Auto merge of rust-lang#128985 - GrigorenkoPV:instantly-dangling-pointer, r=Urgau

Lint against getting pointers from immediately dropped temporaries

Fixes rust-lang#123613

## Changes:
1. New lint: `dangling_pointers_from_temporaries`. Is a generalization of `temporary_cstring_as_ptr` for more types and more ways to get a temporary.
2. `temporary_cstring_as_ptr` is removed and marked as renamed to `dangling_pointers_from_temporaries`.
3. `clippy::temporary_cstring_as_ptr` is marked as renamed to `dangling_pointers_from_temporaries`.
4. Fixed a false positive[^fp] for when the pointer is not actually dangling because of lifetime extension for function/method call arguments.
5. `core::cell::Cell` is now `rustc_diagnostic_item = "Cell"`

## Questions:
- [ ]  Instead of manually checking for a list of known methods and diagnostic items, maybe add some sort of annotation to those methods in library and check for the presence of that annotation? rust-lang#128985 (comment)

## Known limitations:

### False negatives[^fn]:

See the comments in `compiler/rustc_lint/src/dangling.rs`

1. Method calls that are not checked for:
   - `temporary_unsafe_cell.get()`
   - `temporary_sync_unsafe_cell.get()`
2. Ways to get a temporary that are not recognized:
   - `owning_temporary.field`
   - `owning_temporary[index]`
3. No checks for ref-to-ptr conversions:
   - `&raw [mut] temporary`
   - `&temporary as *(const|mut) _`
    - `ptr::from_ref(&temporary)` and friends

[^fn]: lint **should** be emitted, but **is not**

[^fp]: lint **should not** be emitted, but **is**

Author: bors

alloc/tests/boxed.rs

@@ -4,6 +4,7 @@ use core::mem::MaybeUninit;
 use core::ptr::NonNull;
 
 #[test]
+#[cfg_attr(not(bootstrap), expect(dangling_pointers_from_temporaries))]
 fn uninitialized_zero_size_box() {
     assert_eq!(
         &*Box::<()>::new_uninit() as *const _,

core/src/cell.rs

@@ -304,6 +304,7 @@ pub use once::OnceCell;
 /// ```
 ///
 /// See the [module-level documentation](self) for more.
+#[cfg_attr(not(test), rustc_diagnostic_item = "Cell")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[repr(transparent)]
 #[rustc_pub_transparent]

core/src/ffi/c_str.rs

@@ -464,7 +464,9 @@ impl CStr {
     /// behavior when `ptr` is used inside the `unsafe` block:
     ///
     /// ```no_run
-    /// # #![allow(unused_must_use)] #![allow(temporary_cstring_as_ptr)]
+    /// # #![allow(unused_must_use)]
+    /// # #![cfg_attr(bootstrap, expect(temporary_cstring_as_ptr))]
+    /// # #![cfg_attr(not(bootstrap), expect(dangling_pointers_from_temporaries))]
     /// use std::ffi::CString;
     ///
     /// // Do not do this: