Tuple parameters are {unknown}

[Barugon]

rust-analyzer version: rust-analyzer version: 7e95c14 2022-05-17 stable

rustc version: rustc 1.61.0 (fe5b13d68 2022-05-18)

relevant settings: none

Rust Analyzer gets confused about the tuple parameters in the following code (the logic here is dumb but it's simply to illustrate):

fn main() {
    let mut item = None;
    loop {
        if let Some((id, _, _)) = item {
            if id == 1 {
                break;
            }
        }

        item = Some((1, 2.5, String::new()));
    }
}

The result is this:

[ruabmbua]

I started reducing the example to find out what is happening.

I arrived at this, and I think I have found a clue what is going on:

fn main() {
    enum Option<T> {Some(T), None}
    use Option::*;

    let mut item = None;
   // Infers to Option<(i32, {unknown})>

    if let Some((_a, _)) = item {
    }

    item = Some((1, 2));
}

// Other example:
fn main() {
    enum Option<T> {Some(T), None}
    use Option::*;

    let mut item = None;
   // Infers to Option<({unknown}, i32)>

    if let Some((_, _a)) = item {
    }

    item = Some((1, 2));
}

So it has nothing to do with the loop, its just about a pattern match appearing before the first (fully type-resolvable) assignment. It only happens, when tuples are involved (according to my few tests), and when the pattern matching uses the _ placeholder anywhere. If the pattern matching actually binds the value to an identifier like _a there is no problem.

[ruabmbua]

So somehow the _ in a pattern matching stops the inference from working on that particular part of the type.

[Barugon]

So it has nothing to do with the loop, its just about a pattern match appearing before the first (fully type-resolvable) assignment. It only happens, when tuples are involved (according to my few tests), and when the pattern matching uses the _ placeholder anywhere. If the pattern matching actually binds the value to an identifier like _a there is no problem.

Right. If I do this then it works.

fn main() {
    let mut item = None;
    loop {
        item = Some((1, 2.5, String::new()));

        if let Some((id, _, _)) = item {
            if id == 1 {
                break;
            }
        }
    }
}

[flodiebold]

Good catch. I suspect changing

rust-analyzer/crates/hir-ty/src/infer/pat.rs

Line 145 in 84be2ea

expectations.iter().map(|a| a.assert_ty_ref(Interner)).chain(repeat(&err_ty));

to use repeat_with(|| self.table.new_type_var()) might be the fix, but I don't have time to test it right now.

[ruabmbua]

Tested again the original mentioned code, fixed by #12355