Tell unicode escapes can’t be used as bytes earlier/more

src/libsyntax/parse/lexer/mod.rs

@@ -738,26 +738,24 @@ impl<'a> StringReader<'a> {
                         return match e {
                             'n' | 'r' | 't' | '\\' | '\'' | '"' | '0' => true,
                             'x' => self.scan_byte_escape(delim, !ascii_only),
-                            'u' if self.curr_is('{') => {
+                            'u' => {
-                                let valid = self.scan_unicode_escape(delim);
+                                let valid = if self.curr_is('{') {
-                                if valid && ascii_only {
+                                    self.scan_unicode_escape(delim) && !ascii_only
-                                    self.err_span_(
-                                        start,
-                                        self.last_pos,
-                                        "unicode escape sequences cannot be used as a byte or in \
-                                        a byte string"
-                                    );
-                                    false
                                 } else {
-                                   valid
-                                }
-                            }
-                            'u' if !ascii_only => {
                                     self.err_span_(start, self.last_pos,
-                                    "incomplete unicode escape sequence");
+                                        "incorrect unicode escape sequence");
                                     self.help_span_(start, self.last_pos,
                                         "format of unicode escape sequences is `\\u{…}`");
                                     false
+                                };
+                                if ascii_only {
+                                    self.err_span_(start, self.last_pos,
+                                        "unicode escape sequences cannot be used as a byte or in \
+                                        a byte string"
+                                    );
+                                }
+                                valid
+
                             }
                             '\n' if delim == '"' => {
                                 self.consume_whitespace();

src/test/parse-fail/issue-23620-invalid-escapes.rs

@@ -16,7 +16,8 @@ fn main() {
     //~^ ERROR unicode escape sequences cannot be used as a byte or in a byte string
 
     let _ = b'\u';
-    //~^ ERROR unknown byte escape: u
+    //~^ ERROR incorrect unicode escape sequence
+    //~^^ ERROR unicode escape sequences cannot be used as a byte or in a byte string
 
     let _ = b'\x5';
     //~^ ERROR numeric character escape is too short
@@ -35,11 +36,12 @@ fn main() {
     let _ = b"\u{a4a4} \xf \u";
     //~^ ERROR unicode escape sequences cannot be used as a byte or in a byte string
     //~^^ ERROR illegal character in numeric character escape:
-    //~^^^ ERROR unknown byte escape: u
+    //~^^^ ERROR incorrect unicode escape sequence
+    //~^^^^ ERROR unicode escape sequences cannot be used as a byte or in a byte string
 
     let _ = "\u{ffffff} \xf \u";
     //~^ ERROR illegal unicode character escape
     //~^^ ERROR illegal character in numeric character escape:
     //~^^^ ERROR form of character escape may only be used with characters in the range [\x00-\x7f]
-    //~^^^^ ERROR incomplete unicode escape sequence
+    //~^^^^ ERROR incorrect unicode escape sequence
 }