- Adds optional default values to const generic parameters in the AST
  and HIR
- Parses these optional default values
- Adds a `const_generics_defaults` feature gate

See https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Is.20there.20a.20symbol.20for.20the.20empty.20string.3F/near/220054471
for context.

Edition evaluation is kept lazy because it may be expensive.

Currently, the rustc_macros::symbols macro generates two
`macro_rules!` macros as its output. These two macros are
used in rustc_span/src/symbol.rs.

This means that each Symbol that we define is represented
in the AST of rustc_symbols twice: once in the definition
of the `define_symbols!` macro (similarly for the
`keywords! macro), and once in the rustc_span::symbols
definition.

That would be OK if there were only a handful of symbols,
but currently we define over 1100 symbols. The definition
of the `define_symbols!` macro contains the expanded definition
of each symbol, so that's a lot of AST storage wasted on a
macro that is used exactly once.

This commit removes the `define_symbols` macro, and simply
allows the proc macro to directly generate the
`rustc_symbols::symbol::sym` module.

The benefit is mainly in reducing memory wasted during
compilation of rustc itself. It should also reduce memory used
by Rust Analyzer.

This commit also reduces the size of the AST for symbol
definitions, by moving two `#[allow(...)]` attributes from
the symbol constants to the `sym` module. This eliminates 2200+
attribute nodes.

This commit also eliminates the need for the `digits_array`
constant. There's no need to store an array of Symbol values
for digits. We can simply define a constant of the base value,
and add to that base value.

Edit wording of the doc comment for rustc_span::symbol::Symbol::can_be_raw
to match related methods.

This decreases the size of `Item` from 680 to 616 bytes. It also does a
lot less work since it no longer has to copy as much.

This takes care of one "FIXME":
// FIXME: use direct symbol comparison for register class names

Instead of using string literals, this uses Symbol for register
class names.

They were originally called "opt-in, built-in traits" (OIBITs), but
people realized that the name was too confusing and a mouthful, and so
they were renamed to just "auto traits". The feature flag's name wasn't
updated, though, so that's what this PR does.

There are some other spots in the compiler that still refer to OIBITs,
but I don't think changing those now is worth it since they are internal
and not particularly relevant to this PR.

Also see <https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/opt-in.2C.20built-in.20traits.20(auto.20traits).20feature.20name>.

Signed-off-by: NAman Arora <me@aman-arora.com>

Co-authored-by: Nvarkor <github@varkor.com>

The lint checks arguments in calls to `transmute` or functions that have
`Pointer` as a trait bound and displays a warning if the argument is a function
reference. Also checks for `std::fmt::Pointer::fmt` to handle formatting macros
although it doesn't depend on the exact expansion of the macro or formatting
internals. `std::fmt::Pointer` and `std::fmt::Pointer::fmt` were also added as
diagnostic items and symbols.

This change is needed for compiler-builtins to check for this feature
when implementing memcpy/memset. See:
  https://github.com/rust-lang/compiler-builtins/pull/365

The change just does compile-time detection. I think that runtime
detection will have to come in a follow-up CL to std-detect.

Like all the CPU feature flags, this just references #44839
Signed-off-by: NJoe Richey <joerichey@google.com>

allow_internal_unstable is currently used
to side-step feature gate and stability checks.
While it was originally only meant to be used
only on macros, its use was expanded to
const functions.

This commit prepares stricter checks for the usage of allow_internal_unstable (only on macros)
and introduces the rustc_allow_const_fn_unstable attribute for usage on functions.

See rust-lang/rust#69399

The panic message might contain braces which should never be
interpreted as format placeholders, which panic!() will do in a future
edition.

It now only reacts to expansion of those macros, and suggests
inserting `"{}", ` in the right place.

This extends the existing `ident_name_compatibility_hack` to handle the
`tuple_from_req` macro defined in `actix-web` (and its fork
`actori-web`).

Related: https://github.com/rust-lang/rust/issues/66741

Guarded with `#![feature(default_alloc_error_handler)]` a default
`alloc_error_handler` is called, if a custom allocator is used and no
other custom `#[alloc_error_handler]` is defined.

The panic message does not contain the size anymore, because it would
pull in the fmt machinery, which would blow up the code size
significantly.

This patch adds support for the LLVM cmse_nonsecure_entry attribute.
This is a target-dependent attribute that only has sense for the
thumbv8m Rust targets.
You can find more information about this attribute here:
https://developer.arm.com/documentation/ecm0359818/latest/Signed-off-by: NHugues de Valon <hugues.devalon@arm.com>

This was a hack to work around the lack of an escape hatch for the "min
`const fn`" checks in const-stable functions. Now that we have co-opted
`allow_internal_unstable` for this purpose, we no longer need the
bespoke attribute.