Add lint against integer to pointer transmutes
# `integer_to_ptr_transmutes`
*warn-by-default*
The `integer_to_ptr_transmutes` lint detects integer to pointer transmutes where the resulting pointers are undefined behavior to dereference.
### Example
```rust
fn foo(a: usize) -> *const u8 {
unsafe {
std::mem::transmute::<usize, *const u8>(a)
}
}
```
```
warning: transmuting an integer to a pointer creates a pointer without provenance
--> a.rs:1:9
|
158 | std::mem::transmute::<usize, *const u8>(a)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: this is dangerous because dereferencing the resulting pointer is undefined behavior
= note: exposed provenance semantics can be used to create a pointer based on some previously exposed provenance
= help: if you truly mean to create a pointer without provenance, use `std::ptr::without_provenance_mut`
= help: for more information about transmute, see <https://doc.rust-lang.org/std/mem/fn.transmute.html#transmutation-between-pointers-and-integers>
= help: for more information about exposed provenance, see <https://doc.rust-lang.org/std/ptr/index.html#exposed-provenance>
= note: `#[warn(integer_to_ptr_transmutes)]` on by default
help: use `std::ptr::with_exposed_provenance` instead to use a previously exposed provenance
|
158 - std::mem::transmute::<usize, *const u8>(a)
158 + std::ptr::with_exposed_provenance::<u8>(a)
|
```
### Explanation
Any attempt to use the resulting pointers are undefined behavior as the resulting pointers won't have any provenance.
Alternatively, `std::ptr::with_exposed_provenance` should be used, as they do not carry the provenance requirement or if the wanting to create pointers without provenance `std::ptr::without_provenance_mut` should be used.
See [std::mem::transmute] in the reference for more details.
[std::mem::transmute]: https://doc.rust-lang.org/std/mem/fn.transmute.html
--------
People are getting tripped up on this, see https://github.com/rust-lang/rust/issues/128409 and https://github.com/rust-lang/rust/issues/141220. There are >90 cases like these on [GitHub search](https://github.com/search?q=lang%3Arust+%2Ftransmute%3A%3A%3Cu%5B0-9%5D*.*%2C+%5C*const%2F&type=code).
Fixes https://github.com/rust-lang/rust-clippy/issues/13140
Fixes https://github.com/rust-lang/rust/issues/141220
Fixes https://github.com/rust-lang/rust/issues/145523
`@rustbot` labels +I-lang-nominated +T-lang
cc `@traviscross`
r? compiler
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