wrong function name in which-function-mode

I enable which-function-mode. I noticed a bug that I reduced to this:

fn subroutine_type_metadata<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
                                      unique_type_id: UniqueTypeId,
                                      signature: ty::PolyFnSig<'tcx>,
                                      span: Span)
                                      -> MetadataCreationResult
{
    for &argument_type in signature.inputs() {
        signature_metadata.push(type_metadata(cx, argument_type, span));
    }
}

Put point on or just before the closing brace of the function. I see the function name reported as in. This happens on any line after the for.

Start by reproducing the report in which-function-mode with the provided Rust snippet, placing point near the closing brace and after the for loop. Trace how the mode determines the current function name and verify that it reports subroutine_type_metadata rather than in at those positions.