Rollup merge of #31211 - Manishearth:pr-30765, r=nrc

r? @eddyb or @nrc

src/libsyntax/parse/parser.rs

@@ -2218,6 +2218,12 @@ 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 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) &&