Use matches macro in libcore and libstd
This PR replaces matches like
```rust
match var {
value => true,
_ => false,
}
```
with use of `matches!` macro.
r? @Centril
Clean up const-hack PRs now that const if / match exist.
Closes#67627.
Cleans up these merged PRs tagged with `const-hack`:
- #63810
- #63786
- #61635
- #58044
reverting their contents to have the match or if expressions they originally contained.
r? @oli-obk
There's one more PR in those tagged with `const-hack` that originally wasn't merged (#65107). Reading the thread, it looks like it was originally closed because the `const-hack` for the checked arithmetic non-negligibly hurt performance, and because there was no way to manipulate the returned Option at compile time anyway (with neither const if nor const match). Would you like me to add these changes to the changes from this PR here too, now that we have the necessary features?
As discussed in https://github.com/rust-lang/rust/issues/10184
Currently, casting a floating point number to an integer with `as` is Undefined Behavior if the value is out of range. `-Z saturating-float-casts` fixes this soundness hole by making `as` “saturate” to the maximum or minimum value of the integer type (or zero for `NaN`), but has measurable negative performance impact in some benchmarks. There is some consensus in that thread for enabling saturation by default anyway, but provide an `unsafe fn` alternative for users who know through some other mean that their values are in range.
This commit applies rustfmt with default settings to files in
src/libcore *that are not involved in any currently open PR* to minimize
merge conflicts. The list of files involved in open PRs was determined
by querying GitHub's GraphQL API with this script:
https://gist.github.com/dtolnay/aa9c34993dc051a4f344d1b10e4487e8
With the list of files from the script in `outstanding_files`, the
relevant commands were:
$ find src/libcore -name '*.rs' | xargs rustfmt --edition=2018
$ rg libcore outstanding_files | xargs git checkout --
Repeating this process several months apart should get us coverage of
most of the rest of libcore.
make is_power_of_two a const function
This makes `is_power_of_two` a const function by using `&` instead of short-circuiting `&&`; Rust supports bitwise `&` for `bool` and short-circuiting is not required in the existing expression.
I don't think this needs a const-hack label as I don't find the changed code less readable, if anything I prefer that it is clearer that short circuiting is not used.
@oli-obk
