gc: initial BOM is legal.
Fixes #4040.

R=rsc
CC=golang-dev
https://golang.org/cl/6497098

src/cmd/gc/lex.c

diff --git a/src/cmd/gc/lex.c b/src/cmd/gc/lex.c
index f07a23c..0788a61 100644
--- a/src/cmd/gc/lex.c
+++ b/src/cmd/gc/lex.c
@@ -30,6 +30,8 @@
 static int	getlinepragma(void);
 static char *goos, *goarch, *goroot;
 
+#define	BOM	0xFEFF
+
 // Compiler experiments.
 // These are controlled by the GOEXPERIMENT environment
 // variable recorded when the compiler is built.
@@ -319,6 +321,10 @@
 		curio.peekc1 = 0;
 		curio.nlsemi = 0;
 
+		// Skip initial BOM if present.
+		if(Bgetrune(curio.bin) != BOM)
+			Bungetrune(curio.bin);
+
 		block = 1;
 		iota = -1000000;
 
@@ -1200,7 +1206,7 @@
 			rune = getr();
 			// 0xb7 · is used for internal names
 			if(!isalpharune(rune) && !isdigitrune(rune) && (importpkg == nil || rune != 0xb7))
-				yyerror("invalid identifier character 0x%ux", rune);
+				yyerror("invalid identifier character U+%04x", rune);
 			cp += runetochar(cp, &rune);
 		} else if(!yy_isalnum(c) && c != '_')
 			break;
@@ -1583,6 +1589,10 @@
 	if(!fullrune(str, i))
 		goto loop;
 	c = chartorune(&rune, str);
+	if(rune == BOM) {
+		lineno = lexlineno;
+		yyerror("Unicode (UTF-8) BOM in middle of file");
+	}
 	if(rune == Runeerror && c == 1) {
 		lineno = lexlineno;
 		yyerror("illegal UTF-8 sequence");

test/bom.go

diff --git a/test/bom.go b/test/bom.go
new file mode 100644
index 0000000..37f73bc
--- /dev/null
+++ b/test/bom.go
@@ -0,0 +1,26 @@
+// runoutput
+
+// Copyright 2011 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.
+
+// Test source file beginning with a byte order mark.
+
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+func main() {
+	prog = strings.Replace(prog, "BOM", "\uFEFF", -1)
+	fmt.Print(prog)
+}
+
+var prog = `BOM
+package main
+
+func main() {
+}
+`

test/bombad.go

diff --git a/test/bombad.go b/test/bombad.go
new file mode 100644
index 0000000..b894d9b
--- /dev/null
+++ b/test/bombad.go
@@ -0,0 +1,18 @@
+// errorcheck
+
+// Copyright 2012 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.
+
+// Here for reference, but hard to test automatically
+// because the BOM muddles the
+// processing done by ../run.
+
+package main
+
+func main() {
+	// There's a bom here.	// ERROR "BOM"
+	// And here.	// ERROR "BOM"
+	/* And here.*/	// ERROR "BOM"
+	println("hi there") // and here	// ERROR "BOM"
+}