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: