poll_loop!: Improve diagnostics for limit expr

Put the $yield_after parameter into an expression context expecting
an integer, so that use of a wrong type of value would get an
error message that's informative and does not mention a variable
internal to the expansion.

futures-core/src/task/poll.rs

@@ -31,12 +31,12 @@ macro_rules! ready {
 macro_rules! poll_loop {
     {$yield_after:expr, $cx:expr, $body:expr} => {
         {
-            let limit = $yield_after;
+            let range = 0..$yield_after;
             debug_assert!(
-                limit != 0,
+                range.end != 0,
                 "0 used as the repetition limit in a poll loop",
             );
-            for _ in 0..limit {
+            for _ in range {
                 $body
             }