Lint `trivial_casts`: A FnDef->FnPtr cast gets incorrectly flagged as trivial

Code

EDIT: please see https://github.com/rust-lang/rust/issues/161339#issuecomment-5348482270 for the full repro that actually breaks.

// Needed due to the lack of https://github.com/rust-lang/rust/issues/63063
pub(crate) struct MyIterator {
    inner: std::iter::Map<std::iter::Copied<core::slice::Iter<'static, bool>>, fn(bool) -> usize>,
}

impl MyIterator {
    fn new() -> Self {
        Self {
            inner:
                [false].as_slice().into_iter().copied().map(Into::into as fn(bool) -> usize)
        }
    }
}

Current output

warning: trivial cast: `fn(bool) -> usize {<bool as Into<usize>>::into}` as `fn(bool) -> usize`
 --> <source>:9:61
  |
9 |                 [false].as_slice().into_iter().copied().map(Into::into as fn(bool) -> usize)
  |                                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: cast can be replaced by coercion; this might require a temporary variable
  = note: requested on the command line with `-W trivial-casts`

warning: struct `MyIterator` is never constructed
 --> <source>:1:19
  |
1 | pub(crate) struct MyIterator {
  |                   ^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: associated function `new` is never used
 --> <source>:6:8
  |
5 | impl MyIterator {
  | --------------- associated function in this implementation
6 |     fn new() -> Self {
  |        ^^^

warning: 3 warnings emitted

Compiler returned: 0

Desired output

`no-diagnostic`

Rationale and extra context

If as fn(bool) -> usize is dropped, this no longer compiles.

This -w trivial-casts diag seems like a false-positive here,
i'm not quite sure how it can be easily avoided.

Other cases

Rust Version

rustc 1.100.0-nightly (8fa1c96cf 2026-08-17)
binary: rustc
commit-hash: 8fa1c96cfd489e4c27654c144ae871ce2c4db6c6
commit-date: 2026-08-17
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.0

Anything else?

No response

Start by reproducing the reported warning with the Rust code in the issue and inspect the compiler's trivial_casts lint implementation. Verify the behavior around the FnDef-to-FnPtr cast, then add coverage for this example and confirm the lint produces no diagnostic while the code still compiles.