dead_code false positive introduced in rustc 1.78.0-nightly (8ace7ea1f 2024-02-07)

[tamird]

#![deny(warnings)]

#[repr(u8)]
#[derive(Copy, Clone, Debug)]
pub enum RecordField {
    Target = 1,
    Level,
    Module,
    File,
    Line,
    NumArgs,
}

unsafe trait Pod {}

#[repr(transparent)]
struct RecordFieldWrapper(RecordField);

unsafe impl Pod for RecordFieldWrapper {}

fn try_read<T: Pod>(buf: &[u8]) -> T {
    unsafe { std::ptr::read_unaligned(buf.as_ptr() as *const T) }
}

pub fn foo(buf: &[u8]) -> RecordField {
    let RecordFieldWrapper(tag) = try_read(buf);
    tag
}

This code compiles on stable (and nightlies before 2024-02-07) but now produces:

error: struct `RecordFieldWrapper` is never constructed
  --> aya-log/src/lib.rs:79:8
   |
79 | struct RecordFieldWrapper(RecordField);
   |        ^^^^^^^^^^^^^^^^^^
   |
   = note: `RecordFieldWrapper` has a derived impl for the trait `Clone`, but this is intentionally ignored during dead code analysis
   = note: `-D dead-code` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(dead_code)]`

NOTE: playground's nightly is not new enough
playground

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high

[apiraino]

searched nightlies: from nightly-2024-02-01 to nightly-2024-02-08
regressed nightly: nightly-2024-02-08
searched commit range: 256b6fb...8ace7ea
regressed commit: d4f6f9e

bisected with cargo-bisect-rustc v0.6.7

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start=2024-02-01 --regress error --script script.sh 

cc @mu001999 @cjgillot

[Noratrieb]

How would you expect the compiler to figure out that it is constructed? Just because there's a value of that type somewhere doesn't mean it is, that would make the lint basically useless.