Add LocalKey::try_with as an alternative to state

src/libstd/thread/local.rs

@@ -232,6 +232,32 @@ pub enum LocalKeyState {
     Destroyed,
 }
 
+/// An error returned by [`LocalKey::try_with`](struct.LocalKey.html#method.try_with).
+#[unstable(feature = "thread_local_state",
+           reason = "state querying was recently added",
+           issue = "27716")]
+pub struct AccessError {
+    _private: (),
+}
+
+#[unstable(feature = "thread_local_state",
+           reason = "state querying was recently added",
+           issue = "27716")]
+impl fmt::Debug for AccessError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_struct("AccessError").finish()
+    }
+}
+
+#[unstable(feature = "thread_local_state",
+           reason = "state querying was recently added",
+           issue = "27716")]
+impl fmt::Display for AccessError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt("already destroyed", f)
+    }
+}
+
 impl<T: 'static> LocalKey<T> {
     #[doc(hidden)]
     #[unstable(feature = "thread_local_internals",
@@ -331,6 +357,32 @@ impl<T: 'static> LocalKey<T> {
             }
         }
     }
+
+    /// Acquires a reference to the value in this TLS key.
+    ///
+    /// This will lazily initialize the value if this thread has not referenced
+    /// this key yet. If the key has been destroyed (which may happen if this is called
+    /// in a destructor), this function will return a ThreadLocalError.
+    ///
+    /// # Panics
+    ///
+    /// This function will still `panic!()` if the key is uninitialized and the
+    /// key's initializer panics.
+    #[unstable(feature = "thread_local_state",
+               reason = "state querying was recently added",
+               issue = "27716")]
+    pub fn try_with<F, R>(&'static self, f: F) -> Result<R, AccessError>
+                      where F: FnOnce(&T) -> R {
+        unsafe {
+            let slot = (self.inner)().ok_or(AccessError {
+                _private: (),
+            })?;
+            Ok(f(match *slot.get() {
+                Some(ref inner) => inner,
+                None => self.init(slot),
+            }))
+        }
+    }
 }
 
 #[doc(hidden)]

src/test/run-pass/tls-try-with.rs

@@ -0,0 +1,33 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-emscripten no threads support
+
+#![feature(thread_local_state)]
+
+use std::thread;
+
+struct Foo;
+
+thread_local!(static FOO: Foo = Foo {});
+
+impl Drop for Foo {
+    fn drop(&mut self) {
+        assert!(FOO.try_with(|_| panic!("`try_with` closure run")).is_err());
+    }
+}
+
+fn main() {
+    thread::spawn(|| {
+        assert_eq!(FOO.try_with(|_| {
+            132
+        }).expect("`try_with` failed"), 132);
+    }).join().unwrap();
+}