Feature unsized-locals changed with latest nightly

[huitseeker]

Issue

The nightly feature required here:

kompact/core/src/utils.rs

Lines 608 to 669 in b3d31ba

	// this variant requires #![feature(unsized_locals)]
	#[cfg(nightly)]
	mod iter_extras {
	use crate::serialisation::{SerError, TryClone};
	/// Additional iterator functions
	pub trait IterExtras: Iterator {
	/// Iterate over each item in the iterator and apply a function to it and a clone of the given value `t`
	///
	/// Behaves like `iterator.for_each(|item| f(item, t.clone()))`, except that it avoids cloning
	/// in the case where the iterator contains a single item or for the last item in a larger iterator.
	///
	/// Use this when cloning `T` is relatively expensive compared to `f`.
	fn for_each_with<T, F>(mut self, t: T, mut f: F)
	where
	T: Clone,
	F: FnMut(Self::Item, T),
	{
	let mut current: Option<Self::Item> = self.next();
	let mut next: Option<Self::Item> = self.next();
	while next.is_some() {
	let item = current.take().unwrap();
	f(item, t.clone());
	current = next;
	next = self.next();
	}
	if let Some(item) = current.take() {
	f(item, t)
	}
	}
	/// Iterate over each item in the iterator and apply a function to it and a clone of the given value `t`
	/// if such a clone can be created
	///
	/// Behaves like `iterator.for_each(|item| f(item, t.try_clone()))`, except that it avoids cloning
	/// in the case where the iterator contains a single item or for the last item in a larger iterator.
	/// It also shortcircuits on cloning errors.
	///
	/// Use this when trying to clone `T` is relatively expensive compared to `f`.
	fn for_each_try_with<T, F>(mut self, t: T, mut f: F) -> Result<(), SerError>
	where
	T: TryClone,
	F: FnMut(Self::Item, T),
	{
	let mut current: Option<Self::Item> = self.next();
	let mut next: Option<Self::Item> = self.next();
	while next.is_some() {
	let item = current.take().unwrap();
	let cloned = t.try_clone()?;
	f(item, cloned);
	current = next;
	next = self.next();
	}
	if let Some(item) = current.take() {
	f(item, t)
	}
	Ok(())
	}
	}
	impl<T: ?Sized> IterExtras for T where T: Iterator {}
	}

And turned on optionally there:

core/src/lib.rs
74:#![cfg_attr(nightly, feature(unsized_locals))]

breaks with nightly 2020-10-29 because of rust-lang/rust#78152

feature(unsized_locals) was split into two disjoint features:

• the feature(unsized_locals) which is marked as incomplete
• feature(unsized_fn_params) which isn't, and which is what Kompact is actually using here.

So post nightly 2020-10-29, Kompact is not activating the feature it actually needs.

Steps to reproduce

cargo +nightly-2020-10-29-x86_64-unknown-linux-gnu check

possibly preceded by a rustup install to install the nightly in question.
Witness the following output:

error[E0277]: the size for values of type `Self` cannot be known at compilation time
   --> /home/huitseeker/.cargo/git/checkouts/kompact-8e3803f644d2dc0c/b3d31ba/core/src/utils.rs:621:32
    |
621 |         fn for_each_with<T, F>(mut self, t: T, mut f: F)
    |                                ^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: unsized fn params are gated as an unstable feature
help: consider further restricting `Self`
    |
624 |             F: FnMut(Self::Item, T), Self: Sized
    |                                    ^^^^^^^^^^^^^
help: function arguments must have a statically known size, borrowed types always have a known size
    |
621 |         fn for_each_with<T, F>(&mut self, t: T, mut f: F)
    |                                ^

error[E0277]: the size for values of type `Self` cannot be known at compilation time
   --> /home/huitseeker/.cargo/git/checkouts/kompact-8e3803f644d2dc0c/b3d31ba/core/src/utils.rs:647:36
    |
647 |         fn for_each_try_with<T, F>(mut self, t: T, mut f: F) -> Result<(), SerError>
    |                                    ^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: unsized fn params are gated as an unstable feature
help: consider further restricting `Self`
    |
650 |             F: FnMut(Self::Item, T), Self: Sized
    |                                    ^^^^^^^^^^^^^
help: function arguments must have a statically known size, borrowed types always have a known size
    |
647 |         fn for_each_try_with<T, F>(&mut self, t: T, mut f: F) -> Result<(), SerError>
    |                                    ^

error: aborting due to 2 previous errors

Workaround

One simple way to upgrade is to update the activation of the unsized_locals feature to target unsized_fn_params. Except this is not backwards-compatible (feature gate errors are fatal and this triggers E0635) and will break every nightly below 2020-10-29.

That includes most nightlies in interactive (human-facing) use since rustfmt has not compiled on nightly since 2020-10-25 or so. It's advised not to merge until the "main" components are available in a post 10-29 nightly here :
https://rust-lang.github.io/rustup-components-history/