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 

2 files changed, 4 insertions, 2 deletions

diff --git a/src/libstd/sys/windows/os.rs b/src/libstd/sys/windows/os.rs
index e7194df7ac3..0f26e36a80f 100644
--- a/src/libstd/sys/windows/os.rs
+++ b/src/libstd/sys/windows/os.rs
@@ -124,7 +124,9 @@ pub unsafe fn pipe() -> IoResult<(FileDesc, FileDesc)> {
     }
 }
 
-pub fn fill_utf16_buf_and_decode(f: |*mut u16, DWORD| -> DWORD) -> Option<String> {
+pub fn fill_utf16_buf_and_decode<F>(mut f: F) -> Option<String> where
+    F: FnMut(*mut u16, DWORD) -> DWORD,
+{
     unsafe {
         let mut n = TMPBUF_SZ as DWORD;
         let mut res = None;
diff --git a/src/libstd/sys/windows/process.rs b/src/libstd/sys/windows/process.rs
index 0c2c76077dd..b03c62395d1 100644
--- a/src/libstd/sys/windows/process.rs
+++ b/src/libstd/sys/windows/process.rs
@@ -162,7 +162,7 @@ impl Process {
             // Similarly to unix, we don't actually leave holes for the stdio file
             // descriptors, but rather open up /dev/null equivalents. These
             // equivalents are drawn from libuv's windows process spawning.
-            let set_fd = |fd: &Option<P>, slot: &mut HANDLE,
+            let set_fd = |&: fd: &Option<P>, slot: &mut HANDLE,
                           is_stdin: bool| {
                 match *fd {
                     None => {