Add `cmp::{min_by, min_by_key, max_by, max_by_key}`
This adds the following functions to `core::cmp`:
- `min_by`
- `min_by_key`
- `max_by`
- `max_by_key`
`min_by` and `max_by` are somewhat trivial to implement, but not entirely because `min_by` returns the first value in case the two are equal (and `max_by` the second). `min` and `max` can be implemented in terms of `min_by` and `max_by`, but not as easily the other way around.
To give an example of why I think these functions could be useful: the `Iterator::{min_by, min_by_key, max_by, max_by_key}` methods all currently hard-code the behavior mentioned above which is an ever so small duplication of logic. If we delegate them to `cmp::{min_by, max_by}` methods instead, we get the correct behavior for free. (edit: this is now included in the PR)
I added `min_by_key` / `max_by_key` for consistency's sake but I wouldn't mind removing them. I don't have a particular use case in mind for them, and `min_by` / `max_by` seem to be more useful.
Tracking issue: #64460
PR: documentation spin loop hint
The documentation for 'spin loop hint' explains that yield is better if the lock holder is running on the same CPU. I suggest that 'CPU or core' would be clearer.
improve Vec example soundness in mem::transmute docs
The previous version of the `Vec` example had a case of questionable soundness, because at one point `v_orig` was aliased.
r? @RalfJung
documentation for AtomicPtr CAS operations
The examples in the documentation for AtomicPtr CAS operations only show code that does *not* perform the CAS operation. I suggest to change them so that they actually do exchange the AtomicPtr's value.
Make `abs`, `wrapping_abs`, `overflowing_abs` const functions
This makes `abs`, `wrapping_abs` and `overflowing_abs` const functions like #58044 makes `wrapping_neg` and `overflowing_neg` const functions.
`abs` is made const by returning `(self ^ -1) - -1` = `!self + 1` = `-self` for negative numbers and `(self ^ 0) - 0` = `self` for non-negative numbers. The subexpression `self >> ($BITS - 1)` evaluates to `-1` for negative numbers and `0` otherwise. The subtraction overflows when `self` is `min_value()`, as we would be subtracting `max_value() - -1`; this is when `abs` should overflow.
`wrapping_abs` and `overflowing_abs` make use of `wrapping_sub` and `overflowing_sub` instead of the subtraction operator.
Override `StepBy::{try_fold, try_rfold}`
Previous PR: https://github.com/rust-lang/rust/pull/51435
The previous PR was closed in favor of https://github.com/rust-lang/rust/pull/51601, which was later reverted. I don't think these implementations will make it harder to specialize `StepBy<Range<_>>` later, so we should be able to land this without any consequences.
This should fix https://github.com/rust-lang/rust/issues/57517 – in my benchmarks `iter` and `iter.step_by(1)` now perform equally well, provided internal iteration is used.
Stabilize `bind_by_move_pattern_guards` in Rust 1.39.0
Closes https://github.com/rust-lang/rust/issues/15287.
After stabilizing `#![feature(bind_by_move_pattern_guards)]`, you can now use bind-by-move bindings in patterns and take references to those bindings in `if` guards of `match` expressions. For example, the following now becomes legal:
```rust
fn main() {
let array: Box<[u8; 4]> = Box::new([1, 2, 3, 4]);
match array {
nums
// ---- `nums` is bound by move.
if nums.iter().sum::<u8>() == 10
// ^------ `.iter()` implicitly takes a reference to `nums`.
=> {
drop(nums);
// --------- Legal as `nums` was bound by move and so we have ownership.
}
_ => unreachable!(),
}
}
```
r? @matthewjasper
Add Iterator comparison methods that take a comparison function
This PR adds `Iterator::{cmp_by, partial_cmp_by, eq_by, ne_by, lt_by, le_by, gt_by, ge_by}`. We already have `Iterator::{cmp, partial_cmp, ...}` which are less general (but not any simpler) than the ones I'm proposing here.
I'm submitting this PR now because #61505 has been merged, so this change should not have a noticeable effect on the `Iterator` docs page size.
The diff is quite messy, here's what I changed:
- The logic of `cmp` / `partial_cmp` / `eq` is moved to `cmp_by` / `partial_cmp_by` / `eq_by` respectively, changing `x.cmp(&y)` to `cmp(&x, &y)` in the `cmp` method where `cmp` is the given comparison function (and similar for `partial_cmp_by` and `eq_by`).
- `ne_by` / `lt_by` / `le_by` / `gt_by` / `ge_by` are each implemented in terms of one of the three methods above.
- The existing comparison methods are each forwarded to their `_by` counterpart, passing one of `Ord::cmp` / `PartialOrd::partial_cmp` / `PartialEq::eq` as the comparison function.
The corresponding `_by_key` methods aren't included because they're not as fundamental as the `_by` methods and can easily be implemented in terms of them. Is that reasonable, or would adding the `_by_key` methods be desirable for the sake of completeness?
I didn't add any tests – I couldn't think of any that weren't already covered by our existing tests. Let me know if there's a particular test that would be useful to add.
Use unicode-xid crate instead of libcore
This PR proposes to remove `char::is_xid_start` and `char::is_xid_continue` functions from `libcore` and use `unicode_xid` crate from crates.io (note that this crate is already present in rust-lang/rust's Cargo.lock).
Reasons to do this:
* removing rustc-binary-specific stuff from libcore
* making sure that, across the ecosystem, there's a single definition of what rust identifier is (`unicode-xid` has almost 10 million downs, as a `proc_macro2` dependency)
* making it easier to share `rustc_lexer` crate with rust-analyzer: no need to `#[cfg]` if we are building as a part of the compiler
Reasons not to do this:
* increased maintenance burden: we'll need to upgrade unicode version both in libcore and in unicode-xid. However, this shouldn't be a too heavy burden: just running `./unicode.py` after new unicode version. I (@matklad) am ready to be a t-compiler side maintainer of unicode-xid. Moreover, given that xid-unicode is an important dependency of syn, *someone* needs to maintain it anyway.
* xid-unicode implementation is significantly slower. It uses a more compact table with binary search, instead of a trie. However, this shouldn't matter in practice, because we have fast-path for ascii anyway, and code size savings is a plus. Moreover, in #59706 not using libcore turned out to be *faster*, presumably beacause checking for whitespace with match is even faster.
<details>
<summary>old description</summary>
Followup to #59706
r? @eddyb
Note that this doesn't actually remove tables from libcore, to avoid conflict with https://github.com/rust-lang/rust/pull/62641.
cc https://github.com/unicode-rs/unicode-xid/pull/11
</details>
