auto merge of #14644 : alexcrichton/rust/more-no-runtime-use-cases, r=brson

A few notable improvements were implemented to cut down on the number of aborts
triggered by the standard library when a local task is not found.

* Primarily, the unwinding functionality was restructured to support an unsafe
  top-level function, `try`. This function invokes a closure, capturing any
  failure which occurs inside of it. The purpose of this function is to be as
  lightweight of a "try block" as possible for rust, intended for use when the
  runtime is difficult to set up.

  This function is *not* meant to be used by normal rust code, nor should it be
  consider for use with normal rust code.

* When invoking spawn(), a `fail!()` is triggered rather than an abort.

* When invoking LocalIo::borrow(), which is transitively called by all I/O
  constructors, None is returned rather than aborting to indicate that there is
  no local I/O implementation.

A test case was also added showing the variety of things that you can do without
a runtime or task set up now. In general, this is just a refactoring to abort
less quickly in the standard library when a local task is not found.

Author: bors

src/libstd/local_data.rs

@@ -96,22 +96,24 @@ pub type Map = Vec<Option<(*u8, TLSValue, uint)>>;
 type TLSValue = Box<LocalData:Send>;
 
 // Gets the map from the runtime. Lazily initialises if not done so already.
-unsafe fn get_local_map() -> &mut Map {
+unsafe fn get_local_map() -> Option<&mut Map> {
     use rt::local::Local;
 
+    if !Local::exists(None::<Task>) { return None }
+
     let task: *mut Task = Local::unsafe_borrow();
     match &mut (*task).storage {
         // If the at_exit function is already set, then we just need to take
         // a loan out on the TLS map stored inside
         &LocalStorage(Some(ref mut map_ptr)) => {
-            return map_ptr;
+            return Some(map_ptr);
         }
         // If this is the first time we've accessed TLS, perform similar
         // actions to the oldsched way of doing things.
         &LocalStorage(ref mut slot) => {
             *slot = Some(vec!());
             match *slot {
-                Some(ref mut map_ptr) => { return map_ptr }
+                Some(ref mut map_ptr) => { return Some(map_ptr) }
                 None => unreachable!(),
             }
         }
@@ -156,7 +158,10 @@ impl<T: 'static> KeyValue<T> {
     /// assert_eq!(foo.replace(None), Some(4));
     /// ```
     pub fn replace(&'static self, data: Option<T>) -> Option<T> {
-        let map = unsafe { get_local_map() };
+        let map = match unsafe { get_local_map() } {
+            Some(map) => map,
+            None => fail!("must have a local task to insert into TLD"),
+        };
         let keyval = key_to_key_value(self);
 
         // When the task-local map is destroyed, all the data needs to be
@@ -223,7 +228,10 @@ impl<T: 'static> KeyValue<T> {
     /// assert_eq!(*key.get().unwrap(), 3);
     /// ```
     pub fn get(&'static self) -> Option<Ref<T>> {
-        let map = unsafe { get_local_map() };
+        let map = match unsafe { get_local_map() } {
+            Some(map) => map,
+            None => return None,
+        };
 
         self.find(map).map(|(pos, data, loan)| {
             *loan += 1;
@@ -260,7 +268,7 @@ impl<T: 'static> Deref<T> for Ref<T> {
 #[unsafe_destructor]
 impl<T: 'static> Drop for Ref<T> {
     fn drop(&mut self) {
-        let map = unsafe { get_local_map() };
+        let map = unsafe { get_local_map().unwrap() };
 
         let (_, _, ref mut loan) = *map.get_mut(self._index).get_mut_ref();
         *loan -= 1;

src/libstd/rt/rtio.rs

@@ -171,7 +171,10 @@ impl<'a> LocalIo<'a> {
         //
         // In order to get around this, we just transmute a copy out of the task
         // in order to have what is likely a static lifetime (bad).
-        let mut t: Box<Task> = Local::take();
+        let mut t: Box<Task> = match Local::try_take() {
+            Some(t) => t,
+            None => return None,
+        };
         let ret = t.local_io().map(|t| {
             unsafe { mem::transmute_copy(&t) }
         });