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Stabilize `unsafe_attributes`

# Stabilization report

## Summary

This is a tracking issue for the RFC 3325: unsafe attributes

We are stabilizing `#![feature(unsafe_attributes)]`,  which makes certain attributes considered 'unsafe', meaning that they must be surrounded by an `unsafe(...)`, as in `#[unsafe(no_mangle)]`.

RFC: rust-lang/rfcs#3325
Tracking issue: #123757

## What is stabilized

### Summary of stabilization

Certain attributes will now be designated as unsafe attributes, namely, `no_mangle`, `export_name`, and `link_section` (stable only), and these attributes will need to be called by surrounding them in `unsafe(...)` syntax. On editions prior to 2024, this is simply an edition lint, but it will become a hard error in 2024. This also works in `cfg_attr`, but `unsafe` is not allowed for any other attributes, including proc-macros ones.

```rust
#[unsafe(no_mangle)]
fn a() {}

#[cfg_attr(any(), unsafe(export_name = "c"))]
fn b() {}
```

For a table showing the attributes that were considered to be included in the list to require unsafe, and subsequent reasoning about why each such attribute was or was not included, see [this comment here](rust-lang/rust#124214 (comment))

## Tests

The relevant tests are in `tests/ui/rust-2024/unsafe-attributes` and `tests/ui/attributes/unsafe`.