We have `File::create` for creating a file or opening an existing file,
but the secure way to guarantee creating a new file requires a longhand
invocation via `OpenOptions`.

Add `File::create_new` to handle this case, to make it easier for people
to do secure file creation.

...replacing `.cast().wrapping_offset().cast()` & similar code.

Signed-off-by: Alex Saveau <saveau.alexandre@gmail.com>

Add MaybeUninit memset test

Closes rust-lang#96274

Add a `File::create_new` constructor

We have `File::create` for creating a file or opening an existing file,
but the secure way to guarantee creating a new file requires a longhand
invocation via `OpenOptions`.

Add `File::create_new` to handle this case, to make it easier for people
to do secure file creation.

…r-errors

Remove separate indexing of early-bound regions

~Based on rust-lang#99728

This PR copies some modifications from rust-lang#97839 around object lifetime defaults.
These modifications allow to stop counting generic parameters during lifetime resolution, and rely on the indexing given by `rustc_typeck::collect`.

remove an ineffective check in const_prop

Based on rust-lang#100043, only the last two commits are new.

ConstProp has a special check when reading from a local that prevents reading uninit locals. However, if that local flows into `force_allocation`, then no check fires and evaluation proceeds. So this check is not really effective at preventing accesses to uninit locals.

With rust-lang#100043, `read_immediate` and friends always fail when reading uninit locals, so I don't see why ConstProp would need a separate check. Thus I propose we remove it. This is needed to be able to do rust-lang#100085.

…g, r=JohnTitor

Stabilize `std::io::read_to_string`

Closes rust-lang#80218. 🎉

This PR stabilizes the `std::io::read_to_string` function, with the following public API:

```rust
pub fn read_to_string<R: Read>(reader: R) -> Result<String>;
```

It's analogous to `std::fs::read_to_string` for files, but it works on anything that implements `io::Read`, including `io::stdin()`.

See the tracking issue (rust-lang#80218) or documentation for details.

… r=scottmcm

Make use of `[wrapping_]byte_{add,sub}`

These new methods trivially replace old `.cast().wrapping_offset().cast()` & similar code.
Note that [`arith_offset`](https://doc.rust-lang.org/std/intrinsics/fn.arith_offset.html) and `wrapping_offset` are the same thing.

r? ``@scottmcm``

_split off from #100746_

…ark-Simulacrum

Remove a panicking branch from `fmt::builders::PadAdapter`

Separate CountIsStar from CountIsParam in rustc_parse_format.

`rustc_parse_format`'s parser would result in the exact same output for `{:.*}` and `{:.0$}`, making it hard for diagnostics to handle these cases properly.

This splits those cases by adding a new `CountIsStar` enum variant.

This fixes rust-lang#100995

Prerequisite for rust-lang#100996