type annotations needed for if let Some(whatever) = todo!() { not very helpful

[matthiaskrgr]

Code

fn main() {
    if let Some(whatever) = todo!() {
        todo!();
    } else {
        todo!();
    }
}

Current output

error[E0282]: type annotations needed
 --> src/main.rs:2:17
  |
2 |     if let Some(whatever) = todo!() {
  |                 ^^^^^^^^ cannot infer type

For more information about this error, try `rustc --explain E0282`.

It's not really clear how rustc would like to see the type annotations, without adding an extra binding and without type_ascription

What does not work:

if let Some(whatever): Option<i32> = todo!() {
or
if let Some(whatever: i32) = todo!() {

what did end up working:

if let Some(x) = Some(todo!()) {
or
if let Some(x @ ()) = todo!() {

zulip thread: https://rust-lang.zulipchat.com/#narrow/channel/147480-t-compiler.2Fdiagnostics/topic/type.20annotations.20needed.20.60if.20let.20Some.28x.29.20.3D.20todo!.28.29.60/with/521126024

Rust Version

rustc 1.89.0-nightly (6f6971078 2025-05-28)
binary: rustc
commit-hash: 6f69710780d579b180ab38da4c1384d630f7bd31
commit-date: 2025-05-28
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

[jhpratt]

My first thought was to annotate Some, which turns out to work.

fn main() {
    if let Some::<()>(whatever) = todo!() {
        todo!();
    } else {
        todo!();
    }
}

[matthiaskrgr]

mmh maybe it makes sense to make the diagnostic span the entire Some(...) ? 🤔

[jhpratt]

I think so. It is the Option that needs the annotating, technically.

[fmease]

If I'm reading the messages of both of you correctly, you seem to be interpreting the type annotations needed plus the primary highlight as "type annotations needed exactly where the highlight resides" (apologies if that's not the case).

Whereas I'm quite certain that this diagnostic simply points to the location that's in some way associated with a type (type variable) that couldn't be inferred (solved).

And I go as far as saying that that's how you should interpret it. For illustrative/pedagogical purposes only, this is how I read this diagnostic:

error[E0282]: cannot infer a type at this place
 --> src/main.rs:2:17
  |
2 |     if let Some(whatever) = todo!() {
  |                 ^^^^^^^^ cannot infer a type at this place
  |
  = help: consider adding a type annotation somewhere in the vicinity
          where "type annotation" not only includes `: $ty` but
          also `as` casts, generic argument lists or anything really
          that can narrow down the types in some way

---

Don't get me wrong,

1. it would be nice if we could suggest consider specifying a type here: Some::</* Type */>(whatever) here.
2. I think the phrasing of the primary message and of the label could be made a lot better / clearer assuming that I guessed correctly about the misunderstanding it may cause. I'm pretty used to it, so it's hard for me to see and reevaluate it from an outsider's perspective (I'm not implying that you are outsiders lol, you certainly aren't)