Improve floating point documentation
This is my attempt to improve/solve https://github.com/rust-lang/rust/issues/95468 and https://github.com/rust-lang/rust/issues/73328 .
Added/refined explanations:
- Refine the "NaN as a special value" top level explanation of f32
- Refine `const NAN` docstring: add an explanation about there being multitude of NaN bitpatterns and disclaimer about the portability/stability guarantees.
- Refine `fn is_sign_positive` and `fn is_sign_negative` docstrings: add disclaimer about the sign bit of NaNs.
- Refine `fn min` and `fn max` docstrings: explain the semantics and their relationship to the standard and libm better.
- Refine `fn trunc` docstrings: explain the semantics slightly more.
- Refine `fn powi` docstrings: add disclaimer that the rounding behaviour might be different from `powf`.
- Refine `fn copysign` docstrings: add disclaimer about payloads of NaNs.
- Refine `minimum` and `maximum`: add disclaimer that "propagating NaN" doesn't mean that propagating the NaN bit patterns is guaranteed.
- Refine `max` and `min` docstrings: add "ignoring NaN" to bring the one-row explanation to parity with `minimum` and `maximum`.
Cosmetic changes:
- Reword `NaN` and `NAN` as plain "NaN", unless they refer to the specific `const NAN`.
- Reword "a number" to `self` in function docstrings to clarify.
- Remove "Returns NAN if the number is NAN" from `abs`, as this is told to be the default behavior in the top explanation.
Remove `#[rustc_deprecated]`
This removes `#[rustc_deprecated]` and introduces diagnostics to help users to the right direction (that being `#[deprecated]`). All uses of `#[rustc_deprecated]` have been converted. CI is expected to fail initially; this requires #95958, which includes converting `stdarch`.
I plan on following up in a short while (maybe a bootstrap cycle?) removing the diagnostics, as they're only intended to be short-term.
No "weird" floats in const fn {from,to}_bits
I suspect this code is subtly incorrect and that we don't even e.g. use x87-style floats in CTFE, so I don't have to guard against that case. A future PR will be hopefully removing them from concern entirely, anyways. But at the moment I wanted to get this rolling because small questions like that one seem best answered by review.
r? `@oli-obk`
cc `@eddyb` `@thomcc`
Some masks where defined as
```rust
const NONASCII_MASK: usize = 0x80808080_80808080u64 as usize;
```
where it was assumed that `usize` is never wider than 64, which is currently true.
To make those constants valid in a hypothetical 128-bit target, these constants have been redefined in an `usize`-width-agnostic way
```rust
const NONASCII_MASK: usize = usize::from_ne_bytes([0x80; size_of::<usize>()]);
```
There are already some cases where Rust anticipates the possibility of supporting 128-bit targets, such as not implementing `From<usize>` for `u64`.
Faster parsing for lower numbers for radix up to 16 (cont.)
( Continuation of https://github.com/rust-lang/rust/pull/83371 )
With LingMan's change I think this is potentially ready.
Add debug assertions to some unsafe functions
As suggested by https://github.com/rust-lang/rust/issues/51713
~~Some similar code calls `abort()` instead of `panic!()` but aborting doesn't work in a `const fn`, and the intrinsic for doing dispatch based on whether execution is in a const is unstable.~~
This picked up some invalid uses of `get_unchecked` in the compiler, and fixes them.
I can confirm that they do in fact pick up invalid uses of `get_unchecked` in the wild, though the user experience is less-than-awesome:
```
Running unittests (target/x86_64-unknown-linux-gnu/debug/deps/rle_decode_fast-04b7918da2001b50)
running 6 tests
error: test failed, to rerun pass '--lib'
Caused by:
process didn't exit successfully: `/home/ben/rle-decode-helper/target/x86_64-unknown-linux-gnu/debug/deps/rle_decode_fast-04b7918da2001b50` (signal: 4, SIGILL: illegal instruction)
```
~~As best I can tell these changes produce a 6% regression in the runtime of `./x.py test` when `[rust] debug = true` is set.~~
Latest commit (https://github.com/rust-lang/rust/pull/92686/commits/6894d559bdb4365243b3f4bf73f18e4b1bed04d1) brings the additional overhead from this PR down to 0.5%, while also adding a few more assertions. I think this actually covers all the places in `core` that it is reasonable to check for safety requirements at runtime.
Thoughts?
- Refine the "NaN as a special value" top level explanation of f32
- Refine `const NAN` docstring.
- Refine `fn is_sign_positive` and `fn is_sign_negative` docstrings.
- Refine `fn min` and `fn max` docstrings.
- Refine `fn trunc` docstrings.
- Refine `fn powi` docstrings.
- Refine `fn copysign` docstrings.
- Reword `NaN` and `NAN` as plain "NaN", unless they refer to the specific `const NAN`.
- Reword "a number" to `self` in function docstrings to clarify.
- Remove "Returns NAN if the number is NAN" as this is told to be the default behavior in the top explanation.
- Remove "propagating NaNs", as full propagation (preservation of payloads) is not guaranteed.
These debug assertions are all implemented only at runtime using
`const_eval_select`, and in the error path they execute
`intrinsics::abort` instead of being a normal debug assertion to
minimize the impact of these assertions on code size, when enabled.
Of all these changes, the bounds checks for unchecked indexing are
expected to be most impactful (case in point, they found a problem in
rustc).
Make float parsing docs more comprehensive
I was working on some code with some specialized restrictions on float parsing. I noticed the doc comments for `f32::from_str` and `f64::from_str` were missing several cases of valid inputs that are otherwise difficult to discover without looking at source code.
I'm not sure if the doc comments were initially intended to contain a comprehensive description of valid inputs, but I figured it's useful to include these extra cases for reference.
Matthias Krüger