Improve error message for let-in-expr-position

Author: Manishearth

src/libsyntax/parse/parser.rs

@@ -2221,8 +2221,9 @@ impl<'a> Parser<'a> {
                 } else if self.token.is_keyword(keywords::Let) {
                     // Catch this syntax error here, instead of in `check_strict_keywords`, so
                     // that we can explicitly mention that let is not to be used as an expression
-                    let msg = "`let` is not an expression, so it cannot be used in this way";
-                    return Err(self.fatal(&msg));
+                    let mut db = self.fatal("expected expression, found statement (`let`)");
+                    db.note("variable declaration using `let` is a statement");
+                    return Err(db);
                 } else if self.check(&token::ModSep) ||
                         self.token.is_ident() &&
                         !self.check_keyword(keywords::True) &&

src/test/run-fail-fulldeps/qquote.rs

@@ -10,7 +10,7 @@
 
 // ignore-cross-compile
 
-// error-pattern:`let` is not an expression, so it cannot be used in this way
+// error-pattern:expected expression, found statement (`let`)
 
 #![feature(quote, rustc_private)]