			if b.Pos == src.NoXPos {
+				b.Pos = p.Pos // It needs a file, otherwise a no-file non-zero line causes confusion.  See #35652.
+			}
 			b.Pos = b.Pos.WithBogusLine() // Debuggers are not good about infinite loops, force a change in line number
 		}
 		// Emit control flow instructions for block

src/cmd/internal/src/xpos.go

@@ -71,6 +71,10 @@ func (p XPos) WithIsStmt() XPos {
 // gdb chooses not to display the bogus line; delve shows it with a complaint, but the
 // alternative behavior is to hang.
 func (p XPos) WithBogusLine() XPos {
+	if p.index == 0 {
+		// See #35652
+		panic("Assigning a bogus line to XPos with no file will cause mysterious downstream failures.")
+	}
 	p.lico = makeBogusLico()
 	return p
 }

test/fixedbugs/issue35652.go

@@ -0,0 +1,15 @@
+// compile
+
+// Copyright 2019 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 p
+
+func f() {
+	for true {
+		if true {
+			continue
+		}
+	}
+}