cmd/compile: fix label redefinition error column numbers

Ensure that label redefinition error column numbers
print the actual start of the label instead of the
position of the label's delimiting token ":".

For example, given this program:

package main

func main() {

            foo:
   foo:
foo:
foo            :
}

* Before:
main.go:5:13: label foo defined and not used
main.go:6:7: label foo already defined at main.go:5:13
main.go:7:4: label foo already defined at main.go:5:13
main.go:8:16: label foo already defined at main.go:5:13

* After:
main.go:5:13: label foo defined and not used
main.go:6:4: label foo already defined at main.go:5:13
main.go:7:1: label foo already defined at main.go:5:13
main.go:8:1: label foo already defined at main.go:5:13

Fixes #26411

Change-Id: I8eb874b97fdc8862547176d57ac2fa0f075f2367
Reviewed-on: https://go-review.googlesource.com/c/124595
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/syntax/branches.go

@@ -77,7 +77,7 @@ func (ls *labelScope) declare(b *block, s *LabeledStmt) *label {
 		labels = make(map[string]*label)
 		ls.labels = labels
 	} else if alt := labels[name]; alt != nil {
-		ls.err(s.Pos(), "label %s already defined at %s", name, alt.lstmt.Label.Pos().String())
+		ls.err(s.Label.Pos(), "label %s already defined at %s", name, alt.lstmt.Label.Pos().String())
 		return alt
 	}
 	l := &label{b, s, false}

test/fixedbugs/issue26411.go

+// +build !nacl,!js
+// run
+
+// Copyright 2018 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.
+
+// Ensure that label redefinition errors print out
+// a column number that matches the start of the current label's
+// definition instead of the label delimiting token ":"
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+)
+
+func main() {
+	tmpdir, err := ioutil.TempDir("", "issue26411")
+	if err != nil {
+		log.Fatalf("Failed to create temporary directory: %v", err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	samples := []struct {
+		code       string
+		wantOutput []string
+	}{
+		{
+			code: `
+package main
+
+func main() {
+foo:
+foo:
+}
+`,
+			wantOutput: []string{
+				"^.+:5:1: label foo defined and not used\n",
+				".+:6:1: label foo already defined at .+:5:1\n$",
+			},
+		},
+		{
+			code: `
+package main
+
+func main() {
+
+            bar:
+   bar:
+bar:
+bar            :
+}
+`,
+
+			wantOutput: []string{
+				"^.+:6:13: label bar defined and not used\n",
+				".+:7:4: label bar already defined at .+:6:13\n",
+				".+:8:1: label bar already defined at .+:6:13\n",
+				".+:9:1: label bar already defined at .+:6:13\n$",
+			},
+		},
+	}
+
+	for i, sample := range samples {
+		filename := filepath.Join(tmpdir, fmt.Sprintf("%d.go", i))
+		if err := ioutil.WriteFile(filename, []byte(sample.code), 0644); err != nil {
+			log.Printf("#%d: failed to create file %s", i, filename)
+			continue
+		}
+		output, _ := exec.Command("go", "tool", "compile", filename).CombinedOutput()
+
+		// Now match the output
+		for _, regex := range sample.wantOutput {
+			reg := regexp.MustCompile(regex)
+			matches := reg.FindAll(output, -1)
+			for _, match := range matches {
+				index := bytes.Index(output, match)
+				output = bytes.Join([][]byte{output[:index], output[index+len(match):]}, []byte(""))
+			}
+		}
+
+		if len(output) != 0 {
+			log.Printf("#%d: did not match all the output\nResidual output:\n\t%s", i, output)
+		}
+	}
+}