clippy::transmute_undefined_repr: transmute from `main::Foo<u32>` to `main::Foo<i32>`, both of which have an undefined layout

What you see

warning: transmute from `main::Foo<u32>` to `main::Foo<i32>`, both of which have an undefined layout
 --> main.rs:4:18
  |
4 | let _ = unsafe { core::mem::transmute::<Foo<u32>, Foo<i32>>(Foo(0u32, 0u32)) };
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: two instances of the same generic type (`Foo`) may have different layouts
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#transmute_undefined_repr
  = note: `-W clippy::transmute-undefined-repr` implied by `-W clippy::nursery`

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

The results of such a transmute are not defined.

New lints still being finished. Off by default and may have false positives.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

struct Foo<T>(u32, T);
let _ = unsafe { core::mem::transmute::<Foo<u32>, Foo<i32>>(Foo(0u32, 0u32)) };

Write this instead

#[repr(C)]
struct Foo<T>(u32, T);
let _ = unsafe { core::mem::transmute::<Foo<u32>, Foo<i32>>(Foo(0u32, 0u32)) };

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

[lints.clippy]
transmute_undefined_repr = "warn"

#![warn(clippy::transmute_undefined_repr)]

cargo clippy -- -W clippy::transmute-undefined-repr

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.