ex.rs

// ...snip
pub enum ClaimResult<'a> {
  Claimed(&'a SpawnClaim),
  ClaimedAlready(&'a SpawnClaim),
  UnknownSpawn,
}
// ...snip

impl ClaimList {
  // ...snip
  
  fn find_claim(&self, spawn: SpawnItem) -> Option<&SpawnClaim> {
    self.0.iter().find(|claim| claim.spawn.code == spawn.code)
  }

  pub fn claim(
    &mut self,
    spawn_list: &SpawnList,
    spawn_msg: &str,
    message: Message,
  ) -> ClaimResult {
    match spawn_list.find_from_msg(spawn_msg) {
      Some(target_spawn) => {
          if let Some(claim) = self.find_claim(target_spawn.clone()) {
            return ClaimResult::ClaimedAlready(claim)
          }
          else {
            self.0.push(SpawnClaim { message, spawn: target_spawn.clone() });
            //ClaimResult::Claimed(self.0.iter().last().unwrap())
            ClaimResult::UnknownSpawn
          }
      },
      None => ClaimResult::UnknownSpawn
    }
  }
}

// error[E0502]: cannot borrow `self.0` as mutable because `*self` is also borrowed as immutable
//   --> src\spawns.rs:89:13
//    |
// 85 |           if let Some(claim) = self.find_claim(target_spawn.clone()) {
//    |                                ---- immutable borrow occurs here
// ...
// 89 |             self.0.push(SpawnClaim { message, spawn: target_spawn.clone() });
//    |             ^^^^^^ mutable borrow occurs here
// ...
// 96 |   }
//    |   - immutable borrow ends here

ex2.rs

// ...snip
pub enum ClaimResult<'a> {
  Claimed(&'a SpawnClaim),
  ClaimedAlready(&'a SpawnClaim),
  UnknownSpawn,
}
// ...snip

impl ClaimList {
  // ...snip
  
  fn find_claim(&self, spawn: SpawnItem) -> Option<&SpawnClaim> {
    self.0.iter().find(|claim| claim.spawn.code == spawn.code)
  }

pub fn claim(
    &mut self,
    spawn_list: &SpawnList,
    spawn_msg: &str,
    message: Message,
  ) -> ClaimResult {
    match spawn_list.find_from_msg(spawn_msg) {
      Some(target_spawn) => {
          {
            if let Some(claim) = self.find_claim(target_spawn.clone()) {
              return ClaimResult::ClaimedAlready(claim)
            }
          }

          self.0.push(SpawnClaim { message, spawn: target_spawn.clone() });
          //ClaimResult::Claimed(self.0.iter().last().unwrap())
          ClaimResult::UnknownSpawn
      },
      None => ClaimResult::UnknownSpawn
    }
  }
}