I'm looking for a little borrow-checker advice. Here's a reasonably
minimal program that demonstrates the problem:
extern crate collections;
use collections::HashMap;
struct G {
verts: HashMap<int, String>,
edges: Vec<(int, int)>,
next_vert_id: int
}
impl G {
fn new() -> G {
G{verts: HashMap::new(), edges: Vec::new(), next_vert_id: 0}
}
fn add_vert(&mut self, s: &str) -> int {
let id = self.next_vert_id;
self.next_vert_id += 1;
self.verts.insert(id, String::from_str(s));
id
}
fn add_edge(&mut self, v0: int, v1: int) {
self.edges.push((v0, v1))
}
}
fn main() {
let mut g = G::new();
{
let v0 = g.add_vert("vert 0");
let v1 = g.add_vert("vert 1");
g.add_edge(v0, v1);
}
for &(v0, v1) in g.edges.iter() {
g.add_vert("edge vert");
}
}
This fails to compile:
$ rust-nightly-x86_64-unknown-linux-gnu/bin/rustc -v
rustc 0.11.0-pre-nightly (064dbb9 2014-06-01 00:56:42 -0700)
host: x86_64-unknown-linux-gnu
$ rust-nightly-x86_64-unknown-linux-gnu/bin/rustc graph.rs
graph.rs:39:9: 39:10 error: cannot borrow `g` as mutable because
`g.edges` is also borrowed as immutable
graph.rs:39 g.add_vert("edge vert");
^
graph.rs:38:22: 38:29 note: previous borrow of `g.edges` occurs here;
the immutable borrow prevents subsequent moves or mutable borrows of
`g.edges` until the borrow ends
graph.rs:38 for &(v0, v1) in g.edges.iter() {
^~~~~~~
graph.rs:41:2: 41:2 note: previous borrow ends here
graph.rs:38 for &(v0, v1) in g.edges.iter() {
graph.rs:39 g.add_vert("edge vert");
graph.rs:40 }
graph.rs:41 }
^
error: aborting due to previous error
My understanding of the error is: G::add_vert is being given a mutable
reference to "g", which means it could do something naughty like clear
g.edges, which would screw up the loop iteration that is happening in
main().
That seems like a pretty reasonable thing to prevent, but it's not
clear to me how I should restructure the program to work around the
error. In this minimal example I could copy the code out of
G::add_vert and stick it directly inside the loop, but that's clearly
not the general solution.
Thanks,
-Nicholas
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