Non-Deterministic Lifetime Error Message

[maxsnew]

I'm getting this on Fedora 20 with rustc 0.12.0-nightly (b5ba2f5 2014-10-06 20:27:14 +0000).

The following program:

extern crate graphviz;

use graphviz as dot;
use std::str;

type Nd = uint;
type Ed = (uint, uint);
struct Graph<'a> {
  nodes: Vec<&'a str>,
  edges: Vec<Ed>,
}

impl<'a> dot::Labeller<'a, Nd, Ed> for Graph<'a> {
  fn graph_id(&self) -> dot::Id<'a> {
    dot::Id::new("example3")
  }
  fn node_id(&self, n: &Nd) -> dot::Id {
    dot::Id::new(format!("N{:u}", *n))
  }
  fn node_label<'a>(&self, i: &Nd) -> dot::LabelText<'a> {
    dot::LabelStr(str::Slice(self.nodes[*i].as_slice()))
  }
}

When compiled with rustc --crate-type=lib non-deterministically gives one of 2 error messages.
A useful one:

lifetime-error.rs:21:5: 23:6 note: consider using an explicit lifetime parameter as shown: fn node_label<'a>(&'a self, i: &Nd) -> dot::LabelText<'a>

and a buggy one, thats suggested fix is what's already written in the program:

lifetime-error.rs:21:5: 23:6 note: consider using an explicit lifetime parameter as shown: fn node_label<'a>(&self, i: &Nd) -> dot::LabelText<'a>

On my Mac, I always get the worse error message.

I think this is the same issue described here: #13057

[tamird]

This suggestion has actually gotten worse. I now consistently get a nonsense message:

#![feature(rustc_private)]

extern crate graphviz;

use std::borrow::IntoCow;
use graphviz as dot;

type Nd = usize;
type Ed = (usize, usize);
struct Graph<'a> {
    nodes: Vec<&'a str>,
    edges: Vec<Ed>,
}

impl<'a> dot::Labeller<'a, Nd, Ed> for Graph<'a> {
    fn graph_id(&self) -> dot::Id {
        dot::Id::new("example2").unwrap()
    }

    fn node_id(&self, n: &Nd) -> dot::Id {
        dot::Id::new(format!("N{}", n)).unwrap()
    }

    fn node_label<'b>(&self, n: &Nd) -> dot::LabelText<'b> {
        dot::LabelText::LabelStr(self.nodes[*n].into_cow())
        // error: cannot infer an appropriate lifetime for autoref due to conflicting requirements
        // help: consider using an explicit lifetime parameter as shown: fn node_label(&self, n: &Nd) -> dot::LabelText<'a>
    }
}

fn main() { }

...the compiler is suggesting to use a different lifetime parameter, not just an explicit one.

[steveklabnik]

These examples do not compile for other reasons, but in the 18 months since it was filed, we did some updates on shadowing of lifetimes. You now get:

src/lib.rs:20:19: 20:21 error: lifetime name `'a` shadows a lifetime name that is already in scope [E0496]
src/lib.rs:20     fn node_label<'a>(&self, i: &Nd) -> dot::LabelText<'a> {
                                ^~
src/lib.rs:20:19: 20:21 help: run `rustc --explain E0496` to see a detailed explanation
src/lib.rs:13:6: 13:8 note: shadowed lifetime `'a` declared here
src/lib.rs:13 impl<'a> dot::Labeller<'a, Nd, Ed> for Graph<'a> {
                   ^~

Much better. As such I'm going to give it a close!