OccupiedEntry::get returns references bound to the lifetime of the Entry, not the HashMap

[khuey]

Consider the following toy code to implement a simple HashMap-based cache.

struct Table {
    table: HashMap<String, Option<u32>>,
}

impl Table {
    fn lookup(&mut self, s: String) -> Result<&u32> {
        match self.table.entry(s) {
            Entry::Occupied(o) => {
                o.get().as_ref().ok_or_else(|| panic!())
            },
            Entry::Vacant(v) => {
                // TODO: Calculate a value.
                panic!();
            }
        }
    }
}

The borrow checker will fail this.

rustc 1.14.0 (e8a0123 2016-12-16)
error: o does not live long enough
--> :13:17
|
13 | o.get().as_ref().ok_or_else(|| panic!())
| ^ does not live long enough
...
19 | }
| - borrowed value only lives until here
|
note: borrowed value must be valid for the anonymous lifetime #1 defined on the block at 10:52...
--> :10:53
|
10 | fn lookup(&mut self, s: String) -> Result<&u32> {
| ^

It appears that OccupiedEntry::get is returning a reference to the value in the HashMap with the lifetime of the Entry, not the HashMap. This seems like a bug to me.

[khuey]

talchas on IRC introduced me to into_mut