New lint: suggest ptr::read instead of mem::replace(..., uninitialized())

[RalfJung]

I have now at least twice seen code like this:

mem::replace(x, mem::uninitialized());

Most recently here, but I saw this before somewhere (not sure where).

Could Clippy suggest to replace this code by the following:

ptr::read(x)

The latter is more clear, avoids the mess that is uninitialized memory (both of the cases where I saw this were actually UB because of this), and can never be wrong when the former is right. Indeed, LLVM will likely (but not always) compile the former to the latter, because "write of uninitialized data to memory" can be just optimized away to nothing.

[esamudera]

Hello, I'm interested in working on this issue, if this is still available.

All I need to do should be just:

• Add mem::replace(x, mem::uninitialized()); test case to tests/ui/mem_replace.rs
• Implement lint in clippy_lints/src/mem_replace.rs:
  • detect mem::replace fn call with mem::unititialized() param
  • suggest replacing it with ptr::read(x)

Right? If yes, I can start working on this 👍

[RalfJung]

Yes that sounds good. :-)
For bonus points, also detect MaybeUninit::uninit().assume_init() as an equivalent to mem::uninitialized().

[esamudera]

Okay, adding that to the test case 👍