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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: Surprising type inference error

I have following minimal reproducible example (see also on playground):

pub struct Bar {
    value: u32,
}

pub struct Foo {
    pub bars: Vec<Bar>,
}

impl Foo {
    pub fn new(bar: Bar) -> Self {
        let mut bars = vec![];

        if let Some(_) = bars.iter().find(|&item| item.value == bar.value) {
            unreachable!();
        }

        bars.push(bar);

        Self { bars }
    }
}

When running cargo check this fails to compile with following error:

    Checking type-annotation-mre v0.1.0 (/home/akrauze/scratch/type-annotation-mre)
error[E0282]: type annotations needed for `Vec<_>`
  --> src/lib.rs:11:13
   |
11 |         let mut bars = vec![];
   |             ^^^^^^^^
12 |
13 |         if let Some(_) = bars.iter().find(|&item| item.value == bar.value) {
   |                                                   ---------- type must be known at this point
   |
help: consider giving `bars` an explicit type, where the type for type parameter `T` is specified
   |
11 |         let mut bars: Vec<_> = vec![];
   |                     ++++++++

For more information about this error, try `rustc --explain E0282`.
error: could not compile `type-annotation-mre` (lib) due to 1 previous error

I find this very odd, because:

• it should be possible for the compiler to infer type or bars thanks to bars.push(bar) call
• rust-analyzer shows correct types inline

I've tried running this with a couple of rust versions and editions, but the only things that changed were wording of error message.

Is this known limitation of the inference system in the compiler? I don't recall encountering anything similar in the past. If so, is it technically solvable (I suspect it is since rust-analyzer can infer types properly) and is it planned in the future?

Or is it some regression/bug in the compiler that should be reported?

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