cmd/compile/internal/syntax: better error positions for scanner errors

For string/comment not terminated errors, report error at the
start of the string/comment rather than the end of the file.

3 files changed, 22 insertions, 20 deletions

diff --git a/src/cmd/compile/internal/syntax/scanner.go b/src/cmd/compile/internal/syntax/scanner.go
index f146c8cb6f..dcbe49e82a 100644
--- a/src/cmd/compile/internal/syntax/scanner.go
+++ b/src/cmd/compile/internal/syntax/scanner.go
@@ -44,7 +44,7 @@ redo:
 	}
 
 	// token start
-	s.pos, s.line = s.source.pos(), s.source.line
+	s.pos, s.line = s.source.pos0(), s.source.line0
 
 	if isLetter(c) || c >= utf8.RuneSelf && unicode.IsLetter(c) {
 		s.ident()
@@ -461,7 +461,7 @@ func (s *scanner) stdString() {
 			break
 		}
 		if r < 0 {
-			s.error("string not terminated") // TODO(gri) should point at start of string
+			s.error_at(s.pos, s.line, "string not terminated")
 			break
 		}
 	}
@@ -481,7 +481,7 @@ func (s *scanner) rawString() {
 			break
 		}
 		if r < 0 {
-			s.error("string not terminated") // TODO(gri) should point at start of string
+			s.error_at(s.pos, s.line, "string not terminated")
 			break
 		}
 	}
@@ -578,7 +578,7 @@ func (s *scanner) fullComment() {
 			}
 		}
 		if r < 0 {
-			s.error("comment not terminated")
+			s.error_at(s.pos, s.line, "comment not terminated")
 			return
 		}
 	}
diff --git a/src/cmd/compile/internal/syntax/scanner_test.go b/src/cmd/compile/internal/syntax/scanner_test.go
index dca838dafa..69e81aceca 100644
--- a/src/cmd/compile/internal/syntax/scanner_test.go
+++ b/src/cmd/compile/internal/syntax/scanner_test.go
@@ -272,8 +272,8 @@ func TestScanErrors(t *testing.T) {
 		{"foo$bar = 0", "invalid rune '$'", 3, 1},
 		{"const x = 0xyz", "malformed hex constant", 12, 1},
 		{"0123456789", "malformed octal constant", 10, 1},
-		{"0123456789. /*", "comment not terminated", 14, 1},  // valid float constant
-		{"0123456789e0 /*", "comment not terminated", 15, 1}, // valid float constant
+		{"0123456789. /* foobar", "comment not terminated", 12, 1},   // valid float constant
+		{"0123456789e0 /*\nfoobar", "comment not terminated", 13, 1}, // valid float constant
 		{"var a, b = 08, 07\n", "malformed octal constant", 13, 1},
 		{"(x + 1.0e+x)", "malformed floating-point constant exponent", 10, 1},
 
@@ -286,32 +286,33 @@ func TestScanErrors(t *testing.T) {
 		{`'\y'`, "unknown escape sequence", 2, 1},
 		{`'\x0'`, "escape sequence incomplete", 4, 1},
 		{`'\00'`, "escape sequence incomplete", 4, 1},
-		{`'\377' /*`, "comment not terminated", 9, 1}, // valid octal escape
+		{`'\377' /*`, "comment not terminated", 7, 1}, // valid octal escape
 		{`'\378`, "illegal character U+0038 '8' in escape sequence", 4, 1},
 		{`'\400'`, "octal escape value > 255: 256", 5, 1},
 		{`'xx`, "missing '", 2, 1},
 
 		{"\"\n", "newline in string", 1, 1},
-		{`"`, "string not terminated", 1, 1},
-		{`"foo`, "string not terminated", 4, 1},
-		{"`", "string not terminated", 1, 1},
-		{"`foo", "string not terminated", 4, 1},
-		{"/*/", "comment not terminated", 3, 1},
-		{"/*\n\nfoo", "comment not terminated", 7, 3},
-		{`"\`, "string not terminated", 2, 1},
-		{`"\"`, "string not terminated", 3, 1},
-		{`"\x`, "string not terminated", 3, 1},
+		{`"`, "string not terminated", 0, 1},
+		{`"foo`, "string not terminated", 0, 1},
+		{"`", "string not terminated", 0, 1},
+		{"`foo", "string not terminated", 0, 1},
+		{"/*/", "comment not terminated", 0, 1},
+		{"/*\n\nfoo", "comment not terminated", 0, 1},
+		{"/*\n\nfoo", "comment not terminated", 0, 1},
+		{`"\`, "string not terminated", 0, 1},
+		{`"\"`, "string not terminated", 0, 1},
+		{`"\x`, "string not terminated", 0, 1},
 		{`"\x"`, "escape sequence incomplete", 3, 1},
 		{`"\y"`, "unknown escape sequence", 2, 1},
 		{`"\x0"`, "escape sequence incomplete", 4, 1},
 		{`"\00"`, "escape sequence incomplete", 4, 1},
-		{`"\377" /*`, "comment not terminated", 9, 1}, // valid octal escape
+		{`"\377" /*`, "comment not terminated", 7, 1}, // valid octal escape
 		{`"\378"`, "illegal character U+0038 '8' in escape sequence", 4, 1},
 		{`"\400"`, "octal escape value > 255: 256", 5, 1},
 
 		{`s := "foo\z"`, "unknown escape sequence", 10, 1},
 		{`s := "foo\z00\nbar"`, "unknown escape sequence", 10, 1},
-		{`"\x`, "string not terminated", 3, 1},
+		{`"\x`, "string not terminated", 0, 1},
 		{`"\x"`, "escape sequence incomplete", 3, 1},
 		{`var s string = "\x"`, "escape sequence incomplete", 18, 1},
 		{`return "\Uffffffff"`, "escape sequence is invalid Unicode code point", 18, 1},
diff --git a/src/cmd/compile/internal/syntax/source.go b/src/cmd/compile/internal/syntax/source.go
index 577d7cb168..87c22fcc26 100644
--- a/src/cmd/compile/internal/syntax/source.go
+++ b/src/cmd/compile/internal/syntax/source.go
@@ -46,7 +46,7 @@ func (s *source) init(src io.Reader, errh ErrorHandler) {
 }
 
 func (s *source) error(msg string) {
-	s.error_at(s.pos(), s.line, msg)
+	s.error_at(s.pos0(), s.line0, msg)
 }
 
 func (s *source) error_at(pos, line int, msg string) {
@@ -57,7 +57,8 @@ func (s *source) error_at(pos, line int, msg string) {
 	panic(fmt.Sprintf("%d: %s", line, msg))
 }
 
-func (s *source) pos() int {
+// pos0 returns the byte position of the last character read.
+func (s *source) pos0() int {
 	return s.offs + s.r0
 }