CODEWITHSUNDEEP
rustlifetimeborrow
Adrian Willenbücher
Adrian Willenbücher

Reputation: 985

Return mapped data from parking_lot::RwLock

My struct has a vector inside a parking_lot::RwLock, and one member function has to return a guarded element from that vector:

use parking_lot::*;

struct S {
    v: RwLock<Vec<String>>,
}

impl S {
    fn f(&self, i: usize) -> MappedRwLockReadGuard<'_, Option<&String>> {
        RwLockReadGuard::map(self.v.read(), |unlocked| &unlocked.get(i))
    }
}

(this is the simplified core of the problem, not the real code)

The code passes type checks, but I get lifetime errors for f. Is there a way to modify the code so that it is sound and passes the borrow checker?

The error is:

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in function call due to conflicting requirements
  --> src/lib.rs:9:66
   |
9  |         RwLockReadGuard::map(self.v.read(), |unlocked| &unlocked.get(i))
   |                                                                  ^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the body at 9:45...
  --> src/lib.rs:9:45
   |
9  |         RwLockReadGuard::map(self.v.read(), |unlocked| &unlocked.get(i))
   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...so that reference does not outlive borrowed content
  --> src/lib.rs:9:57
   |
9  |         RwLockReadGuard::map(self.v.read(), |unlocked| &unlocked.get(i))
   |                                                         ^^^^^^^^
note: but, the lifetime must be valid for the anonymous lifetime #1 defined on the method body at 8:5...
  --> src/lib.rs:8:5
   |
8  | /     fn f(&self, i: usize) -> MappedRwLockReadGuard<'_, Option<&String>> {
9  | |         RwLockReadGuard::map(self.v.read(), |unlocked| &unlocked.get(i))
10 | |     }
   | |_____^
note: ...so that the expression is assignable
  --> src/lib.rs:9:9
   |
9  |         RwLockReadGuard::map(self.v.read(), |unlocked| &unlocked.get(i))
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: expected  `lock_api::rwlock::MappedRwLockReadGuard<'_, _, std::option::Option<&std::string::String>>`
              found  `lock_api::rwlock::MappedRwLockReadGuard<'_, _, std::option::Option<&std::string::String>>`

Upvotes: 1

Views: 808

Answers (1)

Adrian Willenb&#252;cher
Adrian Willenb&#252;cher

Reputation: 985

The solution is to use RwLockReadGuard::try_map() and to change the return type from MappedRwLockReadGuard<'_, Option<&_>> to Option<MappedRwLockReadGuard<'a, _>>:

impl S {
    fn f(&self, i: usize) -> Option<MappedRwLockReadGuard<'_, String>> {
        RwLockReadGuard::try_map(self.v.read(), |unlocked| unlocked.get(i)).ok()
    }
}

Upvotes: 0

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