Rollup merge of rust-lang#79208 - LeSeulArtichaut:stable-unsafe_op_in_unsafe_fn, r=nikomatsakis

Stabilize `unsafe_op_in_unsafe_fn` lint

This makes it possible to override the level of the `unsafe_op_in_unsafe_fn`, as proposed in rust-lang#71668 (comment).

Tracking issue: rust-lang#71668
r? ```@nikomatsakis``` cc ```@SimonSapin``` ```@RalfJung```

# Stabilization report

This is a stabilization report for `#![feature(unsafe_block_in_unsafe_fn)]`.

## Summary

Currently, the body of unsafe functions is an unsafe block, i.e. you can perform unsafe operations inside.

The `unsafe_op_in_unsafe_fn` lint, stabilized here, can be used to change this behavior, so performing unsafe operations in unsafe functions requires an unsafe block.

For now, the lint is allow-by-default, which means that this PR does not change anything without overriding the lint level.

For more information, see [RFC 2585](https://github.com/rust-lang/rfcs/blob/master/text/2585-unsafe-block-in-unsafe-fn.md)

### Example

```rust
// An `unsafe fn` for demonstration purposes.
// Calling this is an unsafe operation.
unsafe fn unsf() {}

// #[allow(unsafe_op_in_unsafe_fn)] by default,
// the behavior of `unsafe fn` is unchanged
unsafe fn allowed() {
    // Here, no `unsafe` block is needed to
    // perform unsafe operations...
    unsf();

    // ...and any `unsafe` block is considered
    // unused and is warned on by the compiler.
    unsafe {
        unsf();
    }
}

#[warn(unsafe_op_in_unsafe_fn)]
unsafe fn warned() {
    // Removing this `unsafe` block will
    // cause the compiler to emit a warning.
    // (Also, no "unused unsafe" warning will be emitted here.)
    unsafe {
        unsf();
    }
}

#[deny(unsafe_op_in_unsafe_fn)]
unsafe fn denied() {
    // Removing this `unsafe` block will
    // cause a compilation error.
    // (Also, no "unused unsafe" warning will be emitted here.)
    unsafe {
        unsf();
    }
}
```

Author: JohnTitor

alloc/src/lib.rs

@@ -133,7 +133,7 @@
 #![feature(trusted_len)]
 #![feature(unboxed_closures)]
 #![feature(unicode_internals)]
-#![feature(unsafe_block_in_unsafe_fn)]
+#![cfg_attr(bootstrap, feature(unsafe_block_in_unsafe_fn))]
 #![feature(unsize)]
 #![feature(unsized_fn_params)]
 #![feature(allocator_internals)]

core/src/lib.rs

@@ -164,8 +164,8 @@
 #![feature(const_caller_location)]
 #![feature(slice_ptr_get)]
 #![feature(no_niche)] // rust-lang/rust#68303
-#![feature(unsafe_block_in_unsafe_fn)]
 #![feature(int_error_matching)]
+#![cfg_attr(bootstrap, feature(unsafe_block_in_unsafe_fn))]
 #![deny(unsafe_op_in_unsafe_fn)]
 
 #[prelude_import]

core/tests/lib.rs

@@ -72,16 +72,16 @@
 #![feature(peekable_peek_mut)]
 #![cfg_attr(not(bootstrap), feature(ptr_metadata))]
 #![feature(once_cell)]
-#![feature(unsafe_block_in_unsafe_fn)]
 #![feature(unsized_tuple_coercion)]
 #![feature(int_bits_const)]
 #![feature(nonzero_leading_trailing_zeros)]
 #![feature(const_option)]
 #![feature(integer_atomics)]
 #![feature(slice_group_by)]
 #![feature(trusted_random_access)]
-#![deny(unsafe_op_in_unsafe_fn)]
+#![cfg_attr(bootstrap, feature(unsafe_block_in_unsafe_fn))]
 #![cfg_attr(not(bootstrap), feature(unsize))]
+#![deny(unsafe_op_in_unsafe_fn)]
 
 extern crate test;
 

std/src/lib.rs

@@ -327,7 +327,7 @@
 #![feature(try_blocks)]
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
-#![feature(unsafe_block_in_unsafe_fn)]
+#![cfg_attr(bootstrap, feature(unsafe_block_in_unsafe_fn))]
 #![feature(unsafe_cell_raw_get)]
 #![feature(unwind_attributes)]
 #![feature(vec_into_raw_parts)]