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Auto merge of #93176 - danielhenrymantilla:stack-pinning-macro, r=m-ou-se

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Add a stack-`pin!`-ning macro to `core::pin`.

  - https://github.com/rust-lang/rust/issues/93178

`pin!` allows pinning a value to the stack. Thanks to being implemented in the stdlib, which gives access to `macro` macros, and to the private `.pointer` field of the `Pin` wrapper, [it was recently discovered](https://rust-lang.zulipchat.com/#narrow/stream/187312-wg-async-foundations/topic/pin!.20.E2.80.94.20the.20.22definitive.22.20edition.20.28a.20rhs-compatible.20pin-nin.2E.2E.2E/near/268731241) ([archive link](https://zulip-archive.rust-lang.org/stream/187312-wg-async-foundations/topic/A.20rhs-compatible.20pin-ning.20macro.html#268731241)), contrary to popular belief, that it is actually possible to implement and feature such a macro:

```rust
let foo: Pin<&mut PhantomPinned> = pin!(PhantomPinned);
stuff(foo);
```
or, directly:

```rust
stuff(pin!(PhantomPinned));
```

  - For context, historically, this used to require one of the two following syntaxes:

      - ```rust
        let foo = PhantomPinned;
        pin!(foo);
        stuff(foo);
        ```

      -  ```rust
         pin! {
             let foo = PhantomPinned;
         }
         stuff(foo);
         ```

This macro thus allows, for instance, doing things like:

```diff
fn block_on<T>(fut: impl Future<Output = T>) -> T {
    // Pin the future so it can be polled.
-   let mut fut = Box::pin(fut);
+   let mut fut = pin!(fut);

    // Create a new context to be passed to the future.
    let t = thread::current();
    let waker = Arc::new(ThreadWaker(t)).into();
    let mut cx = Context::from_waker(&waker);

    // Run the future to completion.
    loop {
        match fut.as_mut().poll(&mut cx) {
            Poll::Ready(res) => return res,
            Poll::Pending => thread::park(),
        }
    }
}
```

  - _c.f._, https://doc.rust-lang.org/1.58.1/alloc/task/trait.Wake.html

And so on, and so forth.

I don't think such an API can get better than that, barring full featured language support (`&pin` references or something), so I see no reason not to start experimenting with featuring this in the stdlib already 🙂

  - cc `@rust-lang/wg-async-foundations` \[EDIT: this doesn't seem to have pinged anybody 😩, thanks `@yoshuawuyts` for the real ping\]

r? `@joshtriplett`

___

# Docs preview

https://user-images.githubusercontent.com/9920355/150605731-1f45c2eb-c9b0-4ce3-b17f-2784fb75786e.mp4

___

# Implementation

The implementation ends up being dead simple (so much it's embarrassing):

```rust
pub macro pin($value:expr $(,)?) {
    Pin { pointer: &mut { $value } }
}
```

_and voilà_!

  - The key for it working lies in [the rules governing the scope of anonymous temporaries](https://doc.rust-lang.org/1.58.1/reference/destructors.html#temporary-lifetime-extension).

<details><summary>Comments and context</summary>

This is `Pin::new_unchecked(&mut { $value })`, so, for starters, let's
review such a hypothetical macro (that any user-code could define):
```rust
macro_rules! pin {( $value:expr ) => (
    match &mut { $value } { at_value => unsafe { // Do not wrap `$value` in an `unsafe` block.
        $crate::pin::Pin::<&mut _>::new_unchecked(at_value)
    }}
)}
```

Safety:
  - `type P = &mut _`. There are thus no pathological `Deref{,Mut}` impls that would break `Pin`'s invariants.
  - `{ $value }` is braced, making it a _block expression_, thus **moving** the given `$value`, and making it _become an **anonymous** temporary_.
    By virtue of being anonynomous, it can no longer be accessed, thus preventing any attemps to `mem::replace` it or `mem::forget` it, _etc._

This gives us a `pin!` definition that is sound, and which works, but only in certain scenarios:

  - If the `pin!(value)` expression is _directly_ fed to a function call:
    `let poll = pin!(fut).poll(cx);`

  - If the `pin!(value)` expression is part of a scrutinee:

    ```rust
    match pin!(fut) { pinned_fut => {
        pinned_fut.as_mut().poll(...);
        pinned_fut.as_mut().poll(...);
    }} // <- `fut` is dropped here.
    ```

Alas, it doesn't work for the more straight-forward use-case: `let` bindings.

```rust
let pinned_fut = pin!(fut); // <- temporary value is freed at the end of this statement
pinned_fut.poll(...) // error[E0716]: temporary value dropped while borrowed
                     // note: consider using a `let` binding to create a longer lived value
```

  - Issues such as this one are the ones motivating https://github.com/rust-lang/rfcs/pull/66

This makes such a macro incredibly unergonomic in practice, and the reason most macros out there had to take the path of being a statement/binding macro (_e.g._, `pin!(future);`) instead of featuring the more intuitive ergonomics of an expression macro.

Luckily, there is a way to avoid the problem. Indeed, the problem stems from the fact that a temporary is dropped at the end of its enclosing statement when it is part of the parameters given to function call, which has precisely been the case with our `Pin::new_unchecked()`!

For instance,

```rust
let p = Pin::new_unchecked(&mut <temporary>);
```

becomes:

```rust
let p = { let mut anon = <temporary>; &mut anon };
```

However, when using a literal braced struct to construct the value, references to temporaries can then be taken. This makes Rust change the lifespan of such temporaries so that they are, instead, dropped _at the end of the enscoping block_.

For instance,
```rust
let p = Pin { pointer: &mut <temporary> };
```

becomes:

```rust
let mut anon = <temporary>;
let p = Pin { pointer: &mut anon };
```

which is *exactly* what we want.

Finally, we don't hit problems _w.r.t._ the privacy of the `pointer` field, or the unqualified `Pin` name, thanks to `decl_macro`s being _fully_ hygienic (`def_site` hygiene).

</details>

___

# TODO

  - [x] Add compile-fail tests with attempts to break the `Pin` invariants thanks to the macro (_e.g._, try to access the private `.pointer` field, or see what happens if such a pin is used outside its enscoping scope (borrow error));
  - [ ] Follow-up stuff:
      - [ ] Try to experiment with adding `pin!` to the prelude: this may require to be handled with some extra care, as it may lead to issues reminiscent of those of `assert_matches!`: https://github.com/rust-lang/rust/issues/82913
      - [x] Create the tracking issue.
This commit is contained in:
bors 2022-02-15 09:32:03 +00:00
commit 6421a499a5
12 changed files with 404 additions and 1 deletions
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3
compiler/rustc_feature/src/active.rs
View file

@ -161,6 +161,9 @@ declare_features! (
(active, staged_api, "1.0.0", None, None), (active, staged_api, "1.0.0", None, None),
/// Added for testing E0705; perma-unstable. /// Added for testing E0705; perma-unstable.
(active, test_2018_feature, "1.31.0", None, Some(Edition::Edition2018)), (active, test_2018_feature, "1.31.0", None, Some(Edition::Edition2018)),
/// Allows non-`unsafe` —and thus, unsound— access to `Pin` constructions.
/// Marked `incomplete` since perma-unstable and unsound.
(incomplete, unsafe_pin_internals, "1.60.0", None, None),
/// Use for stable + negative coherence and strict coherence depending on trait's /// Use for stable + negative coherence and strict coherence depending on trait's
/// rustc_strict_coherence value. /// rustc_strict_coherence value.
(active, with_negative_coherence, "1.60.0", None, None), (active, with_negative_coherence, "1.60.0", None, None),

1
compiler/rustc_span/src/symbol.rs
View file

@ -1455,6 +1455,7 @@ symbols! {
unsafe_block_in_unsafe_fn, unsafe_block_in_unsafe_fn,
unsafe_cell, unsafe_cell,
unsafe_no_drop_flag, unsafe_no_drop_flag,
unsafe_pin_internals,
unsize, unsize,
unsized_fn_params, unsized_fn_params,
unsized_locals, unsized_locals,

249
library/core/src/pin.rs
View file

@ -406,7 +406,14 @@ use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};
#[repr(transparent)] #[repr(transparent)]
#[derive(Copy, Clone)] #[derive(Copy, Clone)]
pub struct Pin<P> { pub struct Pin<P> {
pointer: P, // FIXME(#93176): this field is made `#[unstable] #[doc(hidden)] pub` to:
// - deter downstream users from accessing it (which would be unsound!),
// - let the `pin!` macro access it (such a macro requires using struct
// literal syntax in order to benefit from lifetime extension).
// Long-term, `unsafe` fields or macro hygiene are expected to offer more robust alternatives.
#[unstable(feature = "unsafe_pin_internals", issue = "none")]
#[doc(hidden)]
pub pointer: P,
} }
// The following implementations aren't derived in order to avoid soundness // The following implementations aren't derived in order to avoid soundness
@ -909,3 +916,243 @@ impl<P, U> CoerceUnsized<Pin<U>> for Pin<P> where P: CoerceUnsized<U> {}
#[stable(feature = "pin", since = "1.33.0")] #[stable(feature = "pin", since = "1.33.0")]
impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U> {} impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U> {}
/// Constructs a <code>[Pin]<[&mut] T></code>, by pinning[^1] a `value: T` _locally_[^2].
///
/// Unlike [`Box::pin`], this does not involve a heap allocation.
///
/// [^1]: If the (type `T` of the) given value does not implement [`Unpin`], then this
/// effectively pins the `value` in memory, where it will be unable to be moved.
/// Otherwise, <code>[Pin]<[&mut] T></code> behaves like <code>[&mut] T</code>, and operations such
/// as [`mem::replace()`][crate::mem::replace] will allow extracting that value, and therefore,
/// moving it.
/// See [the `Unpin` section of the `pin` module][self#unpin] for more info.
///
/// [^2]: This is usually dubbed "stack"-pinning. And whilst local values are almost always located
/// in the stack (_e.g._, when within the body of a non-`async` function), the truth is that inside
/// the body of an `async fn` or block —more generally, the body of a generator— any locals crossing
/// an `.await` point —a `yield` point— end up being part of the state captured by the `Future` —by
/// the `Generator`—, and thus will be stored wherever that one is.
///
/// ## Examples
///
/// ### Basic usage
///
/// ```rust
/// #![feature(pin_macro)]
/// # use core::marker::PhantomPinned as Foo;
/// use core::pin::{pin, Pin};
///
/// fn stuff(foo: Pin<&mut Foo>) {
/// // …
/// # let _ = foo;
/// }
///
/// let pinned_foo = pin!(Foo { /* … */ });
/// stuff(pinned_foo);
/// // or, directly:
/// stuff(pin!(Foo { /* … */ }));
/// ```
///
/// ### Manually polling a `Future` (wihout `Unpin` bounds)
///
/// ```rust
/// #![feature(pin_macro)]
/// use std::{
/// future::Future,
/// pin::pin,
/// task::{Context, Poll},
/// thread,
/// };
/// # use std::{sync::Arc, task::Wake, thread::Thread};
///
/// # /// A waker that wakes up the current thread when called.
/// # struct ThreadWaker(Thread);
/// #
/// # impl Wake for ThreadWaker {
/// # fn wake(self: Arc<Self>) {
/// # self.0.unpark();
/// # }
/// # }
/// #
/// /// Runs a future to completion.
/// fn block_on<Fut: Future>(fut: Fut) -> Fut::Output {
/// let waker_that_unparks_thread = // …
/// # Arc::new(ThreadWaker(thread::current())).into();
/// let mut cx = Context::from_waker(&waker_that_unparks_thread);
/// // Pin the future so it can be polled.
/// let mut pinned_fut = pin!(fut);
/// loop {
/// match pinned_fut.as_mut().poll(&mut cx) {
/// Poll::Pending => thread::park(),
/// Poll::Ready(res) => return res,
/// }
/// }
/// }
/// #
/// # assert_eq!(42, block_on(async { 42 }));
/// ```
///
/// ### With `Generator`s
///
/// ```rust
/// #![feature(generators, generator_trait, pin_macro)]
/// use core::{
/// ops::{Generator, GeneratorState},
/// pin::pin,
/// };
///
/// fn generator_fn() -> impl Generator<Yield = usize, Return = ()> /* not Unpin */ {
/// // Allow generator to be self-referential (not `Unpin`)
/// // vvvvvv so that locals can cross yield points.
/// static || {
/// let foo = String::from("foo"); // --+
/// yield 0; // | <- crosses yield point!
/// println!("{}", &foo); // <----------+
/// yield foo.len();
/// }
/// }
///
/// fn main() {
/// let mut generator = pin!(generator_fn());
/// match generator.as_mut().resume(()) {
/// GeneratorState::Yielded(0) => {},
/// _ => unreachable!(),
/// }
/// match generator.as_mut().resume(()) {
/// GeneratorState::Yielded(3) => {},
/// _ => unreachable!(),
/// }
/// match generator.resume(()) {
/// GeneratorState::Yielded(_) => unreachable!(),
/// GeneratorState::Complete(()) => {},
/// }
/// }
/// ```
///
/// ## Remarks
///
/// Precisely because a value is pinned to local storage, the resulting <code>[Pin]<[&mut] T></code>
/// reference ends up borrowing a local tied to that block: it can't escape it.
///
/// The following, for instance, fails to compile:
///
/// ```rust,compile_fail
/// #![feature(pin_macro)]
/// use core::pin::{pin, Pin};
/// # use core::{marker::PhantomPinned as Foo, mem::drop as stuff};
///
/// let x: Pin<&mut Foo> = {
/// let x: Pin<&mut Foo> = pin!(Foo { /* … */ });
/// x
/// }; // <- Foo is dropped
/// stuff(x); // Error: use of dropped value
/// ```
///
/// <details><summary>Error message</summary>
///
/// ```console
/// error[E0716]: temporary value dropped while borrowed
/// --> src/main.rs:9:28
/// |
/// 8 | let x: Pin<&mut Foo> = {
/// | - borrow later stored here
/// 9 | let x: Pin<&mut Foo> = pin!(Foo { /* … */ });
/// | ^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
/// 10 | x
/// 11 | }; // <- Foo is dropped
/// | - temporary value is freed at the end of this statement
/// |
/// = note: consider using a `let` binding to create a longer lived value
/// ```
///
/// </details>
///
/// This makes [`pin!`] **unsuitable to pin values when intending to _return_ them**. Instead, the
/// value is expected to be passed around _unpinned_ until the point where it is to be consumed,
/// where it is then useful and even sensible to pin the value locally using [`pin!`].
///
/// If you really need to return a pinned value, consider using [`Box::pin`] instead.
///
/// On the other hand, pinning to the stack[<sup>2</sup>](#fn2) using [`pin!`] is likely to be
/// cheaper than pinning into a fresh heap allocation using [`Box::pin`]. Moreover, by virtue of not
/// even needing an allocator, [`pin!`] is the main non-`unsafe` `#![no_std]`-compatible [`Pin`]
/// constructor.
///
/// [`Box::pin`]: ../../std/boxed/struct.Box.html#method.pin
#[unstable(feature = "pin_macro", issue = "93178")]
#[rustc_macro_transparency = "semitransparent"]
#[allow_internal_unstable(unsafe_pin_internals)]
pub macro pin($value:expr $(,)?) {
// This is `Pin::new_unchecked(&mut { $value })`, so, for starters, let's
// review such a hypothetical macro (that any user-code could define):
//
// ```rust
// macro_rules! pin {( $value:expr ) => (
// match &mut { $value } { at_value => unsafe { // Do not wrap `$value` in an `unsafe` block.
// $crate::pin::Pin::<&mut _>::new_unchecked(at_value)
// }}
// )}
// ```
//
// Safety:
// - `type P = &mut _`. There are thus no pathological `Deref{,Mut}` impls
// that would break `Pin`'s invariants.
// - `{ $value }` is braced, making it a _block expression_, thus **moving**
// the given `$value`, and making it _become an **anonymous** temporary_.
// By virtue of being anonynomous, it can no longer be accessed, thus
// preventing any attemps to `mem::replace` it or `mem::forget` it, _etc._
//
// This gives us a `pin!` definition that is sound, and which works, but only
// in certain scenarios:
// - If the `pin!(value)` expression is _directly_ fed to a function call:
// `let poll = pin!(fut).poll(cx);`
// - If the `pin!(value)` expression is part of a scrutinee:
// ```rust
// match pin!(fut) { pinned_fut => {
// pinned_fut.as_mut().poll(...);
// pinned_fut.as_mut().poll(...);
// }} // <- `fut` is dropped here.
// ```
// Alas, it doesn't work for the more straight-forward use-case: `let` bindings.
// ```rust
// let pinned_fut = pin!(fut); // <- temporary value is freed at the end of this statement
// pinned_fut.poll(...) // error[E0716]: temporary value dropped while borrowed
// // note: consider using a `let` binding to create a longer lived value
// ```
// - Issues such as this one are the ones motivating https://github.com/rust-lang/rfcs/pull/66
//
// This makes such a macro incredibly unergonomic in practice, and the reason most macros
// out there had to take the path of being a statement/binding macro (_e.g._, `pin!(future);`)
// instead of featuring the more intuitive ergonomics of an expression macro.
//
// Luckily, there is a way to avoid the problem. Indeed, the problem stems from the fact that a
// temporary is dropped at the end of its enclosing statement when it is part of the parameters
// given to function call, which has precisely been the case with our `Pin::new_unchecked()`!
// For instance,
// ```rust
// let p = Pin::new_unchecked(&mut <temporary>);
// ```
// becomes:
// ```rust
// let p = { let mut anon = <temporary>; &mut anon };
// ```
//
// However, when using a literal braced struct to construct the value, references to temporaries
// can then be taken. This makes Rust change the lifespan of such temporaries so that they are,
// instead, dropped _at the end of the enscoping block_.
// For instance,
// ```rust
// let p = Pin { pointer: &mut <temporary> };
// ```
// becomes:
// ```rust
// let mut anon = <temporary>;
// let p = Pin { pointer: &mut anon };
// ```
// which is *exactly* what we want.
//
// See https://doc.rust-lang.org/1.58.1/reference/destructors.html#temporary-lifetime-extension
// for more info.
$crate::pin::Pin::<&mut _> { pointer: &mut { $value } }
}

2
library/core/tests/lib.rs
View file

@ -45,6 +45,7 @@
#![feature(inline_const)] #![feature(inline_const)]
#![feature(is_sorted)] #![feature(is_sorted)]
#![feature(pattern)] #![feature(pattern)]
#![feature(pin_macro)]
#![feature(sort_internals)] #![feature(sort_internals)]
#![feature(slice_take)] #![feature(slice_take)]
#![feature(maybe_uninit_uninit_array)] #![feature(maybe_uninit_uninit_array)]
@ -122,6 +123,7 @@ mod ops;
mod option; mod option;
mod pattern; mod pattern;
mod pin; mod pin;
mod pin_macro;
mod ptr; mod ptr;
mod result; mod result;
mod simd; mod simd;

33
library/core/tests/pin_macro.rs Normal file
View file

@ -0,0 +1,33 @@
// edition:2021
use core::{
marker::PhantomPinned,
mem::{drop as stuff, transmute},
pin::{pin, Pin},
};
#[test]
fn basic() {
let it: Pin<&mut PhantomPinned> = pin!(PhantomPinned);
stuff(it);
}
#[test]
fn extension_works_through_block() {
let it: Pin<&mut PhantomPinned> = { pin!(PhantomPinned) };
stuff(it);
}
#[test]
fn extension_works_through_unsafe_block() {
// "retro-type-inference" works as well.
let it: Pin<&mut PhantomPinned> = unsafe { pin!(transmute(())) };
stuff(it);
}
#[test]
fn unsize_coercion() {
let slice: Pin<&mut [PhantomPinned]> = pin!([PhantomPinned; 2]);
stuff(slice);
let dyn_obj: Pin<&mut dyn Send> = pin!([PhantomPinned; 2]);
stuff(dyn_obj);
}

2
src/test/rustdoc-js-std/typed-query.js
View file

@ -8,5 +8,7 @@ const EXPECTED = {
{ 'path': 'std', 'name': 'eprint' }, { 'path': 'std', 'name': 'eprint' },
{ 'path': 'std', 'name': 'println' }, { 'path': 'std', 'name': 'println' },
{ 'path': 'std', 'name': 'eprintln' }, { 'path': 'std', 'name': 'eprintln' },
{ 'path': 'std::pin', 'name': 'pin' },
{ 'path': 'core::pin', 'name': 'pin' },
], ],
}; };

17
src/test/ui/feature-gates/feature-gate-unsafe_pin_internals.rs Normal file
View file

@ -0,0 +1,17 @@
// edition:2018
#![forbid(incomplete_features, unsafe_code)]
#![feature(unsafe_pin_internals)]
//~^ ERROR the feature `unsafe_pin_internals` is incomplete and may not be safe to use
use core::{marker::PhantomPinned, pin::Pin};
/// The `unsafe_pin_internals` is indeed unsound.
fn non_unsafe_pin_new_unchecked<T>(pointer: &mut T) -> Pin<&mut T> {
Pin { pointer }
}
fn main() {
let mut self_referential = PhantomPinned;
let _: Pin<&mut PhantomPinned> = non_unsafe_pin_new_unchecked(&mut self_referential);
core::mem::forget(self_referential); // move and disable drop glue!
}

14
src/test/ui/feature-gates/feature-gate-unsafe_pin_internals.stderr Normal file
View file

@ -0,0 +1,14 @@
error: the feature `unsafe_pin_internals` is incomplete and may not be safe to use and/or cause compiler crashes
--> $DIR/feature-gate-unsafe_pin_internals.rs:3:12
|
LL | #![feature(unsafe_pin_internals)]
| ^^^^^^^^^^^^^^^^^^^^
|
note: the lint level is defined here
--> $DIR/feature-gate-unsafe_pin_internals.rs:2:11
|
LL | #![forbid(incomplete_features, unsafe_code)]
| ^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error

13
src/test/ui/pin-macro/cant_access_internals.rs Normal file
View file

@ -0,0 +1,13 @@
// edition:2018
#![feature(pin_macro)]
use core::{
marker::PhantomPinned,
mem,
pin::{pin, Pin},
};
fn main() {
let mut phantom_pinned = pin!(PhantomPinned);
mem::take(phantom_pinned.pointer); //~ ERROR use of unstable library feature 'unsafe_pin_internals'
}

11
src/test/ui/pin-macro/cant_access_internals.stderr Normal file
View file

@ -0,0 +1,11 @@
error[E0658]: use of unstable library feature 'unsafe_pin_internals'
--> $DIR/cant_access_internals.rs:12:15
|
LL | mem::take(phantom_pinned.pointer);
| ^^^^^^^^^^^^^^^^^^^^^^
|
= help: add `#![feature(unsafe_pin_internals)]` to the crate attributes to enable
error: aborting due to previous error
For more information about this error, try `rustc --explain E0658`.

29
src/test/ui/pin-macro/lifetime_errors_on_promotion_misusage.rs Normal file
View file

@ -0,0 +1,29 @@
// edition:2018
#![feature(pin_macro)]
use core::{
convert::identity,
marker::PhantomPinned,
mem::drop as stuff,
pin::pin,
};
fn function_call_stops_borrow_extension() {
let phantom_pinned = identity(pin!(PhantomPinned));
//~^ ERROR temporary value dropped while borrowed
stuff(phantom_pinned)
}
fn promotion_only_works_for_the_innermost_block() {
let phantom_pinned = {
let phantom_pinned = pin!(PhantomPinned);
//~^ ERROR temporary value dropped while borrowed
phantom_pinned
};
stuff(phantom_pinned)
}
fn main() {
function_call_stops_borrow_extension();
promotion_only_works_for_the_innermost_block();
}

31
src/test/ui/pin-macro/lifetime_errors_on_promotion_misusage.stderr Normal file
View file

@ -0,0 +1,31 @@
error[E0716]: temporary value dropped while borrowed
--> $DIR/lifetime_errors_on_promotion_misusage.rs:12:35
|
LL | let phantom_pinned = identity(pin!(PhantomPinned));
| ^^^^^^^^^^^^^^^^^^^ - temporary value is freed at the end of this statement
| |
| creates a temporary which is freed while still in use
LL |
LL | stuff(phantom_pinned)
| -------------- borrow later used here
|
= note: consider using a `let` binding to create a longer lived value
= note: this error originates in the macro `pin` (in Nightly builds, run with -Z macro-backtrace for more info)
error[E0716]: temporary value dropped while borrowed
--> $DIR/lifetime_errors_on_promotion_misusage.rs:19:30
|
LL | let phantom_pinned = {
| -------------- borrow later stored here
LL | let phantom_pinned = pin!(PhantomPinned);
| ^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
...
LL | };
| - temporary value is freed at the end of this statement
|
= note: consider using a `let` binding to create a longer lived value
= note: this error originates in the macro `pin` (in Nightly builds, run with -Z macro-backtrace for more info)
error: aborting due to 2 previous errors
For more information about this error, try `rustc --explain E0716`.