Rollup merge of #137725 - oli-obk:i-want-to-move-it-move-it, r=compiler-errors,traviscross

Add `iter` macro

See related discussion in https://rust-lang.zulipchat.com/#narrow/channel/481571-t-lang.2Fgen/topic/iter!.20macro/near/500784563

very little error case testing so far, but the success path works.

There is also no `IterFn` trait yet, as T-lang didn't consider it something urgently needed I think we can implement it in follow-up PRs.

r? lang for the tests, `@compiler-errors` for the impl

Author: matthiaskrgr

tests/pass/iter_macro.rs

@@ -0,0 +1,22 @@
+#![feature(iter_macro, yield_expr)]
+
+use std::iter::iter;
+
+fn main() {
+    let i = iter! { || {
+        yield 0;
+        for x in 5..10 {
+            yield x * 2;
+        }
+    } };
+    let mut i = i();
+    assert_eq!(i.next(), Some(0));
+    assert_eq!(i.next(), Some(10));
+    assert_eq!(i.next(), Some(12));
+    assert_eq!(i.next(), Some(14));
+    assert_eq!(i.next(), Some(16));
+    assert_eq!(i.next(), Some(18));
+    assert_eq!(i.next(), None);
+    assert_eq!(i.next(), None);
+    assert_eq!(i.next(), None);
+}