Make iter::Empty<T> Send and Sync for any T
Continuing from #57682
It's quite funny, when I initially submitted this pull request, I said "Likely nobody will be using that property of `iter::empty`", but then a year later I got a compilation error because it wasn't `Send` and `Sync`.
Unfortunately, `PhantomData<fn() -> T>` still errors out. Oh well. I proposed `
struct PhantomFnWorkaround<T>(fn() -> T);`, but dtolnay did not like it, so using explicit implementations.
Implement `DebugStruct::non_exhaustive`.
This patch adds a function (finish_non_exhaustive) to add ellipsis before the closing brace when formatting using `DebugStruct`.
## Example
```rust
#![feature(debug_non_exhaustive)]
use std::fmt;
struct Bar {
bar: i32,
hidden: f32,
}
impl fmt::Debug for Bar {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("Bar")
.field("bar", &self.bar)
.non_exhaustive(true) // Show that some other field(s) exist.
.finish()
}
}
assert_eq!(
format!("{:?}", Bar { bar: 10, hidden: 1.0 }),
"Bar { bar: 10, .. }",
);
```
Clean up some diagnostics by making them more consistent
In general:
- Diagnostic should start with a lowercase letter.
- Diagnostics should not end with a full stop.
- Ellipses contain three dots.
- Backticks should encode Rust code.
I also reworded a couple of messages to make them read more clearly.
It might be sensible to create a style guide for diagnostics, so these informal conventions are written down somewhere, after which we could audit the existing diagnostics.
r? @Centril
Optimize size/speed of Unicode datasets
The overall implementation has the same general idea as the prior approach,
which was based on a compressed trie structure, but modified to use less space
(and, coincidentally, be an overall performance improvement).
Sizes | Old | New | New/current
-- | -- | -- | --
Alphabetic | 4616 | 2982 | 64.60%
Case_Ignorable | 3144 | 2112 | 67.18%
Cased | 2376 | 934 | 39.31%
Cc | 19 | 43 | 226.32%
Grapheme_Extend | 3072 | 1734 | 56.45%
Lowercase | 2328 | 985 | 42.31%
N | 2648 | 1239 | 46.79%
Uppercase | 1978 | 934 | 47.22%
White_Space | 241 | 140 | 58.09%
| | |
Total | 20422 | 11103 | 54.37%
This table shows the size of the old and new tables in bytes. The most important
of these tables is "Grapheme_Extend", as it is present in essentially all Rust
programs due to being called from `str`'s Debug impl (`char::escape_debug`). In
a representative case given by this [blog post] for the embedded world, the
shrinking in this PR shrinks the final binary by 1,604 bytes, from 14,440 to
12,836.
The performance of these new tables, based on the (rough) benchmark of linearly
scanning the entire valid set of chars, querying for each `is_*`, is roughly
~50% better, though in some cases is either on par or slightly (3-5%) worse. In
practice, I believe the size benefits of this PR are the main concern. The new
implementation has been tested to be equivalent to the current nightly in terms
of returned values on the set of valid chars.
A (relatively) high-level explanation of the specific compression scheme used
can be found [in the generator].
This is split into three commits -- the first adds the generator which produces
the Rust code for the tables, the second adds support code for the lookup, and
the third actually swaps the current implementation out for the new one.
[blog post]: https://jamesmunns.com/blog/fmt-unreasonably-expensive/
[in the generator]: https://github.com/Mark-Simulacrum/rust/blob/unicode-tables/src/tools/unicode-table-generator/src/raw_emitter.rs
Reset Formatter flags on exit from pad_integral
This fixes a bug where after calling pad_integral with appropriate flags, the
fill and alignment flags would be set to '0' and 'Right' and left as such even
after exiting pad_integral, which meant that future calls on the same Formatter
would get incorrect flags reported.
This is quite difficult to observe in practice, as almost all formatting
implementations in practice don't call `Display::fmt` directly, but rather use
`write!` or a similar macro, which means that they cannot observe the effects of
the wrong flags (as `write!` creates a fresh Formatter instance). However, we
include a test case.
A manual check leads me to believe this is the only case where we failed to reset the flags appropriately, but I could have missed something.
Don't require `allow_internal_unstable` unless `staged_api` is enabled.
#63770 changed `qualify_min_const_fn` to require `allow_internal_unstable` for *all* crates that used an unstable feature, regardless of whether `staged_api` was enabled or the `fn` that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added `#![feature(const_fn)]`, which skips `qualify_min_const_fn` entirely.
After this PR, crates that do not have `#![feature(staged_api)]` will only need to enable the feature they are interested in. For example, `#![feature(const_if_match)]` will be enough to enable `if` and `match` in constants. Crates with `staged_api` (e.g., `libstd`) require `#[allow_internal_unstable]` to be added to a function if it uses nightly features unless that function is also marked `#[rustc_const_unstable]`. This prevents proliferation of `#[allow_internal_unstable]` into functions that are not callable in a `const` context on stable.
r? @oli-obk (author of #63770)
cc @Centril
This flag opts out of the min-const-fn checks entirely, which is usually
not what we want. The few cases where the flag is still necessary have
been annotated.
The as_ref method already has a Null-unchecked version section, its
example is a modification of the example in the main as_ref section.
Similarly the example in this commit is a modification of the example
in main as_mut section.
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
`Option::{expect,unwrap}` and `Result::{expect, expect_err, unwrap, unwrap_err}` have `#[track_caller]`
The annotated functions now produce panic messages pointing to the location where they were called, rather than `core`'s internals.
Remove wrong advice about spin locks from `spin_loop_hint` docs
Using a pure spin lock for a critical section in a preemptable thread
is always wrong, however short the critical section may be. The thread
might be preempted, which will cause all other threads to hammer
busily at the core for the whole quant. Moreover, if threads have
different priorities, this might lead to a priority inversion problem
and a deadlock. More generally, a spinlock is not more efficient than
a well-written mutex, which typically does several spin iterations at
the start anyway.
The advise about UP vs SMP is also irrelevant in the context of
preemptive threads.
See also accompanying piece: https://matklad.github.io/2020/01/02/spinlocs-considered-harmful.html
And another, independent piece: https://probablydance.com/2019/12/30/measuring-mutexes-spinlocks-and-how-bad-the-linux-scheduler-really-is
EDIT: obligatory disclosure that I am not an expert in these things, and might be terribly wrong :)
Dylan DPC