rust - Using a borrow as an associated trait type -


this code works:

struct test {     val: string, }  impl test {     fn mut_out(&mut self) -> &string {         self.val = string::from("something");         &self.val     } }

however, more generic implementation not work:

struct test {     val: string, }  trait mutateout {     type out;     fn mut_out(&mut self) -> self::out; }  impl mutateout test {     type out = &string;      fn mut_out(&mut self) -> self::out {         self.val = string::from("something");         &self.val     } }

the compiler cannot infer lifetime string borrow:

error[e0106]: missing lifetime specifier   --> src/main.rs:13:16    | 11 |     type out = &string;    |                ^ expected lifetime parameter

i cannot figure out way explicitly state lifetime of borrow, depends on function itself.

taking inspiration deref trait, can remove reference associated type , instead note in trait want return reference associated type:

trait mutateout {     type out;     fn mut_out(&mut self) -> &self::out; }  impl mutateout test {     type out = string;      fn mut_out(&mut self) -> &self::out {         self.val = string::from("something");         &self.val     } }

here it in playground. given function name mut_out, if mutable reference after, here playground example well.


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