auto merge of #20363 : japaric/rust/moar-uc, r=nmatsakis

The the last argument of the `ItemDecorator::expand` method has changed to `Box<FnMut>`. Syntax extensions will break.

[breaking-change]

---

This PR removes pretty much all the remaining uses of boxed closures from the libraries. There are still boxed closures under the `test` directory, but I think those should be removed or replaced with unboxed closures at the same time we remove boxed closures from the language.

In a few places I had to do some contortions (see the first commit for an example) to work around issue #19596. I have marked those workarounds with FIXMEs. In the future when `&mut F where F: FnMut` implements the `FnMut` trait, we should be able to remove those workarounds. I've take care to avoid placing the workaround functions in the public API.

Since `let f = || {}` always gets type checked as a boxed closure, I have explictly annotated those closures (with e.g. `|&:| {}`) to force the compiler to type check them as unboxed closures.

Instead of removing the type aliases (like `GetCrateDataCb`), I could have replaced them with newtypes. But this seemed like overcomplicating things for little to no gain.

I think we should be able to remove the boxed closures from the languge after this PR lands. (I'm being optimistic here)

r? @alexcrichton or @aturon 
cc @nikomatsakis 

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstd/rt/task.rs b/src/libstd/rt/task.rs
index 773322e4f57..41e91d1b6ef 100644
--- a/src/libstd/rt/task.rs
+++ b/src/libstd/rt/task.rs
@@ -174,7 +174,7 @@ impl Task {
     ///
     /// It is invalid to call this function with a thread that has been previously
     /// destroyed via a failed call to `run`.
-    pub fn run(mut self: Box<Task>, f: ||) -> Box<Task> {
+    pub fn run<F>(mut self: Box<Task>, f: F) -> Box<Task> where F: FnOnce() {
         assert!(!self.is_destroyed(), "cannot re-use a destroyed thread");
 
         // First, make sure that no one else is in TLS. This does not allow