Rollup merge of #127921 - spastorino:stabilize-unsafe-extern-blocks, r=compiler-errors

Stabilize unsafe extern blocks (RFC 3484)

# Stabilization report

## Summary

This is a tracking issue for the RFC 3484: Unsafe Extern Blocks

We are stabilizing `#![feature(unsafe_extern_blocks)]`, as described in [Unsafe Extern Blocks RFC 3484](https://github.com/rust-lang/rfcs/pull/3484). This feature makes explicit that declaring an extern block is unsafe. Starting in Rust 2024, all extern blocks must be marked as unsafe. In all editions, items within unsafe extern blocks may be marked as safe to use.

RFC: https://github.com/rust-lang/rfcs/pull/3484
Tracking issue: #123743

## What is stabilized

### Summary of stabilization

We now need extern blocks to be marked as unsafe and items inside can also have safety modifiers (unsafe or safe), by default items with no modifiers are unsafe to offer easy migration without surprising results.

```rust
unsafe extern {
    // sqrt (from libm) may be called with any `f64`
    pub safe fn sqrt(x: f64) -> f64;

    // strlen (from libc) requires a valid pointer,
    // so we mark it as being an unsafe fn
    pub unsafe fn strlen(p: *const c_char) -> usize;

    // this function doesn't say safe or unsafe, so it defaults to unsafe
    pub fn free(p: *mut core::ffi::c_void);

    pub safe static IMPORTANT_BYTES: [u8; 256];

    pub safe static LINES: SyncUnsafeCell<i32>;
}
```

## Tests

The relevant tests are in `tests/ui/rust-2024/unsafe-extern-blocks`.

## History

- https://github.com/rust-lang/rust/pull/124482
- https://github.com/rust-lang/rust/pull/124455
- https://github.com/rust-lang/rust/pull/125077
- https://github.com/rust-lang/rust/pull/125522
- https://github.com/rust-lang/rust/issues/126738
- https://github.com/rust-lang/rust/issues/126749
- https://github.com/rust-lang/rust/issues/126755
- https://github.com/rust-lang/rust/pull/126757
- https://github.com/rust-lang/rust/pull/126758
- https://github.com/rust-lang/rust/issues/126756
- https://github.com/rust-lang/rust/pull/126973
- https://github.com/rust-lang/rust/pull/127535
- https://github.com/rust-lang/rustfmt/pull/6204

## Unresolved questions

I am not aware of any unresolved questions.

4 files changed, 4 insertions, 34 deletions

diff --git a/tests/ui/parser/unsafe-foreign-mod-2.rs b/tests/ui/parser/unsafe-foreign-mod-2.rs
index 0b63a993c5b..6d339cd9088 100644
--- a/tests/ui/parser/unsafe-foreign-mod-2.rs
+++ b/tests/ui/parser/unsafe-foreign-mod-2.rs
@@ -1,8 +1,6 @@
 extern "C" unsafe {
     //~^ ERROR expected `{`, found keyword `unsafe`
-    //~| ERROR extern block cannot be declared unsafe
     unsafe fn foo();
-    //~^ ERROR items in unadorned `extern` blocks cannot have safety qualifiers
 }
 
 fn main() {}
diff --git a/tests/ui/parser/unsafe-foreign-mod-2.stderr b/tests/ui/parser/unsafe-foreign-mod-2.stderr
index 8bd592b5d43..0625e3362ed 100644
--- a/tests/ui/parser/unsafe-foreign-mod-2.stderr
+++ b/tests/ui/parser/unsafe-foreign-mod-2.stderr
@@ -4,21 +4,5 @@ error: expected `{`, found keyword `unsafe`
 LL | extern "C" unsafe {
    |            ^^^^^^ expected `{`
 
-error: extern block cannot be declared unsafe
-  --> $DIR/unsafe-foreign-mod-2.rs:1:12
-   |
-LL | extern "C" unsafe {
-   |            ^^^^^^
-   |
-   = note: see issue #123743 <https://github.com/rust-lang/rust/issues/123743> for more information
-   = help: add `#![feature(unsafe_extern_blocks)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-
-error: items in unadorned `extern` blocks cannot have safety qualifiers
-  --> $DIR/unsafe-foreign-mod-2.rs:4:5
-   |
-LL |     unsafe fn foo();
-   |     ^^^^^^^^^^^^^^^^
-
-error: aborting due to 3 previous errors
+error: aborting due to 1 previous error
 
diff --git a/tests/ui/parser/unsafe-foreign-mod.rs b/tests/ui/parser/unsafe-foreign-mod.rs
index eab134a4a4d..623c3bb81e4 100644
--- a/tests/ui/parser/unsafe-foreign-mod.rs
+++ b/tests/ui/parser/unsafe-foreign-mod.rs
@@ -1,5 +1,5 @@
-unsafe extern "C" {
-    //~^ ERROR extern block cannot be declared unsafe
-}
+//@ check-pass
+
+unsafe extern "C" {}
 
 fn main() {}
diff --git a/tests/ui/parser/unsafe-foreign-mod.stderr b/tests/ui/parser/unsafe-foreign-mod.stderr
deleted file mode 100644
index 60b918a89b3..00000000000
--- a/tests/ui/parser/unsafe-foreign-mod.stderr
+++ /dev/null
@@ -1,12 +0,0 @@
-error: extern block cannot be declared unsafe
-  --> $DIR/unsafe-foreign-mod.rs:1:1
-   |
-LL | unsafe extern "C" {
-   | ^^^^^^
-   |
-   = note: see issue #123743 <https://github.com/rust-lang/rust/issues/123743> for more information
-   = help: add `#![feature(unsafe_extern_blocks)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-
-error: aborting due to 1 previous error
-