cmd/compile: omit file:pos for non-existent errors

Omits printing the file:line:column when trying to
open non-existent files

Given:
    go tool compile x.go

* Before:
    x.go:0: open x.go: no such file or directory

* After:
    open x.go: no such file or directory

Reverts the revert in CL 231043 by only fixing the case
of non-existent errors which is what the original bug
was about. The fix for "permission errors" will come later
on when I have bandwidth to investigate the differences
between running with root and why os.Open works for some
builders and not others.

Fixes #36437

Change-Id: I9c8a0981ad708b504bb43990a4105b42266fa41f
Reviewed-on: https://go-review.googlesource.com/c/go/+/230941
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -44,7 +44,7 @@ func parseFiles(filenames []string) uint {
 
 			f, err := os.Open(filename)
 			if err != nil {
-				p.error(syntax.Error{Pos: syntax.MakePos(base, 0, 0), Msg: err.Error()})
+				p.error(syntax.Error{Msg: err.Error()})
 				return
 			}
 			defer f.Close()

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

@@ -60,9 +60,15 @@ func adderrorname(n *Node) {
 }
 
 func adderr(pos src.XPos, format string, args ...interface{}) {
+	msg := fmt.Sprintf(format, args...)
+	// Only add the position if know the position.
+	// See issue golang.org/issue/11361.
+	if pos.IsKnown() {
+		msg = fmt.Sprintf("%v: %s", linestr(pos), msg)
+	}
 	errors = append(errors, Error{
 		pos: pos,
-		msg: fmt.Sprintf("%v: %s\n", linestr(pos), fmt.Sprintf(format, args...)),
+		msg: msg + "\n",
 	})
 }
 

test/fixedbugs/issue36437.go

@@ -0,0 +1,49 @@
+// run
+
+// +build !nacl,!js
+
+// Copyright 2020 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.
+
+// Tests that when non-existent files are passed to the
+// compiler, such as in:
+//    go tool compile foo
+// we don't print the beginning position:
+//    foo:0: open foo: no such file or directory
+// but instead omit it and print out:
+//    open foo: no such file or directory
+
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"regexp"
+)
+
+func main() {
+	tmpDir, err := ioutil.TempDir("", "issue36437")
+	if err != nil {
+		panic(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	msgOrErr := func(msg []byte, err error) string {
+		if len(msg) == 0 && err != nil {
+			return err.Error()
+		}
+		return string(msg)
+	}
+
+	filename := "non-existent.go"
+	output, err := exec.Command("go", "tool", "compile", filename).CombinedOutput()
+	got := msgOrErr(output, err)
+
+	regFilenamePos := regexp.MustCompile(filename + ":\\d+")
+	if regFilenamePos.MatchString(got) {
+		fmt.Printf("Error message must not contain filename:pos, but got:\n%q\n", got)
+	}
+}