Remove manual unrolling from slice::Iter(Mut)::try_fold
While this definitely helps sometimes (particularly for trivial closures), it's also a pessimization sometimes, so it's better to leave this to (hypothetical) future LLVM improvements instead of forcing this on everyone.
I think it's better for the advice to be that sometimes you need to unroll manually than you sometimes need to not-unroll manually (like #64545).
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For context see https://github.com/rust-lang/rust/pull/64572#issuecomment-532961046
Improve wording in documentation of MaybeUninit
Changes
> variables are properly initialized **at** their respective type
into
> variables are properly initialized **as** their respective type
reserve `impl<T> From<!> for T`
this is necessary for never-type stabilization.
cc #57012#35121
I think we wanted a crater run for this @nikomatsakis?
r? @nikomatsakis
use `sign` variable in abs and wrapping_abs methods
This also makes the code easier to understand by hinting at the significance of `self >> ($BITS - 1)`.
Also, now `overflowing_abs` simply uses `wrapping_abs`, which is clearer and avoids a potential performance regression in the LLVM IR.
This PR follows from the discussion from #63786.
r? @eddyb
cc @nikic
Document the unstable iter_order_by library feature
Tracking issue: #64295
Follow-up for: #62205
References the tracking issue and adds a page to the unstable book for the new unstable `iter_order_by` feature.
While this definitely helps sometimes (particularly for trivial closures), it's also a pessimization sometimes, so it's better to leave this to (hypothetical) future LLVM improvements instead of forcing this on everyone.
I think it's better for the advice to be that sometimes you need to unroll manually than you sometimes need to not-unroll manually (like #64545).
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
This also makes the code easier to understand by hinting at the
significance of `self >> ($BITS - 1)` and by including an explanation
in the comments.
Also, now overflowing_abs simply uses wrapping_abs, which is clearer
and avoids a potential performance regression in the LLVM IR.
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.
