1
#![feature(rustc_private)]
extern crate rustc;
use rustc::hir::intravisit as hir_visit;
use rustc::hir;
use std::marker::PhantomData;

// ----------------------------------------------------------------------------
// Why does this compile?
// ----------------------------------------------------------------------------
pub struct Map<'a> {
    pub _m: PhantomData<&'a ()>,
}
pub struct SomeVisitor<'a, 'tcx: 'a> {
    pub map: &'a Map<'tcx>,
}
pub enum NestedVisitorMap<'this, 'tcx: 'this> {
    None,
    OnlyBodies(&'this Map<'tcx>),
    All(&'this Map<'tcx>),
}
pub trait Visitor<'v>: Sized {
    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v>;
}
impl<'v, 'tcx> Visitor<'v> for SomeVisitor<'v, 'tcx> {
    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
        NestedVisitorMap::All(self.map)
    }
}
// ----------------------------------------------------------------------------
// Why does this *not* compile?
// ----------------------------------------------------------------------------
pub struct SomeVisitor2<'a, 'tcx: 'a> {
    pub map: &'a hir::map::Map<'tcx>,
}
impl<'v, 'tcx> hir_visit::Visitor<'v> for SomeVisitor2<'v, 'tcx> {
    fn nested_visit_map<'this>(&'this mut self) -> hir_visit::NestedVisitorMap<'this, 'v> {
        hir_visit::NestedVisitorMap::All(self.map)
    }
}
fn main() {}

playground

NestedVisitorMap and Visitor

I recently ran into a lifetime issue and I decided to recreate it without any dependencies. The odd thing is that I can not recreate the lifetime error. To me both implementations look the same from the outside, but only one compiles successfully. Why is that?

rustc 1.21.0-nightly (e26688824 2017-08-27)

Update:

The problem seems to be the RefCell inside Map.

#[derive(Clone)]
pub struct Map<'hir> {
    inlined_bodies: RefCell<rustc::util::nodemap::DefIdMap<&'hir hir::Body>>,
}

If there is a RefCell with a inner lifetime, it will trigger an error.

playground

Update2:

It turns out that I just mixed up lifetime subtyping. playground

I still don't know why only RefCell causes the error.

Maik Klein
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    I don't know enough to answer. I will say in the example replacing your `Map` struct with `type Map<'a> = hir::map::Map<'a>;` shows the same error, so it seems somehow your struct and the core one are different? – loganfsmyth Aug 29 '17 at 00:16
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    It is likely because `RefCell` forces `'hir` to be invariant. https://stackoverflow.com/q/43210387/ – kennytm Aug 29 '17 at 07:13

0 Answers0