Remove spotlight
I had a few comments saying that this feature was at best misunderstood or not even used so I decided to organize a poll about on [twitter](https://twitter.com/imperioworld_/status/1232769353503956994). After 87 votes, the result is very clear: it's not useful. Considering the amount of code we have just to run it, I think it's definitely worth it to remove it.
r? @kinnison
cc @ollie27
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#69524.
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 https://github.com/rust-lang/rust/issues/68618
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.
Compile address sanitizer test with debuginfo
This makes error-pattern to match regardless of current
configuration of `rust.debuginfo-level-tests` in `config.toml`.
Miri visitor: detect primitive types based on type, not layout (also, more tests)
I also converted the union-based transmutes to use `mem::transmute` for increased readability.
r? @eddyb @oli-obk
