cmd/compile: fix line numbers for index panics

In the statement x = a[i], the index panic should appear to come from
the line number of the '['. Previous to this CL we sometimes used the
line number of the '=' instead.

Fixes #29504

Change-Id: Ie718fd303c1ac2aee33e88d52c9ba9bcf220dea1
Reviewed-on: https://go-review.googlesource.com/c/go/+/174617
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -2732,6 +2732,8 @@ func (s *state) assign(left *Node, right *ssa.Value, deref bool, skip skipMask)
 			return
 		}
 		if left.Op == OINDEX && left.Left.Type.IsArray() {
+			s.pushLine(left.Pos)
+			defer s.popLine()
 			// We're assigning to an element of an ssa-able array.
 			// a[i] = v
 			t := left.Left.Type
@@ -3894,6 +3896,11 @@ func etypesign(e types.EType) int8 {
 // If bounded is true then this address does not require a nil check for its operand
 // even if that would otherwise be implied.
 func (s *state) addr(n *Node, bounded bool) *ssa.Value {
+	if n.Op != ONAME {
+		s.pushLine(n.Pos)
+		defer s.popLine()
+	}
+
 	t := types.NewPtr(n.Type)
 	switch n.Op {
 	case ONAME:

test/fixedbugs/issue29504.go

+// run
+
+// 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.
+
+// Make sure that in code involving indexing, the bounds
+// check always fails at the line number of the '[' token.
+
+package main
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+)
+
+type T struct{ a, b, c, d, e int } // unSSAable
+
+func main() {
+	shouldPanic(func() {
+		var a [1]int
+		sink = a /*line :999999:1*/ [ /*line :100:1*/ i]
+	})
+	shouldPanic(func() {
+		var a [3]int
+		sink = a /*line :999999:1*/ [ /*line :200:1*/ i]
+	})
+	shouldPanic(func() {
+		var a []int
+		sink = a /*line :999999:1*/ [ /*line :300:1*/ i]
+	})
+	shouldPanic(func() {
+		var a [1]int
+		a /*line :999999:1*/ [ /*line :400:1*/ i] = 1
+	})
+	shouldPanic(func() {
+		var a [3]int
+		a /*line :999999:1*/ [ /*line :500:1*/ i] = 1
+	})
+	shouldPanic(func() {
+		var a []int
+		a /*line :999999:1*/ [ /*line :600:1*/ i] = 1
+	})
+
+	shouldPanic(func() {
+		var a [3]T
+		sinkT = a /*line :999999:1*/ [ /*line :700:1*/ i]
+	})
+	shouldPanic(func() {
+		var a []T
+		sinkT = a /*line :999999:1*/ [ /*line :800:1*/ i]
+	})
+	shouldPanic(func() {
+		var a [3]T
+		a /*line :999999:1*/ [ /*line :900:1*/ i] = T{}
+	})
+	shouldPanic(func() {
+		var a []T
+		a /*line :999999:1*/ [ /*line :1000:1*/ i] = T{}
+	})
+
+	shouldPanic(func() {
+		var a [3]int
+		sinkS = a /*line :999999:1*/ [ /*line :1100:1*/ i:]
+	})
+	shouldPanic(func() {
+		var a []int
+		sinkS = a /*line :999999:1*/ [ /*line :1200:1*/ i:]
+	})
+	shouldPanic(func() {
+		var a [3]int
+		sinkS = a /*line :999999:1*/ [: /*line :1300:1*/ i]
+	})
+	shouldPanic(func() {
+		var a []int
+		sinkS = a /*line :999999:1*/ [: /*line :1400:1*/ i]
+	})
+
+	shouldPanic(func() {
+		var a [3]T
+		sinkST = a /*line :999999:1*/ [ /*line :1500:1*/ i:]
+	})
+	shouldPanic(func() {
+		var a []T
+		sinkST = a /*line :999999:1*/ [ /*line :1600:1*/ i:]
+	})
+	shouldPanic(func() {
+		var a [3]T
+		sinkST = a /*line :999999:1*/ [: /*line :1700:1*/ i]
+	})
+	shouldPanic(func() {
+		var a []T
+		sinkST = a /*line :999999:1*/ [: /*line :1800:1*/ i]
+	})
+
+	shouldPanic(func() {
+		s := "foo"
+		sinkB = s /*line :999999:1*/ [ /*line :1900:1*/ i]
+	})
+	shouldPanic(func() {
+		s := "foo"
+		sinkStr = s /*line :999999:1*/ [ /*line :2000:1*/ i:]
+	})
+	shouldPanic(func() {
+		s := "foo"
+		sinkStr = s /*line :999999:1*/ [: /*line :2100:1*/ i]
+	})
+
+	if bad {
+		panic("ERRORS")
+	}
+}
+
+var i = 9
+var sink int
+var sinkS []int
+var sinkT T
+var sinkST []T
+var sinkB byte
+var sinkStr string
+
+var bad = false
+
+func shouldPanic(f func()) {
+	defer func() {
+		if recover() == nil {
+			panic("did not panic")
+		}
+		var pcs [10]uintptr
+		n := runtime.Callers(1, pcs[:])
+		iter := runtime.CallersFrames(pcs[:n])
+		buf := ""
+		for {
+			frame, more := iter.Next()
+			buf += fmt.Sprintf("%s:%d %s\n", frame.File, frame.Line, frame.Function)
+			if !more {
+				break
+			}
+		}
+		if !strings.Contains(buf, "999999") {
+			fmt.Printf("could not find marker line in traceback:\n%s\n", buf)
+			bad = true
+		}
+	}()
+	f()
+}