cannot infer an appropriate lifetime for borrow expression due to conflicting requirements

Question

pub struct FooStruct<'a> {
  pub bars: Vec<&'a str>,
}

pub trait FooTrait<'a> {
  fn getBars(&self) -> &'a Vec<&'a str>;
}

impl<'a> FooTrait<'a> for FooStruct<'a> {
  fn getBars(&self) -> &'a Vec<&'a str> {
    &self.bars // cannot infer an appropriate lifetime for borrow expression due to conflicting requirements
  }
}

Run it: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=3211c32dd5b9244ff91777f1820ffed5

I do not understand where the requirement conflict comes from. Afaik there is no conflict since everything lives as long as the FooStruct lives.

Answer 1

Let's take it apart:

pub struct FooStruct<'a> {
  pub bars: Vec<&'a str>,
}

FooStruct holds a container containing string slices with lifetime 'a . The container's lifetime corresponds to the lifetime of FooStruct .

pub trait FooTrait<'a> {
  fn getBars(&self) -> &'a Vec<&'a str>;
}

FooTrait wants getBars to return a reference to a container holding string slices with lifetime 'a . The lifetime of the returned reference should be 'a , too.

impl<'a> FooTrait<'a> for FooStruct<'a> {
  fn getBars(&self) -> &'a Vec<&'a str> {
    &self.bars
  }
}

Here, getBars returns a reference to self.bars which is a container of string slices with lifetime 'a . So far, so good.

• However, what is the lifetime of &self.bars ? It corresponds to the lifetime of self (ie the respective FooStruct ).
• What is the lifetime of self ? It is 'self (an implicit lifetime).

However, the FooTrait requires that the returned reference lifetime is 'a , so that does not match FooTrait 's declaration.

One solution is to separate the lifetimes in FooTrait :

pub trait FooTrait<'a> {
  fn getBars<'s>(&'s self) -> &'s Vec<&'a str>;
}

impl<'a> FooTrait<'a> for FooStruct<'a> {
  fn getBars<'s>(&'s self) -> &'s Vec<&'a str> {
    &self.bars
  }
}