Lifetime error when creating mutable iterator - rust

Question

I had some problems when I tried to create a custom iterator. The non-mutable version worked fine but when copying the same function to create a mutable version, a lifetime error appeared. This is a simplified version of my problem:
struct Test {
map: HashMap<u32, String>
}
impl Test {
pub fn iter(&self, start: u32, end: u32) -> impl Iterator<Item = &String> {
(start..=end).filter_map(move |i| {
self.map.get(&i)
})
}
pub fn iter_mut(&mut self, start: u32, end: u32) -> impl Iterator<Item = &mut String> {
(start..=end).filter_map(move |i| {
self.map.get_mut(&i)
})
}
}
The iter function works fine but the iter_mut function doesn't compile with this error:
error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
--> src/main.rs:21:22
|
21 | self.map.get_mut(&i)
| ^^^^^^^
|
note: first, the lifetime cannot outlive the lifetime `'_` as defined on the body at 20:34...
--> src/main.rs:20:34
|
20 | (start..=end).filter_map(|i| {
| ^^^
note: ...so that closure can access `self`
--> src/main.rs:21:13
|
21 | self.map.get_mut(&i)
| ^^^^^^^^
note: but, the lifetime must be valid for the anonymous lifetime defined on the method body at 19:21...
--> src/main.rs:19:21
|
19 | pub fn iter_mut(&mut self, start: u32, end: u32) -> impl Iterator<Item = &mut String> {
| ^^^^^^^^^
note: ...so that the types are compatible
--> src/main.rs:19:57
|
19 | pub fn iter_mut(&mut self, start: u32, end: u32) -> impl Iterator<Item = &mut String> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= note: expected `std::option::Option<&mut std::string::String>`
found `std::option::Option<&mut std::string::String>`

Answer 1

As said Todd, compile error on iter_mut likely occurs due to creation many mutable references to the same HashMap, but I'm not sure about that. You can do something like that:
struct Test {
map: HashMap<u32, String>
}
impl Test {
pub fn iter(&self, start: u32, end: u32) -> impl Iterator<Item=&String> {
self.map
.iter()
.filter_map(move |k| {
if (start..=end).contains(k.0) {
Some(k.1)
} else {
None
}
})
}
pub fn iter_mut(&mut self, start: u32, end: u32) -> impl Iterator<Item=&mut String> {
self.map
.iter_mut()
.filter_map(move |k| {
if (start..=end).contains(k.0) {
Some(k.1)
} else {
None
}
})
}
}