E0308 for Unit as Associated Type in a function that returns the Associated Type.

[m-rph]

What I saw:

expected <UnitVariableCollector as Visitor<'tcx>>::Result, found ()rust-analyzer[E0308](https://doc.rust-lang.org/stable/error_codes/E0308.html)

What I expected: Nothing

Why did I expect that: The program compiles. The trait has associated type Result, which is set to unit / ().

rust-analyzer version: rust-analyzer version: 0.4.1904-standalone (f5a9250 2024-03-29)

rustc version: rustc 1.79.0-nightly (1388d7a06 2024-03-20)

editor or extension: VSCode / v0.4.1904 (pre-release)

relevant settings: ?

repository link (if public, optional): https://github.com/rust-lang/rust-clippy/blob/005b05f441b3082e9923e44f9dedcf416b1288e7/clippy_lints/src/unit_types/let_unit_value.rs#L114-L124

code snippet to reproduce:

struct UnitVariableCollector {
    id: HirId,
    spans: Vec<rustc_span::Span>,
}

impl UnitVariableCollector {
    fn new(id: HirId) -> Self {
        Self { id, spans: vec![] }
    }
}

impl<'tcx> Visitor<'tcx> for UnitVariableCollector {
    fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) -> Self::Result {
        if let ExprKind::Path(QPath::Resolved(None, path)) = ex.kind
            && let Res::Local(id) = path.res
            && id == self.id
        {
            self.spans.push(path.span);
        }
        rustc_hir::intravisit::walk_expr(self, ex);
    }
}

[Veykril]

This is due to the use of assoc type defaults here https://github.com/rust-lang/rust/blob/3d5528c287860b918e178a34f04ff903325571b3/compiler/rustc_hir/src/intravisit.rs#L221 which iirc we can't support without the new trait solver.

The diagnostic will be silenced by #16968

[roife]

This issue has been resolved in #16968, so it can be closed now.

[RalfJung]

I see a similar issue that does not involve a defaulted associated type; can I expect that to be the same problem?

expected <Result<{unknown}, InterpErrorInfo<'static>> as Try>::Output, found ()

That's for this code here. Notice the lack of a semicolon in the first arm. (That's an accident but rustc accepts it.)

[Veykril]

#16968 will silence that error as well. Unsure what the problem there for the {unknown} is