MIR borrow checker panics on a field projection from a slice

rust-analyzer version: rust-analyzer 1.99.0-nightly (af3d9558 2026-07-09)

rustc version: rustc 1.99.0-nightly (af3d95584 2026-07-09)

editor or extension: VsCode extension. rust-analyzer v0.3.2963. Observed through LSP textDocument/diagnostic and textDocument/codeAction requests. The behavior should not specific to an editor.

relevant settings: N/A

repository link (if public, optional): Overseer

code snippet to reproduce:

#[derive(Clone, Copy)]
enum Row {
    A { x: usize },
    B { x: usize, y: usize, z: bool, w: usize },
}

struct S {
    rows: Vec<Row>,
}

impl S {
    fn f(&mut self, idx: Option<usize>) -> bool {
        let rows = &self.rows;
        let Some(row) = idx.and_then(|i| rows.get(i)).copied() else {
            return false;
        };

        match row {
            Row::B { y, .. } => {
                let _ = y;
                false
            }
            Row::A { x } => {
                let _ = x;
                false
            }
        }
    }
}

The reduced standalone source does not preserve every aspect of the original inference context. The panic itself can be reproduced directly in the hir-ty test harness by applying a MIR Field projection to a slice-typed PlaceTy.

The panic is:

​text thread 'Worker3' panicked at crates/hir-ty/src/mir.rs:1277:22: can't project out of [Row] ​

borrowck_query reaches PlaceTy::field_ty with a ProjectionElem::Field, no downcast variant, and a slice base type. field_ty only accepts non-enum ADTs, closures, and tuples in this case.

A field projection does not contain the element index needed to project through an array or slice, so it cannot derive a meaningful element-field type. This malformed MIR should not crash diagnostic or code-action requests. Propagating an error type allows MIR diagnostic recovery to stop safely.

This is rust-analyzer's MIR-based diagnostic borrow checker, not rustc's borrow checker, so the panic does not prevent cargo build.

Start in crates/hir-ty/src/mir.rs at PlaceTy::field_ty and inspect how borrowck_query handles ProjectionElem::Field on slice-typed places. Use the MIR test harness to reproduce the slice Field projection, then exercise the diagnostic and code-action paths. Done means the malformed projection recovers without a panic and the existing valid projection cases still pass.