Update safety documentation for `CString::from_ptr` and `str::from_boxed_utf8_unchecked`
## PR Description
This PR addresses missing safety documentation for two APIs:
**1. alloc::ffi::CStr::from_raw**
- `Alias`: The pointer must not be aliased (accessed via other pointers) during the reconstructed CString's lifetime.
- `Owning`: Calling this function twice on the same pointer and creating two objects with overlapping lifetimes, introduces two alive owners of the same memory. This may result in a double-free.
- `Dangling`: The prior documentation required the pointer to originate from CString::into_raw, but this constraint is incomplete. A validly sourced pointer can also cause undefined behavior (UB) if it becomes dangling. A simple Poc for this situation:
```
use std::ffi::CString;
use std::os::raw::c_char;
fn create_dangling() -> *mut c_char {
let local_ptr: *mut c_char = {
let valid_data = CString::new("valid").unwrap();
valid_data.into_raw()
};
unsafe {
let _x = CString::from_raw(local_ptr);
}
local_ptr
}
fn main() {
let dangling = create_dangling();
unsafe {let _y = CString::from_raw(dangling);} // Cause UB!
}
```
**2. alloc::str::from_boxed_utf8_unchecked**
- `ValidStr`: Bytes must contain a valid UTF-8 sequence.
Stabilise `std::ffi::c_str`
This finished FCP in #112134 but never actually got a stabilisation PR. Since the FCP in #120048 recently passed to add the `os_str` module, it would be nice to also merge this too, to ensure that both get added in the next version.
Note: The added stability attributes which *somehow* were able to be omitted before (rustc bug?) were added based on the fact that they were added in 302551388b, which ended up in 1.85.0.
Closes: https://github.com/rust-lang/rust/issues/112134
r? libs-api
This PR lets `impl Default for Rc<str>` re-use the implementation
for `Rc::<[u8]>::default()`. The previous version only calculted the
memory layout at runtime, even though it should be known at compile
time, resulting in an additional function call.
The same optimization is done for `Rc<CStr>`.
Generated byte code: <https://godbolt.org/z/dfq73jsoP>.
Resolves <https://github.com/rust-lang/rust/issues/135784>.
Implemented `FromStr` for `CString` and `TryFrom<CString>` for `String`
The motivation of this change is making it possible to use `CString` in generic methods with `FromStr` and `TryInto<String>` trait bounds. The same traits are already implemented for `OsString` which is an FFI type too.
warning: casting raw pointers to the same type and constness is unnecessary (`*mut V` -> `*mut V`)
--> library\alloc\src\collections\btree\map\entry.rs:357:31
|
357 | let val_ptr = root.borrow_mut().push(self.key, value) as *mut V;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `root.borrow_mut().push
(self.key, value)`
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
warning: casting to the same type is unnecessary (`usize` -> `usize`)
--> library\alloc\src\ffi\c_str.rs:411:56
|
411 | let slice = slice::from_raw_parts_mut(ptr, len as usize);
| ^^^^^^^^^^^^ help: try: `len`
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
warning: casting raw pointers to the same type and constness is unnecessary (`*mut T` -> `*mut T`)
--> library\alloc\src\slice.rs:516:25
|
516 | (buf.as_mut_ptr() as *mut T).add(buf.len()),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `buf.as_mut_ptr()`
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
warning: casting raw pointers to the same type and constness is unnecessary (`*mut T` -> `*mut T`)
--> library\alloc\src\slice.rs:537:21
|
537 | (buf.as_mut_ptr() as *mut T).add(buf.len()),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `buf.as_mut_ptr()`
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
warning: casting raw pointers to the same type and constness is unnecessary (`*const ()` -> `*const ()`)
--> library\alloc\src\task.rs:151:13
|
151 | waker as *const (),
| ^^^^^^^^^^^^^^^^^^ help: try: `waker`
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
warning: casting raw pointers to the same type and constness is unnecessary (`*const ()` -> `*const ()`)
--> library\alloc\src\task.rs:323:13
|
323 | waker as *const (),
| ^^^^^^^^^^^^^^^^^^ help: try: `waker`
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
warning: casting to the same type is unnecessary (`usize` -> `usize`)
--> library\std\src\sys_common\net.rs:110:21
|
110 | assert!(len as usize >= mem::size_of::<c::sockaddr_in>());
| ^^^^^^^^^^^^ help: try: `len`
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
warning: casting to the same type is unnecessary (`usize` -> `usize`)
--> library\std\src\sys_common\net.rs:116:21
|
116 | assert!(len as usize >= mem::size_of::<c::sockaddr_in6>());
| ^^^^^^^^^^^^ help: try: `len`
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
detects redundant imports that can be eliminated.
for #117772 :
In order to facilitate review and modification, split the checking code and
removing redundant imports code into two PR.
Stabilize the `core_c_str` and `alloc_c_string` feature gates.
Change `std::ffi` to re-export these types rather than creating type
aliases, since they now have matching stability.
It looks like the last time had left some remaining cfg's -- which made me think
that the stage0 bump was actually successful. This brings us to a released 1.62
beta though.
