`clippy::option_if_let_else` suggests invalid code fix

[u9g]

Summary

The code produced by clippy is in theory correct in my eyes, but in reality the type checker doesn't understand it.

Reproducer

I tried this code:

if let Some(id) = &v.as_class().unwrap().id {
    Box::new(std::iter::once(Vertex::Span(id.span)))
} else {
    Box::new(std::iter::empty())
}

I expected to see this happen: to get working code after a quick fix.
Instead, this happened: The quick fix produces the following non-working code:

v.as_class().unwrap().id.as_ref().map_or_else(
    || Box::new(std::iter::empty()),
    |id| Box::new(std::iter::once(Vertex::Span(id.span))),
)

Which is invalid because:

error[E0308]: mismatched types
   --> crates\oxc_linter\src\lint_adapter2.rs:217:35
    |
217 |                     |id| Box::new(std::iter::once(Vertex::Span(id.span))),
    |                          -------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Empty<_>`, found `Once<Vertex<'_>>`
    |                          |
    |                          arguments to this function are incorrect
    |
    = note: expected struct `std::iter::Empty<_>`
               found struct `std::iter::Once<lint_adapter2::Vertex<'_>>`
note: associated function defined here
   --> C:\Users\jason\.rustup\toolchains\nightly-2023-06-02-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\alloc\src\boxed.rs:215:12
    |
215 |     pub fn new(x: T) -> Self {
    |            ^^^

Version

rustc 1.72.0-nightly (d59363ad0 2023-06-01)
binary: rustc
commit-hash: d59363ad0b6391b7fc5bbb02c9ccf9300eef3753
commit-date: 2023-06-01
host: x86_64-pc-windows-msvc
release: 1.72.0-nightly
LLVM version: 16.0.4

Additional Labels

@rustbot label +I-suggestion-causes-error

[zaneduffield]

Hi @u9g, I think your original code doesn't type-check either because the if and else branches have incompatible types

  | |_____- `if` and `else` have incompatible types
  |
  = note: expected struct `std::boxed::Box<std::iter::Once<&_>>`
             found struct `std::boxed::Box<std::iter::Empty<_>>`

[y21]

@zaneduffield it depends in which position this piece of code is. This for example does pass typeck, because the two types are coerced into trait objects because of the return type:

use std::fmt::Debug;

fn foo(o: Option<i32>) -> Box<dyn Debug> {
  if let Some(o) = o {
    Box::new(o)
  } else {
    Box::new("foo")
  }
}

But this coercion doesn't happen implicitly in Option::map_or_else and would need an explicit as-cast to Box<dyn Debug>

[zaneduffield]

@y21 good point. I'd argue that perhaps it would be better to skip the lint in such cases, because a suggestion with the map_or_else and casts seems overly complicated.