redundant_reference

redundant_reference

What it does

Checks for fields that are references used only to read one copyable subfield, and whose lifetimes are not used elsewhere.

Why is this bad?

Storing the reference instead of a copy of the subfield adds an unnecessary lifetime parameter to the struct. It also creates an unnecessary pointer dereference at runtime.

Example

struct V<'cx, 'tcx> {
    cx: &'cx LateContext<'tcx>,
}

impl<'cx, 'tcx> Visitor<'tcx> for V<'cx, 'tcx> {
    type Map = rustc_middle::hir::map::Map<'tcx>;
    type NestedFilter = rustc_middle::hir::nested_filter::All;

    fn nested_visit_map(&mut self) -> Self::Map {
        self.cx.tcx.hir()
    }
}

Use instead:

struct V<'tcx> {
    tcx: TyCtxt<'tcx>,
}

impl<'tcx> Visitor<'tcx> for V<'tcx> {
    type Map = rustc_middle::hir::map::Map<'tcx>;
    type NestedFilter = rustc_middle::hir::nested_filter::All;

    fn nested_visit_map(&mut self) -> Self::Map {
        self.tcx.hir()
    }
}

Configuration

• lifetime_check: bool (default true): Setting this to false disables the check that the lifetime use is unique. That is, the lint becomes a check for: fields that are references used only to read one copyable subfield.