Add E0766 error for unterminated double quote byte string

src/librustc_error_codes/error_codes.rs

@@ -446,6 +446,7 @@ E0762: include_str!("./error_codes/E0762.md"),
 E0763: include_str!("./error_codes/E0763.md"),
 E0764: include_str!("./error_codes/E0764.md"),
 E0765: include_str!("./error_codes/E0765.md"),
+E0766: include_str!("./error_codes/E0766.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

src/librustc_error_codes/error_codes/E0766.md

@@ -0,0 +1,13 @@
+A double quote byte string (`b"`) was not terminated.
+
+Erroneous code example:
+
+```compile_fail,E0766
+let s = b"; // error!
+```
+
+To fix this error, add the missing double quote at the end of the string:
+
+```
+let s = b""; // ok!
+```

src/librustc_parse/lexer/mod.rs

@@ -367,12 +367,15 @@ impl<'a> StringReader<'a> {
             }
             rustc_lexer::LiteralKind::ByteStr { terminated } => {
                 if !terminated {
-                    self.fatal_span_(
-                        start + BytePos(1),
-                        suffix_start,
-                        "unterminated double quote byte string",
-                    )
-                    .raise()
+                    self.sess
+                        .span_diagnostic
+                        .struct_span_fatal_with_code(
+                            self.mk_sp(start + BytePos(1), suffix_start),
+                            "unterminated double quote byte string",
+                            error_code!(E0766),
+                        )
+                        .emit();
+                    FatalError.raise();
                 }
                 (token::ByteStr, Mode::ByteStr, 2, 1) // b" "
             }