Lots of incorrect useless_conversion warnings when libc is involved

I tried this code:

    let cmsg_len = match usize::try_from(hdr.cmsg_len) {
        Ok(l) => l,
        _ => return einval(),
    };

I expected to see this happen: Nothing, as cmsg_len is u32 on many platforms.

Instead, this happened:

warning: useless conversion to the same type: `usize`
  --> uapi/src/socket/cmsg.rs:71:26
   |
71 |     let cmsg_len = match usize::try_from(hdr.cmsg_len) {
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: consider removing `usize::try_from()`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion

Meta

• cargo clippy -V: clippy 0.0.212 (7eac88a 2020-11-16)
• rustc -Vv:

   rustc 1.48.0 (7eac88abb 2020-11-16)
   binary: rustc
   commit-hash: 7eac88abb2e57e752f3302f02be5f3ce3d7adfb4
   commit-date: 2020-11-16
   host: x86_64-unknown-linux-gnu
   release: 1.48.0
   LLVM version: 11.0
  

Start with the reported example in uapi/src/socket/cmsg.rs at line 71 and reproduce the useless_conversion warning with the listed Rust and Clippy versions. Trace how the lint handles libc-dependent types across platforms; done means the warning is no longer emitted when the conversion is necessary while remaining for genuinely useless conversions.