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.

…nitialized Scalar

…rminal

The UNIX and WASI implementations use `isatty`. The Windows
implementation uses the same logic the `atty` crate uses, including the
hack needed to detect msys terminals.

Implement this trait for `File` and for `Stdin`/`Stdout`/`Stderr` and
their locked counterparts on all platforms. On UNIX and WASI, implement
it for `BorrowedFd`/`OwnedFd`. On Windows, implement it for
`BorrowedHandle`/`OwnedHandle`.

Based on rust-lang#91121

Co-authored-by: Matt Wilkinson <mattwilki17@gmail.com>

If a process has no console, it'll have NULL in place of a console
handle, so return early with `false` in that case without making any OS
calls.

Rather than referencing a slice's pointer and then creating a new slice
with a longer length, offset from the base structure pointer instead.
This makes some choices of Rust semantics happier.

Continuation of rust-lang#100938 and rust-lang#101010. This rule was added to support the old,
table-based style for displaying enum variants, which are now displayed using
headers and paragraphs.

… r=thomcc

Add `IsTerminal` trait to determine if a descriptor or handle is a terminal

The UNIX implementation uses `isatty`. The Windows implementation uses
the same logic the `atty` crate uses, including the hack needed to
detect msys terminals.

Implement this trait for `Stdin`/`Stdout`/`Stderr`/`File` on all
platforms. On Unix, implement it for `BorrowedFd`/`OwnedFd`. On Windows,
implement it for `BorrowedHandle`/`OwnedHandle`.

Based on rust-lang#91121

Co-authored-by: Matt Wilkinson <mattwilki17@gmail.com>

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.

Add comments about stdout locking

This is the source of some confusion regarding the `println!` macro:
* https://llogiq.github.io/2017/06/01/perf-pitfalls.html#unbuffered-io
* https://news.ycombinator.com/item?id=18794930
* https://reddit.com/r/rust/comments/5puyx2/why_is_println_so_slow/dcua5g5/
* https://reddit.com/r/rust/comments/ab7hsi/comparing_pythagorean_triples_in_c_d_and_rust/ecy7ql8/

In some of these cases it's not the locking behavior where the bottleneck lies, but it's still mentioned as a surprise when, eg, benchmarking a million `println!`'s in a very tight loop.

If there's any stylistic problems please feel free to correct me! This is my first contribution and I want to get it right 🦀

…-obk

interpret: remove support for uninitialized scalars

With Miri no longer supporting `-Zmiri-allow-uninit-numbers`, we no longer need to support storing uninit data in a `Scalar`. We anyway already only use this representation for types with *initialized* `Scalar` layout (and we have to, due to partial initialization), so let's get rid of the `ScalarMaybeUninit` type entirely.

I tried to stage this into meaningful commits, but the one that changes `read_immediate` to always trigger UB on uninit is the largest chunk of the PR and I don't see how it could be subdivided.

Fixes rust-lang/miri#2187
r? ``@oli-obk``

…tch-err, r=oli-obk

Improve const mismatch `FulfillmentError`

Fixes rust-lang#100414

…-session-diagnostic, r=davidtwco

Migrate ast lowering to session diagnostic

I migrated the whole rustc_ast_lowering crate to session diagnostic *except* the for the use of `span_fatal` at /compiler/rustc_ast_lowering/src/expr.rs#L1268 because `#[fatal(...)]` is not yet supported (see rust-lang#100694).

extra sanity check against consts pointing to mutable memory

This should be both unreachable and redundant (since we already ensure that validation only reads from read-only memory, when validating consts), but I feel like we cannot be paranoid enough here, and also if this ever fails it'll be a nicer error than the "cannot read from mutable memory" error.

…, r=jsha

rustdoc: remove unused CSS for `.variants_table`

Continuation of rust-lang#100938 and rust-lang#101010. This rule was added to support the old, table-based style for displaying enum variants, which are now displayed using headers and paragraphs.