Implement nth_back for RChunks(Exact)(Mut)
Part of #54054.
These implementations may not be optimal because of the use of `self.len()`, but it's quite cheap and simplifies the code a lot.
There's quite some duplication going on here, I wouldn't mind cleaning this up later. A good next step would probably be to add private `split_off_up_to`/`split_off_from` helper methods for slices since their behavior is commonly useful throughout the `Chunks` types.
r? @scottmcm
Add Step::sub_usize
Required for #54054.
I'm aware that the `Step` trait needs a rework, but this still seems like a reasonable addition?
This currently doesn't compile because Chalk contains a type that implement this trait, and this is a breaking change. How can that be fixed?
Implement `iter::Sum` and `iter::Product` for `Option`
This is similar to the existing implementation for `Result`. It will take each item into the accumulator unless a `None` is returned.
I based a lot of this on #38580. From that discussion it didn't seem like this addition would be too controversial or difficult. One thing I still don't understand is picking the values for the `stable` attribute. This is my first non-documentation PR for rust so I am open to any feedback on improvements.
Updated the Iterator docs with information about overriding methods.
# Description
Updated the Iterator docs with information about overriding methods.
closes#60223
Revert "Add implementations of last in terms of next_back on a bunch of DoubleEndedIterators."
This changed observable behavior for several iterator types.
r? @alexcrichton
Simplify RefCell minimum_spanning_tree example
This simplifies the implementation of the `minimum_spanning_tree` example of `RefCell` in the `cell` module-level docs, avoiding an unnecessary recursive call. This also eliminates the need for a block to contain the scope of the borrow in this example. But since that use of a block served an important didactic purpose, we make up for this by instead introducing a block in the initial, simpler example of `RefCell`, where the point will hopefully be conveyed to the reader more easily.
This allows types like Option<NonZeroU8> to be used in FFI without triggering the improper_ctypes lint. This works by changing the is_repr_nullable_ptr function to consider an enum E to be FFI-safe if:
- E has no explicit #[repr(...)].
- It only has two variants.
- One of those variants is empty (meaning it has no fields).
- The other variant has only one field.
- That field is one of the following:
- &T
- &mut T
- extern "C" fn
- core::num::NonZero*
- core::ptr::NonNull<T>
- #[repr(transparent)] struct wrapper around one of the types in this list.
- The size of E and its field are both known and are both the same size (implying E is participating in the nonnull optimization).
remove confusing remarks about mixed volatile and non-volatile accesses
These comments were originally added by @ecstatic-morse in 911d35f0bf and then later edited by me. The intention, I think, was to make sure people do both their reads and writes with these methods if the affected memory really is used for communication with external devices.
However, [people read this as saying that mixed volatile/non-volatile accesses are UB](https://github.com/rust-lang/rust/issues/58599#issuecomment-493791130), which -- to my knowledge -- they are not. So better remove this.
Cc @rkruppe @rust-lang/wg-unsafe-code-guidelines
Fix intra-doc link resolution failure on re-exporting libstd
Currently, re-exporting libstd items as below will [occur a lot of failures](https://gist.github.com/taiki-e/e33e0e8631ef47f65a74a3b69f456366).
```rust
pub use std::*;
```
Until the underlying issue (#56922) fixed, we can fix that so they don't propagate to downstream crates.
Related: https://github.com/rust-lang/rust/pull/56941 (That PR fixed failures that occur when re-exporting from libcore to libstd.)
r? @QuietMisdreavus
