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Rollup merge of #118983 - Urgau:invalid_ref_casting-bigger-layout, r=oli-obk

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Warn on references casting to bigger memory layout

This PR extends the [`invalid_reference_casting`](https://doc.rust-lang.org/rustc/lints/listing/deny-by-default.html#invalid-reference-casting) lint (*deny-by-default*) which currently lint on `&T -> &mut T` casting to also lint on `&(mut) A -> &(mut) B` where `size_of::<B>() > size_of::<A>()` (bigger memory layout requirement).

The goal is to detect such cases:

```rust
let u8_ref: &u8 = &0u8;
let u64_ref: &u64 = unsafe { &*(u8_ref as *const u8 as *const u64) };
//~^ ERROR casting references to a bigger memory layout is undefined behavior

let mat3 = Mat3 { a: Vec3(0i32, 0, 0), b: Vec3(0, 0, 0), c: Vec3(0, 0, 0) };
let mat3 = unsafe { &*(&mat3 as *const _ as *const [[i64; 3]; 3]) };
//~^ ERROR casting references to a bigger memory layout is undefined behavior
```

This is added to help people who write unsafe code, especially when people have matrix struct that they cast to simple array of arrays.

EDIT: One caveat, due to the [`&Header`](https://github.com/rust-lang/unsafe-code-guidelines/issues/256) uncertainty the lint only fires when it can find the underline allocation.

~~I have manually tested all the new expressions that warn against Miri, and they all report immediate UB.~~

r? ``@est31``
This commit is contained in:
Matthias Krüger 2024-02-12 23:18:52 +01:00 committed by GitHub
commit dbccecf375
3 changed files with 11 additions and 11 deletions
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6
tests/ui/transmute_ptr_to_ptr.fixed
View file

@ -35,7 +35,7 @@ fn transmute_ptr_to_ptr() {
// ref-ref transmutes; bad
let _: &f32 = &*(&1u32 as *const u32 as *const f32);
//~^ ERROR: transmute from a reference to a reference
let _: &f64 = &*(&1f32 as *const f32 as *const f64);
let _: &f32 = &*(&1f64 as *const f64 as *const f32);
//~^ ERROR: transmute from a reference to a reference
//:^ this test is here because both f32 and f64 are the same TypeVariant, but they are not
// the same type
@ -43,8 +43,8 @@ fn transmute_ptr_to_ptr() {
//~^ ERROR: transmute from a reference to a reference
let _: &GenericParam<f32> = &*(&GenericParam { t: 1u32 } as *const GenericParam<u32> as *const GenericParam<f32>);
//~^ ERROR: transmute from a reference to a reference
let u8_ref: &u8 = &0u8;
let u64_ref: &u64 = unsafe { &*(u8_ref as *const u8 as *const u64) };
let u64_ref: &u64 = &0u64;
let u8_ref: &u8 = unsafe { &*(u64_ref as *const u64 as *const u8) };
//~^ ERROR: transmute from a reference to a reference
}

6
tests/ui/transmute_ptr_to_ptr.rs
View file

@ -35,7 +35,7 @@ fn transmute_ptr_to_ptr() {
// ref-ref transmutes; bad
let _: &f32 = std::mem::transmute(&1u32);
//~^ ERROR: transmute from a reference to a reference
let _: &f64 = std::mem::transmute(&1f32);
let _: &f32 = std::mem::transmute(&1f64);
//~^ ERROR: transmute from a reference to a reference
//:^ this test is here because both f32 and f64 are the same TypeVariant, but they are not
// the same type
@ -43,8 +43,8 @@ fn transmute_ptr_to_ptr() {
//~^ ERROR: transmute from a reference to a reference
let _: &GenericParam<f32> = std::mem::transmute(&GenericParam { t: 1u32 });
//~^ ERROR: transmute from a reference to a reference
let u8_ref: &u8 = &0u8;
let u64_ref: &u64 = unsafe { std::mem::transmute(u8_ref) };
let u64_ref: &u64 = &0u64;
let u8_ref: &u8 = unsafe { std::mem::transmute(u64_ref) };
//~^ ERROR: transmute from a reference to a reference
}

10
tests/ui/transmute_ptr_to_ptr.stderr
View file

@ -22,8 +22,8 @@ LL | let _: &f32 = std::mem::transmute(&1u32);
error: transmute from a reference to a reference
--> $DIR/transmute_ptr_to_ptr.rs:38:23
|
LL | let _: &f64 = std::mem::transmute(&1f32);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*(&1f32 as *const f32 as *const f64)`
LL | let _: &f32 = std::mem::transmute(&1f64);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*(&1f64 as *const f64 as *const f32)`
error: transmute from a reference to a reference
--> $DIR/transmute_ptr_to_ptr.rs:42:27
@ -38,10 +38,10 @@ LL | let _: &GenericParam<f32> = std::mem::transmute(&GenericParam { t:
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*(&GenericParam { t: 1u32 } as *const GenericParam<u32> as *const GenericParam<f32>)`
error: transmute from a reference to a reference
--> $DIR/transmute_ptr_to_ptr.rs:47:38
--> $DIR/transmute_ptr_to_ptr.rs:47:36
|
LL | let u64_ref: &u64 = unsafe { std::mem::transmute(u8_ref) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*(u8_ref as *const u8 as *const u64)`
LL | let u8_ref: &u8 = unsafe { std::mem::transmute(u64_ref) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*(u64_ref as *const u64 as *const u8)`
error: aborting due to 7 previous errors