auto merge of #20578 : japaric/rust/no-more-bc, r=nmatsakis

This PR removes boxed closures from the language, the closure type syntax (`let f: |int| -> bool = /* ... */`) has been obsoleted. Move all your uses of closures to the new unboxed closure system (i.e. `Fn*` traits).

[breaking-change] patterns

- `lef f = || {}`

This binding used to type check to a boxed closure. Now that boxed closures are gone, you need to annotate the "kind" of the unboxed closure, i.e. you need pick one of these: `|&:| {}`, `|&mut:| {}` or `|:| {}`.

In the (near) future we'll have closure "kind" inference, so the compiler will infer which `Fn*` trait to use based on how the closure is used. Once this inference machinery is in place, we'll be able to remove the kind annotation from most closures.

- `type Alias<'a> = |int|:'a -> bool`

Use a trait object: `type Alias<'a> = Box<FnMut(int) -> bool + 'a>`. Use the `Fn*` trait that makes sense for your use case.

- `fn foo(&self, f: |uint| -> bool)`

In this case you can use either a trait object or an unboxed closure:

``` rust
fn foo(&self, f: F) where F: FnMut(uint) -> bool;
// or
fn foo(&self, f: Box<FnMut(uint) -> bool>);
```

- `struct Struct<'a> { f: |uint|:'a -> bool }`

Again, you can use either a trait object or an unboxed closure:

``` rust
struct Struct<F> where F: FnMut(uint) -> bool { f: F }
// or
struct Struct<'a> { f: Box<FnMut(uint) -> bool + 'a> }
```

- Using `|x, y| f(x, y)` for closure "borrows"

This comes up in recursive functions, consider the following (contrived) example:

``` rust
fn foo(x: uint, f: |uint| -> bool) -> bool {
    //foo(x / 2, f) && f(x)  // can't use this because `f` gets moved away in the `foo` call
    foo(x / 2, |x| f(x)) && f(x)  // instead "borrow" `f` in the `foo` call
}
```

If you attempt to do the same with unboxed closures you'll hit ""error: reached the recursion limit during monomorphization" (see #19596):

``` rust
fn foo<F>(x: uint, mut f: F) -> bool where F: FnMut(uint) -> bool {
    foo(x / 2, |x| f(x)) && f(x)
    //~^ error: reached the recursion limit during monomorphization
}
```

Instead you *should* be able to write this:

``` rust
fn foo<F>(x: uint, mut f: F) -> bool where F: FnMut(uint) -> bool {
    foo(x / 2, &mut f) && f(x)
    //~^ error: the trait `FnMut` is not implemented for the type `&mut F`
}
```

But as you see above `&mut F` doesn't implement the `FnMut` trait. `&mut F` *should* implement the `FnMut` and the above code *should* work, but due to a bug (see #18835) it doesn't (for now).

You can work around the issue by rewriting the function to take `&mut F` instead of `F`:

``` rust
fn foo<F>(x: uint, f: &mut F) -> bool where F: FnMut(uint) -> bool {
    foo(x / 2, f) && (*f)(x)
}
```

This finally works! However writing `foo(0, &mut |x| x == 0)` is unergonomic. So you can use a private helper function to avoid this:

``` rust
// public API function
pub fn foo<F>(x: uint, mut f: F) -> bool where F: FnMut(uint) -> bool {
    foo_(x, &mut f)
}

// private helper function
fn foo_<F>(x: uint, f: &mut F) -> bool where F: FnMut(uint) -> bool {
    foo_(x / 2, f) && (*f)(x)
}
```

Closes #14798

---

There is more cleanup to do: like renaming functions/types from `unboxed_closure` to just `closure`, removing more dead code, simplify functions which now have unused arguments, update the documentation, etc. But that can be done in another PR.

r? @nikomatsakis @aturon (You probably want to focus on the deleted/modified tests.)
cc @eddyb

Author: bors

src/compiletest/compiletest.rs

@@ -339,8 +339,9 @@ pub fn is_test(config: &Config, testfile: &Path) -> bool {
     return valid;
 }
 
-pub fn make_test(config: &Config, testfile: &Path, f: || -> test::TestFn)
-                 -> test::TestDescAndFn {
+pub fn make_test<F>(config: &Config, testfile: &Path, f: F) -> test::TestDescAndFn where
+    F: FnOnce() -> test::TestFn,
+{
     test::TestDescAndFn {
         desc: test::TestDesc {
             name: make_test_name(config, testfile),

src/compiletest/header.rs

@@ -220,7 +220,9 @@ pub fn is_test_ignored(config: &Config, testfile: &Path) -> bool {
     !val
 }
 
-fn iter_header(testfile: &Path, it: |&str| -> bool) -> bool {
+fn iter_header<F>(testfile: &Path, mut it: F) -> bool where
+    F: FnMut(&str) -> bool,
+{
     use std::io::{BufferedReader, File};
 
     let mut rdr = BufferedReader::new(File::open(testfile).unwrap());

src/compiletest/runtest.rs

@@ -1233,12 +1233,14 @@ enum TargetLocation {
     ThisDirectory(Path),
 }
 
-fn make_compile_args(config: &Config,
-                     props: &TestProps,
-                     extras: Vec<String> ,
-                     xform: |&Config, &Path| -> TargetLocation,
-                     testfile: &Path)
-                     -> ProcArgs {
+fn make_compile_args<F>(config: &Config,
+                        props: &TestProps,
+                        extras: Vec<String> ,
+                        xform: F,
+                        testfile: &Path)
+                        -> ProcArgs where
+    F: FnOnce(&Config, &Path) -> TargetLocation,
+{
     let xform_file = xform(config, testfile);
     let target = if props.force_host {
         config.host.as_slice()

src/doc/guide-testing.md

@@ -537,7 +537,8 @@ computation entirely. This could be done for the example above by adjusting the
 `b.iter` call to
 
 ```rust
-# struct X; impl X { fn iter<T>(&self, _: || -> T) {} } let b = X;
+# struct X;
+# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
 b.iter(|| {
     // note lack of `;` (could also use an explicit `return`).
     range(0u, 1000).fold(0, |old, new| old ^ new)
@@ -552,7 +553,8 @@ argument as used.
 extern crate test;
 
 # fn main() {
-# struct X; impl X { fn iter<T>(&self, _: || -> T) {} } let b = X;
+# struct X;
+# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
 b.iter(|| {
     test::black_box(range(0u, 1000).fold(0, |old, new| old ^ new));
 });

src/doc/guide.md

@@ -4232,7 +4232,7 @@ arguments, really powerful things are possible.
 Let's make a closure:
 
 ```{rust}
-let add_one = |x| { 1 + x };
+let add_one = |&: x| { 1 + x };
 
 println!("The sum of 5 plus 1 is {}.", add_one(5));
 ```
@@ -4244,8 +4244,8 @@ binding name and two parentheses, just like we would for a named function.
 Let's compare syntax. The two are pretty close:
 
 ```{rust}
-let add_one = |x: i32| -> i32 { 1 + x };
-fn  add_one   (x: i32) -> i32 { 1 + x }
+let add_one = |&: x: i32| -> i32 { 1 + x };
+fn  add_one      (x: i32) -> i32 { 1 + x }
 ```
 
 As you may have noticed, closures infer their argument and return types, so you
@@ -4258,9 +4258,9 @@ this:
 
 ```{rust}
 fn main() {
-    let x = 5;
+    let x: i32 = 5;
 
-    let printer = || { println!("x is: {}", x); };
+    let printer = |&:| { println!("x is: {}", x); };
 
     printer(); // prints "x is: 5"
 }
@@ -4276,7 +4276,7 @@ defined. The closure borrows any variables it uses, so this will error:
 fn main() {
     let mut x = 5;
 
-    let printer = || { println!("x is: {}", x); };
+    let printer = |&:| { println!("x is: {}", x); };
 
     x = 6; // error: cannot assign to `x` because it is borrowed
 }
@@ -4298,12 +4298,12 @@ now. We'll talk about them more in the "Threads" section of the guide.
 Closures are most useful as an argument to another function. Here's an example:
 
 ```{rust}
-fn twice(x: i32, f: |i32| -> i32) -> i32 {
+fn twice<F: Fn(i32) -> i32>(x: i32, f: F) -> i32 {
     f(x) + f(x)
 }
 
 fn main() {
-    let square = |x: i32| { x * x };
+    let square = |&: x: i32| { x * x };
 
     twice(5, square); // evaluates to 50
 }
@@ -4312,15 +4312,15 @@ fn main() {
 Let's break the example down, starting with `main`:
 
 ```{rust}
-let square = |x: i32| { x * x };
+let square = |&: x: i32| { x * x };
 ```
 
 We've seen this before. We make a closure that takes an integer, and returns
 its square.
 
 ```{rust}
-# fn twice(x: i32, f: |i32| -> i32) -> i32 { f(x) + f(x) }
-# let square = |x: i32| { x * x };
+# fn twice<F: Fn(i32) -> i32>(x: i32, f: F) -> i32 { f(x) + f(x) }
+# let square = |&: x: i32| { x * x };
 twice(5, square); // evaluates to 50
 ```
 
@@ -4343,8 +4343,8 @@ how the `|i32| -> i32` syntax looks a lot like our definition of `square`
 above, if we added the return type in:
 
 ```{rust}
-let square = |x: i32| -> i32 { x * x };
-//           |i32|    -> i32
+let square = |&: x: i32| -> i32 { x * x };
+//           |i32|       -> i32
 ```
 
 This function takes an `i32` and returns an `i32`.
@@ -4358,7 +4358,7 @@ Finally, `twice` returns an `i32` as well.
 Okay, let's look at the body of `twice`:
 
 ```{rust}
-fn twice(x: i32, f: |i32| -> i32) -> i32 {
+fn twice<F: Fn(i32) -> i32>(x: i32, f: F) -> i32 {
   f(x) + f(x)
 }
 ```
@@ -4376,7 +4376,7 @@ If we didn't want to give `square` a name, we could just define it inline.
 This example is the same as the previous one:
 
 ```{rust}
-fn twice(x: i32, f: |i32| -> i32) -> i32 {
+fn twice<F: Fn(i32) -> i32>(x: i32, f: F) -> i32 {
     f(x) + f(x)
 }
 
@@ -4389,7 +4389,7 @@ A named function's name can be used wherever you'd use a closure. Another
 way of writing the previous example:
 
 ```{rust}
-fn twice(x: i32, f: |i32| -> i32) -> i32 {
+fn twice<F: Fn(i32) -> i32>(x: i32, f: F) -> i32 {
     f(x) + f(x)
 }
 

src/doc/reference.md

@@ -1563,7 +1563,7 @@ functions](#generic-functions).
 trait Seq<T> {
    fn len(&self) -> uint;
    fn elt_at(&self, n: uint) -> T;
-   fn iter(&self, |T|);
+   fn iter<F>(&self, F) where F: Fn(T);
 }
 ```
 
@@ -3218,7 +3218,7 @@ In this example, we define a function `ten_times` that takes a higher-order
 function argument, and call it with a lambda expression as an argument.
 
 ```
-fn ten_times(f: |int|) {
+fn ten_times<F>(f: F) where F: Fn(int) {
     let mut i = 0;
     while i < 10 {
         f(i);
@@ -3828,7 +3828,7 @@ fn add(x: int, y: int) -> int {
 
 let mut x = add(5,7);
 
-type Binop<'a> = |int,int|: 'a -> int;
+type Binop = fn(int, int) -> int;
 let bo: Binop = add;
 x = bo(5,7);
 ```
@@ -3852,14 +3852,14 @@ An example of creating and calling a closure:
 ```rust
 let captured_var = 10i;
 
-let closure_no_args = || println!("captured_var={}", captured_var);
+let closure_no_args = |&:| println!("captured_var={}", captured_var);
 
-let closure_args = |arg: int| -> int {
+let closure_args = |&: arg: int| -> int {
   println!("captured_var={}, arg={}", captured_var, arg);
   arg // Note lack of semicolon after 'arg'
 };
 
-fn call_closure(c1: ||, c2: |int| -> int) {
+fn call_closure<F: Fn(), G: Fn(int) -> int>(c1: F, c2: G) {
   c1();
   c2(2);
 }

src/libcollections/bit.rs

@@ -164,21 +164,6 @@ pub struct Bitv {
     nbits: uint
 }
 
-// NOTE(stage0): remove impl after a snapshot
-#[cfg(stage0)]
-// FIXME(Gankro): NopeNopeNopeNopeNope (wait for IndexGet to be a thing)
-impl Index<uint,bool> for Bitv {
-    #[inline]
-    fn index(&self, i: &uint) -> &bool {
-        if self.get(*i).expect("index out of bounds") {
-            &TRUE
-        } else {
-            &FALSE
-        }
-    }
-}
-
-#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 // FIXME(Gankro): NopeNopeNopeNopeNope (wait for IndexGet to be a thing)
 impl Index<uint> for Bitv {
     type Output = bool;

src/libcollections/btree/map.rs

@@ -877,18 +877,6 @@ impl<K: Show, V: Show> Show for BTreeMap<K, V> {
     }
 }
 
-// NOTE(stage0): remove impl after a snapshot
-#[cfg(stage0)]
-#[stable]
-impl<K: Ord, Sized? Q, V> Index<Q, V> for BTreeMap<K, V>
-    where Q: BorrowFrom<K> + Ord
-{
-    fn index(&self, key: &Q) -> &V {
-        self.get(key).expect("no entry found for key")
-    }
-}
-
-#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 #[stable]
 impl<K: Ord, Sized? Q, V> Index<Q> for BTreeMap<K, V>
     where Q: BorrowFrom<K> + Ord
@@ -900,18 +888,6 @@ impl<K: Ord, Sized? Q, V> Index<Q> for BTreeMap<K, V>
     }
 }
 
-// NOTE(stage0): remove impl after a snapshot
-#[cfg(stage0)]
-#[stable]
-impl<K: Ord, Sized? Q, V> IndexMut<Q, V> for BTreeMap<K, V>
-    where Q: BorrowFrom<K> + Ord
-{
-    fn index_mut(&mut self, key: &Q) -> &mut V {
-        self.get_mut(key).expect("no entry found for key")
-    }
-}
-
-#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 #[stable]
 impl<K: Ord, Sized? Q, V> IndexMut<Q> for BTreeMap<K, V>
     where Q: BorrowFrom<K> + Ord

src/libcollections/ring_buf.rs

@@ -1360,17 +1360,6 @@ impl<S: Writer, A: Hash<S>> Hash<S> for RingBuf<A> {
     }
 }
 
-// NOTE(stage0): remove impl after a snapshot
-#[cfg(stage0)]
-#[stable]
-impl<A> Index<uint, A> for RingBuf<A> {
-    #[inline]
-    fn index<'a>(&'a self, i: &uint) -> &'a A {
-        self.get(*i).expect("Out of bounds access")
-    }
-}
-
-#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 #[stable]
 impl<A> Index<uint> for RingBuf<A> {
     type Output = A;
@@ -1381,17 +1370,6 @@ impl<A> Index<uint> for RingBuf<A> {
     }
 }
 
-// NOTE(stage0): remove impl after a snapshot
-#[cfg(stage0)]
-#[stable]
-impl<A> IndexMut<uint, A> for RingBuf<A> {
-    #[inline]
-    fn index_mut<'a>(&'a mut self, i: &uint) -> &'a mut A {
-        self.get_mut(*i).expect("Out of bounds access")
-    }
-}
-
-#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 #[stable]
 impl<A> IndexMut<uint> for RingBuf<A> {
     type Output = A;

src/libcollections/vec.rs

@@ -1190,17 +1190,6 @@ impl<S: hash::Writer, T: Hash<S>> Hash<S> for Vec<T> {
     }
 }
 
-// NOTE(stage0): remove impl after a snapshot
-#[cfg(stage0)]
-#[experimental = "waiting on Index stability"]
-impl<T> Index<uint,T> for Vec<T> {
-    #[inline]
-    fn index<'a>(&'a self, index: &uint) -> &'a T {
-        &self.as_slice()[*index]
-    }
-}
-
-#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 #[experimental = "waiting on Index stability"]
 impl<T> Index<uint> for Vec<T> {
     type Output = T;
@@ -1211,16 +1200,6 @@ impl<T> Index<uint> for Vec<T> {
     }
 }
 
-// NOTE(stage0): remove impl after a snapshot
-#[cfg(stage0)]
-impl<T> IndexMut<uint,T> for Vec<T> {
-    #[inline]
-    fn index_mut<'a>(&'a mut self, index: &uint) -> &'a mut T {
-        &mut self.as_mut_slice()[*index]
-    }
-}
-
-#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 impl<T> IndexMut<uint> for Vec<T> {
     type Output = T;
 

src/libcollections/vec_map.rs

@@ -517,17 +517,6 @@ impl<V> Extend<(uint, V)> for VecMap<V> {
     }
 }
 
-// NOTE(stage0): remove impl after a snapshot
-#[cfg(stage0)]
-#[stable]
-impl<V> Index<uint, V> for VecMap<V> {
-    #[inline]
-    fn index<'a>(&'a self, i: &uint) -> &'a V {
-        self.get(i).expect("key not present")
-    }
-}
-
-#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 impl<V> Index<uint> for VecMap<V> {
     type Output = V;
 
@@ -537,17 +526,6 @@ impl<V> Index<uint> for VecMap<V> {
     }
 }
 
-// NOTE(stage0): remove impl after a snapshot
-#[cfg(stage0)]
-#[stable]
-impl<V> IndexMut<uint, V> for VecMap<V> {
-    #[inline]
-    fn index_mut<'a>(&'a mut self, i: &uint) -> &'a mut V {
-        self.get_mut(i).expect("key not present")
-    }
-}
-
-#[cfg(not(stage0))]  // NOTE(stage0): remove cfg after a snapshot
 #[stable]
 impl<V> IndexMut<uint> for VecMap<V> {
     type Output = V;