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`.

compiler/rustc_builtin_macros/src/cfg_accessible.rs

@@ -47,7 +47,6 @@ impl MultiItemModifier for Expander {
     ) -> ExpandResult<Vec<Annotatable>, Annotatable> {
         let template = AttributeTemplate { list: Some("path"), ..Default::default() };
         validate_attr::check_builtin_meta_item(
-            &ecx.ecfg.features,
             &ecx.sess.psess,
             meta_item,
             ast::AttrStyle::Outer,

compiler/rustc_builtin_macros/src/derive.rs

@@ -38,7 +38,6 @@ impl MultiItemModifier for Expander {
                 let template =
                     AttributeTemplate { list: Some("Trait1, Trait2, ..."), ..Default::default() };
                 validate_attr::check_builtin_meta_item(
-                    features,
                     &sess.psess,
                     meta_item,
                     ast::AttrStyle::Outer,

compiler/rustc_builtin_macros/src/util.rs

@@ -17,7 +17,6 @@ pub(crate) fn check_builtin_macro_attribute(ecx: &ExtCtxt<'_>, meta_item: &MetaI
     // All the built-in macro attributes are "words" at the moment.
     let template = AttributeTemplate { word: true, ..Default::default() };
     validate_attr::check_builtin_meta_item(
-        &ecx.ecfg.features,
         &ecx.sess.psess,
         meta_item,
         AttrStyle::Outer,