How can I explicitly specify a lifetime when implementing a trait? - rust

Question

Given the implementation below, where essentially I have some collection of items that can be looked up via either a i32 id field or a string field. To be able to use either interchangeably, a trait "IntoKey" is used, and a match dispatches to the appropriate lookup map; this all works fine for my definition of get within the MapCollection impl:
use std::collections::HashMap;
use std::ops::Index;
enum Key<'a> {
I32Key(&'a i32),
StringKey(&'a String),
}
trait IntoKey<'a> {
fn into_key(&'a self) -> Key<'a>;
}
impl<'a> IntoKey<'a> for i32 {
fn into_key(&'a self) -> Key<'a> { Key::I32Key(self) }
}
impl<'a> IntoKey<'a> for String {
fn into_key(&'a self) -> Key<'a> { Key::StringKey(self) }
}
#[derive(Debug)]
struct Bar {
i: i32,
n: String,
}
struct MapCollection
{
items: Vec<Bar>,
id_map: HashMap<i32, usize>,
name_map: HashMap<String, usize>,
}
impl MapCollection {
fn new(items: Vec<Bar>) -> MapCollection {
let mut is = HashMap::new();
let mut ns = HashMap::new();
for (idx, item) in items.iter().enumerate() {
is.insert(item.i, idx);
ns.insert(item.n.clone(), idx);
}
MapCollection {
items: items,
id_map: is,
name_map: ns,
}
}
fn get<'a, K>(&self, key: &'a K) -> Option<&Bar>
where K: IntoKey<'a> //'
{
match key.into_key() {
Key::I32Key(i) => self.id_map.get(i).and_then(|idx| self.items.get(*idx)),
Key::StringKey(s) => self.name_map.get(s).and_then(|idx| self.items.get(*idx)),
}
}
}
fn main() {
let bars = vec![Bar { i:1, n:"foo".to_string() }, Bar { i:2, n:"far".to_string() }];
let map = MapCollection::new(bars);
if let Some(bar) = map.get(&1) {
println!("{:?}", bar);
}
if map.get(&3).is_none() {
println!("no item numbered 3");
}
if let Some(bar) = map.get(&"far".to_string()) {
println!("{:?}", bar);
}
if map.get(&"baz".to_string()).is_none() {
println!("no item named baz");
}
}
However, if I then want to implement std::ops::Index for this struct, if I attempt to do the below:
impl<'a, K> Index<K> for MapCollection
where K: IntoKey<'a> {
type Output = Bar;
fn index<'b>(&'b self, k: &K) -> &'b Bar {
self.get(k).expect("no element")
}
}
I hit a compiler error:
src/main.rs:70:18: 70:19 error: cannot infer an appropriate lifetime for automatic coercion due to conflicting requirements
src/main.rs:70 self.get(k).expect("no element")
^
src/main.rs:69:5: 71:6 help: consider using an explicit lifetime parameter as shown: fn index<'b>(&'b self, k: &'a K) -> &'b Bar
src/main.rs:69 fn index<'b>(&'b self, k: &K) -> &'b Bar {
src/main.rs:70 self.get(k).expect("no element")
src/main.rs:71 }
I can find no way to specify a distinct lifetime here; following the compiler's recommendation is not permitted as it changes the function signature and no longer matches the trait, and anything else I try fails to satisfy the lifetime specification.
I understand that I can implement the trait for each case (i32, String) separately instead of trying to implement it once for IntoKey, but I am more generally trying to understand lifetimes and appropriate usage. Essentially:
Is there actually an issue the compiler is preventing? Is there something unsound about this approach?
Am I specifying my lifetimes incorrectly? To me, the lifetime 'a in Key/IntoKey is dictating that the reference need only live long enough to do the lookup; the lifetime 'b associated with the index fn is stating that the reference resulting from the lookup will live as long as the containing MapCollection.
Or am I simply not utilizing the correct syntax to specify the needed information?
(using rustc 1.0.0-nightly (b63cee4a1 2015-02-14 17:01:11 +0000))

Answer 1

Do you intend on implementing IntoKey on struct's that are going to store references of lifetime 'a? If not, you can change your trait and its implementations to:
trait IntoKey {
fn into_key<'a>(&'a self) -> Key<'a>;
}
This is the generally recommended definition style, if you can use it. If you can't...
Let's look at this smaller reproduction:
use std::collections::HashMap;
use std::ops::Index;
struct Key<'a>(&'a u8);
trait IntoKey<'a> { //'
fn into_key(&'a self) -> Key<'a>;
}
struct MapCollection;
impl MapCollection {
fn get<'a, K>(&self, key: &'a K) -> &u8
where K: IntoKey<'a> //'
{
unimplemented!()
}
}
impl<'a, K> Index<K> for MapCollection //'
where K: IntoKey<'a> //'
{
type Output = u8;
fn index<'b>(&'b self, k: &K) -> &'b u8 { //'
self.get(k)
}
}
fn main() {
}
The problem lies in get:
fn get<'a, K>(&self, key: &'a K) -> &u8
where K: IntoKey<'a>
Here, we are taking a reference to K that must live as long as the Key we get out of it. However, the Index trait doesn't guarantee that:
fn index<'b>(&'b self, k: &K) -> &'b u8
You can fix this by simply giving a fresh lifetime to key:
fn get<'a, 'b, K>(&self, key: &'b K) -> &u8
where K: IntoKey<'a>
Or more succinctly:
fn get<'a, K>(&self, key: &K) -> &u8
where K: IntoKey<'a>