Auto merge of #128771 - carbotaniuman:stabilize_unsafe_attr, r=nnethercote

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](https://github.com/rust-lang/rust/pull/124214#issuecomment-2124753464)

## Tests

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

1 files changed, 1 insertions, 3 deletions

diff --git a/compiler/rustc_parse/src/parser/attr.rs b/compiler/rustc_parse/src/parser/attr.rs
index 3d06017fcf3..6391ff901cb 100644
--- a/compiler/rustc_parse/src/parser/attr.rs
+++ b/compiler/rustc_parse/src/parser/attr.rs
@@ -4,7 +4,7 @@ use rustc_ast::token::{self, Delimiter};
 use rustc_errors::codes::*;
 use rustc_errors::{Diag, PResult};
 use rustc_span::symbol::kw;
-use rustc_span::{sym, BytePos, Span};
+use rustc_span::{BytePos, Span};
 use thin_vec::ThinVec;
 use tracing::debug;
 
@@ -265,7 +265,6 @@ impl<'a> Parser<'a> {
             let is_unsafe = this.eat_keyword(kw::Unsafe);
             let unsafety = if is_unsafe {
                 let unsafe_span = this.prev_token.span;
-                this.psess.gated_spans.gate(sym::unsafe_attributes, unsafe_span);
                 this.expect(&token::OpenDelim(Delimiter::Parenthesis))?;
                 ast::Safety::Unsafe(unsafe_span)
             } else {
@@ -406,7 +405,6 @@ impl<'a> Parser<'a> {
         };
         let unsafety = if is_unsafe {
             let unsafe_span = self.prev_token.span;
-            self.psess.gated_spans.gate(sym::unsafe_attributes, unsafe_span);
             self.expect(&token::OpenDelim(Delimiter::Parenthesis))?;
 
             ast::Safety::Unsafe(unsafe_span)