Previously, attributes on 'if' expressions (e.g. #[attr] if true {})
were disallowed during parsing. This made it impossible for macros to
perform any custom handling of such attributes (e.g. stripping them
away), since a compilation error would be emitted before they ever had a
chance to run.

This PR permits attributes on 'if' expressions ('if-attrs' from here on).
Both built-in attributes (e.g. `#[allow]`, `#[cfg]`) are supported.

We still do *not* accept attributes on 'other parts' of an if-else
chain. That is, the following code snippet still fails to parse:

```rust
if true {} #[attr] else if false {} else #[attr] if false {} #[attr]
else {}
```

Co-Authored-By: bjorn3 <bjorn3@users.noreply.github.com>

Additionally whenever possible match C API provided by the LLVM.

Although `stack_overflow::init` runs very early in the process, even
before `main`, there may already be signal handlers installed for things
like the address sanitizer. In that case, just leave it alone, and don't
bother trying to allocate our own signal stacks either.

…=Centril

Permit attributes on 'if' expressions

Previously, attributes on 'if' expressions (e.g. `#[attr] if true {}`)
were disallowed during parsing. This made it impossible for macros to
perform any custom handling of such attributes (e.g. stripping them
away), since a compilation error would be emitted before they ever had a
chance to run.

This PR permits attributes on 'if' expressions ('if-attrs' from here on).
Both built-in attributes (e.g. `#[allow]`, `#[cfg]`) and proc-macro attributes are supported.

We still do *not* accept attributes on 'other parts' of an if-else
chain. That is, the following code snippet still fails to parse:

```rust
if true {} #[attr] else if false {} else #[attr] if false {} #[attr]
else {}
```

Closes rust-lang#68618

unix: Don't override existing SIGSEGV/BUS handlers

Although `stack_overflow::init` runs very early in the process, even
before `main`, there may already be signal handlers installed for things
like the address sanitizer. In that case, just leave it alone, and don't
bother trying to allocate our own signal stacks either.

Fixes rust-lang#69524.

Ensure that validity only raises validity errors

For now, only as a debug-assertion (similar to const-prop detecting errors that allocate).

Now includes rust-lang#69646.
[Relative diff](RalfJung/rust@layout-visitor...RalfJung:validity-errors).

r? @oli-obk

librustc_codegen_llvm: Use slices in preference to 0-terminated strings

Additionally whenever possible match C API provided by the LLVM.

rustc_parse: Remove `Parser::normalized(_prev)_token`

Perform the "normalization" (renamed to "uninterpolation") on the fly when necessary.

The final part of rust-lang#69579 rust-lang#69384 rust-lang#69376 rust-lang#69211 rust-lang#69034 rust-lang#69006.
r? @Centril

Add more regression tests

Closes rust-lang#54239
Closes rust-lang#57200
Closes rust-lang#57201
Closes rust-lang#60473
Closes rust-lang#64620
Closes rust-lang#67166

r? @Centril