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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`` |
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