Error on using yield without also using #[coroutine] on the closure

And suggest adding the `#[coroutine]` to the closure

Author: oli-obk

core/src/iter/sources/from_coroutine.rs

@@ -14,7 +14,7 @@ use crate::pin::Pin;
 /// #![feature(coroutines)]
 /// #![feature(iter_from_coroutine)]
 ///
-/// let it = std::iter::from_coroutine(|| {
+/// let it = std::iter::from_coroutine(#[cfg_attr(not(bootstrap), coroutine)] || {
 ///     yield 1;
 ///     yield 2;
 ///     yield 3;

core/src/ops/coroutine.rs

@@ -40,12 +40,13 @@ pub enum CoroutineState<Y, R> {
 /// ```rust
 /// #![feature(coroutines)]
 /// #![feature(coroutine_trait)]
+/// #![feature(stmt_expr_attributes)]
 ///
 /// use std::ops::{Coroutine, CoroutineState};
 /// use std::pin::Pin;
 ///
 /// fn main() {
-///     let mut coroutine = || {
+///     let mut coroutine = #[cfg_attr(not(bootstrap), coroutine)] || {
 ///         yield 1;
 ///         "foo"
 ///     };

core/src/pin.rs

@@ -1809,7 +1809,7 @@ impl<Ptr, U> DispatchFromDyn<Pin<U>> for Pin<Ptr> where Ptr: DispatchFromDyn<U>
 /// fn coroutine_fn() -> impl Coroutine<Yield = usize, Return = ()> /* not Unpin */ {
 ///  // Allow coroutine to be self-referential (not `Unpin`)
 ///  // vvvvvv        so that locals can cross yield points.
-///     static || {
+///     #[cfg_attr(not(bootstrap), coroutine)] static || {
 ///         let foo = String::from("foo");
 ///         let foo_ref = &foo; // ------+
 ///         yield 0;                  // | <- crosses yield point!