Rollup of 5 pull requests
Successful merges:
- #67797 (Query-ify Instance::resolve)
- #70777 (Don't import integer and float modules, use assoc consts)
- #70795 (Keep track of position when deleting from a BTreeMap)
- #70812 (Do not use "nil" to refer to `()`)
- #70815 (Enable layout debugging for `impl Trait` type aliases)
Failed merges:
r? @ghost
Do not use "nil" to refer to `()`
"nil" is not used in the [book](https://doc.rust-lang.org/book) or in the [standard library](https://doc.rust-lang.org/std) anywhere else. Because "nil" is often used in programming languages to refer to "None" or "null" I think it could be a little confusing for newcomers to see this type referred to as "nil".
Keep track of position when deleting from a BTreeMap
This improves the performance of drain_filter and is needed for future Cursor support for BTreeMap.
cc @ssomers
r? @Mark-Simulacrum
Don't import integer and float modules, use assoc consts
Stop importing the standard library integer and float modules to reach the `MIN`, `MAX` and other constants. They are available directly on the primitive types now.
This PR is a follow up of #69860 which made sure we use the new constants in documentation.
This type of change touches a lot of files, and previously all my assoc int consts PRs had collisions and were accepted only after a long delay. So I'd prefer to do it in smaller steps now. Just removing these imports seem like a good next step.
r? @dtolnay
Query-ify Instance::resolve
Split off from #65989
Instance::resolve is now a wrapper for a new `resolve_instance` query.
This greatly improves performance on several benchmarks
clarify comment in RawVec::into_box
On first reading I almost thought "len <= cap" would be all that there is to check here. Expand the comment to clarify that that is not the case.
Fix some aliasing issues in Vec
`Vec::extend` and `Vec::truncate` invalidated references into the vector even without reallocation, because they (implicitly) created a mutable reference covering the *entire* initialized part of the vector.
Fixes https://github.com/rust-lang/rust/issues/70301
I verified the fix by adding some new tests here that I ran in Miri.
Fix performance regression in debuginfo file_metadata.
Fixes performance regression caused by #69718.
Finding the `SourceFile` associated with a `FileName` called `get_source_file` on the `SourceMap`, which does a linear search through all files in the `SourceMap`.
This resolves the issue by passing the `SourceFile` in from the caller (which already had it available) instead of the `FileName`
Fixes#70785.
Finding the `SourceFile` associated with a `FileName` called `get_source_file` on
the `SourceMap`, which does a linear search through all files in the `SourceMap`.
This resolves the issue by passing the SourceFile in from the caller (which already
had it available).
Rollup of 6 pull requests
Successful merges:
- #70635 (rustc_target: Some cleanup to `no_default_libraries`)
- #70748 (Do not disable field reordering on enums with big discriminant)
- #70752 (Add slice::fill)
- #70766 (use ManuallyDrop instead of forget inside collections)
- #70768 (macro_rules: `NtLifetime` cannot start with an identifier)
- #70783 (comment refers to removed type)
Failed merges:
r? @ghost
use ManuallyDrop instead of forget inside collections
This PR changes some usage of `mem::forget` into `mem::ManuallyDrop` in some `Vec`, `VecDeque`, `BTreeMap` and `Box` methods.
Before the commit, the generated IR for some of the methods was longer, and even after optimization, some unwinding artifacts were still present.
Add slice::fill
Adds the `slice::fill` method to fill a slice with an item. This replaces manual for loops where items are copied one-by-one. This is a counterpart to C++20's [`std::fill`](https://en.cppreference.com/w/cpp/algorithm/fill) function.
## Usage
```rust
let mut buf = vec![0; 10];
buf.fill(1);
assert_eq!(buf, vec![1; 10]);
```
## Performance
When compiling in release mode, for `[u8]` and `[u16]` this method will optimize to a `memset(3)` call ([godbolt](https://godbolt.org/z/85El_c)). The initial implementation relies on LLVM's optimizer to make it as fast as possible for any given input. But as @jonas-schievink [pointed out](https://twitter.com/sheevink/status/1245756597453885442) this can later be optimized through specialization to guarantee it has a specific performance profile.
## Why now?
Conversations about adding `slice::fill` are not new. In fact, https://github.com/rust-lang/rfcs/issues/2067 was opened 3 years ago about this exact topic. However discussion stranded while discussing implementation details, and it's not seen much forward motion since.
In ["The Hunt for the Fastest Zero"](https://travisdowns.github.io/blog/2020/01/20/zero.html) Travis Downs provides disects C++'s `std::fill` performance profile on gcc, comparing it among others to `memset(3)`. Even though `memset(3)` outperforms `std::fill` in their tests, the author notes the following:
> That the optimization fails, perhaps unexpectedly, in some cases is unfortunate but it’s nice that you can fix it yourself. [...] Do we throw out modern C++ idioms, at least where performance matters, for example by replacing std::fill with memset? I don’t think so.
Much of the article focuses on how how to fix the performance of `std::fill` by providing specializations for specific input. In Rust we don't have any dedicated methods to fill slices with values, so it either needs to be optimized at the MIR layer, or more likely rely on LLVM's optimizer.
By adding a dedicated method for filling slices with values it opens up the ability for us to in the future guarantee that e.g. `Vec<u8>` will always optimize to `memset` even in debug mode. Or perhaps provide stronger guarantees about memory when zeroing values when a certain flag is passed. But regardless of that, it improves general ergonomics of working with slices by providing a dedicated method with documentation and examples.
## References
- [slice-fill prototype on docs.rs](https://docs.rs/slice-fill/1.0.1/slice_fill/)
- [The Hunt For The Fastest Zero](https://travisdowns.github.io/blog/2020/01/20/zero.html)
- [Safe memset for slices](https://github.com/rust-lang/rfcs/issues/2067)
- [C++20 std::fill](https://en.cppreference.com/w/cpp/algorithm/fill)
- [ASM output on Godbolt](https://godbolt.org/z/5-XU66)
