cmd/compile: add more information to the bisect-verbose report

running on cmd/compile/internal/testdata/inlines now shows:
```
--- change set #1 (enabling changes causes failure)
b/b.go:16:6: loop variable i now per-iteration (loop inlined into b/b.go:10)
b/b.go:16:6: loop variable i now per-iteration
./b/b.go:16:6: loop variable b.i now per-iteration (loop inlined into a/a.go:18)
./b/b.go:16:6: loop variable b.i now per-iteration (loop inlined into ./main.go:37)
./b/b.go:16:6: loop variable b.i now per-iteration (loop inlined into ./main.go:38)
---
```
and
```
--- change set #2 (enabling changes causes failure)
./main.go:27:6: loop variable i now per-iteration
./main.go:27:6: loop variable i now per-iteration (loop inlined into ./main.go:35)
---
```

Still unsure about the utility of mentioning the inlined occurrence, but better
than mysteriously repeating the line over and over again.

Change-Id: I357f5d419ab4928fa316f4612eec3b75e7f8ac34
Reviewed-on: https://go-review.googlesource.com/c/go/+/494296
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: David Chase <drchase@google.com>

src/cmd/compile/internal/loopvar/loopvar.go

@@ -48,6 +48,16 @@ func ForCapture(fn *ir.Func) []VarAndLoop {
 	// if a loop variable is transformed it is appended to this slice for later logging
 	var transformed []VarAndLoop
 
+	describe := func(n *ir.Name) string {
+		pos := n.Pos()
+		inner := base.Ctxt.InnermostPos(pos)
+		outer := base.Ctxt.OutermostPos(pos)
+		if inner == outer {
+			return fmt.Sprintf("loop variable %v now per-iteration", n)
+		}
+		return fmt.Sprintf("loop variable %v now per-iteration (loop inlined into %s:%d)", n, outer.Filename(), outer.Line())
+	}
+
 	forCapture := func() {
 		seq := 1
 
@@ -91,7 +101,10 @@ func ForCapture(fn *ir.Func) []VarAndLoop {
 		// subject to hash-variable debugging.
 		maybeReplaceVar := func(k ir.Node, x *ir.RangeStmt) ir.Node {
 			if n, ok := k.(*ir.Name); ok && possiblyLeaked[n] {
-				if base.LoopVarHash.MatchPos(n.Pos(), nil) {
+				desc := func() string {
+					return describe(n)
+				}
+				if base.LoopVarHash.MatchPos(n.Pos(), desc) {
 					// Rename the loop key, prefix body with assignment from loop key
 					transformed = append(transformed, VarAndLoop{n, x, lastPos})
 					tk := typecheck.Temp(n.Type())
@@ -198,8 +211,11 @@ func ForCapture(fn *ir.Func) []VarAndLoop {
 				// Collect the leaking variables for the much-more-complex transformation.
 				forAllDefInInit(x, func(z ir.Node) {
 					if n, ok := z.(*ir.Name); ok && possiblyLeaked[n] {
+						desc := func() string {
+							return describe(n)
+						}
 						// Hash on n.Pos() for most precise failure location.
-						if base.LoopVarHash.MatchPos(n.Pos(), nil) {
+						if base.LoopVarHash.MatchPos(n.Pos(), desc) {
 							leaked = append(leaked, n)
 						}
 					}

src/cmd/compile/internal/loopvar/loopvar_test.go

@@ -8,6 +8,7 @@ import (
 	"internal/testenv"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"runtime"
 	"strings"
 	"testing"
@@ -159,6 +160,11 @@ func TestLoopVarInlines(t *testing.T) {
 	}
 }
 
+func countMatches(s, re string) int {
+	slice := regexp.MustCompile(re).FindAllString(s, -1)
+	return len(slice)
+}
+
 func TestLoopVarHashes(t *testing.T) {
 	switch runtime.GOOS {
 	case "linux", "darwin":
@@ -195,7 +201,7 @@ func TestLoopVarHashes(t *testing.T) {
 		m := f(arg)
 		t.Logf(m)
 
-		mCount := strings.Count(m, "loopvarhash triggered cmd/compile/internal/loopvar/testdata/inlines/main.go:27:6 001100110110110010100100")
+		mCount := countMatches(m, "loopvarhash triggered cmd/compile/internal/loopvar/testdata/inlines/main.go:27:6: .* 001100110110110010100100")
 		otherCount := strings.Count(m, "loopvarhash")
 		if mCount < 1 {
 			t.Errorf("%s: did not see triggered main.go:27:6", arg)
@@ -203,8 +209,7 @@ func TestLoopVarHashes(t *testing.T) {
 		if mCount != otherCount {
 			t.Errorf("%s: too many matches", arg)
 		}
-
+		mCount = countMatches(m, "cmd/compile/internal/loopvar/testdata/inlines/main.go:27:6: .* \\[bisect-match 0x7802e115b9336ca4\\]")
-		mCount = strings.Count(m, "cmd/compile/internal/loopvar/testdata/inlines/main.go:27:6 [bisect-match 0x7802e115b9336ca4]")
 		otherCount = strings.Count(m, "[bisect-match ")
 		if mCount < 1 {
 			t.Errorf("%s: did not see bisect-match for main.go:27:6", arg)