libsyntax: note that let a = (let b = something) is invalid

src/libsyntax/parse/parser.rs

@@ -2211,6 +2211,11 @@ impl<'a> Parser<'a> {
                         ex = ExprBreak(None);
                     }
                     hi = self.last_span.hi;
+                } 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));
                 } 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:expected identifier, found keyword `let`
+// error-pattern:`let` is not an expression, so it cannot be used in this way
 
 #![feature(quote, rustc_private)]