Auto merge of #59500 - crlf0710:boxed-closure-impls, r=cramertj

Unsized rvalues: implement boxed closure impls. (2nd try)

This is a rebase of S-blocked-closed PR #55431 to current master. LLVM has moved forward since then, so maybe we can check whether the new LLVM 8.0 version unblocked this work.

src/doc/unstable-book/src/library-features/fnbox.md

@@ -0,0 +1,32 @@
+# `fnbox`
+
+The tracking issue for this feature is [#28796]
+
+[#28796]: https://github.com/rust-lang/rust/issues/28796
+
+------------------------
+
+This had been a temporary alternative to the following impls:
+
+```rust,ignore
+impl<A, F> FnOnce for Box<F> where F: FnOnce<A> + ?Sized {}
+impl<A, F> FnMut for Box<F> where F: FnMut<A> + ?Sized {}
+impl<A, F> Fn for Box<F> where F: Fn<A> + ?Sized {}
+```
+
+The impls are parallel to these (relatively old) impls:
+
+```rust,ignore
+impl<A, F> FnOnce for &mut F where F: FnMut<A> + ?Sized {}
+impl<A, F> FnMut for &mut F where F: FnMut<A> + ?Sized {}
+impl<A, F> Fn for &mut F where F: Fn<A> + ?Sized {}
+impl<A, F> FnOnce for &F where F: Fn<A> + ?Sized {}
+impl<A, F> FnMut for &F where F: Fn<A> + ?Sized {}
+impl<A, F> Fn for &F where F: Fn<A> + ?Sized {}
+```
+
+Before the introduction of [`unsized_locals`][unsized_locals], we had been unable to provide the former impls. That means, unlike `&dyn Fn()` or `&mut dyn FnMut()` we could not use `Box<dyn FnOnce()>` at that time.
+
+[unsized_locals]: language-features/unsized-locals.html
+
+`FnBox()` is an alternative approach to `Box<dyn FnBox()>` is delegated to `FnBox::call_box` which doesn't need unsized locals. As we now have `Box<dyn FnOnce()>` working, the `fnbox` feature is going to be removed.

src/liballoc/boxed.rs

@@ -694,6 +694,28 @@ impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
 #[stable(feature = "fused", since = "1.26.0")]
 impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
 
+#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
+impl<A, F: FnOnce<A> + ?Sized> FnOnce<A> for Box<F> {
+    type Output = <F as FnOnce<A>>::Output;
+
+    extern "rust-call" fn call_once(self, args: A) -> Self::Output {
+        <F as FnOnce<A>>::call_once(*self, args)
+    }
+}
+
+#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
+impl<A, F: FnMut<A> + ?Sized> FnMut<A> for Box<F> {
+    extern "rust-call" fn call_mut(&mut self, args: A) -> Self::Output {
+        <F as FnMut<A>>::call_mut(self, args)
+    }
+}
+
+#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
+impl<A, F: Fn<A> + ?Sized> Fn<A> for Box<F> {
+    extern "rust-call" fn call(&self, args: A) -> Self::Output {
+        <F as Fn<A>>::call(self, args)
+    }
+}
 
 /// `FnBox` is a version of the `FnOnce` intended for use with boxed
 /// closure objects. The idea is that where one would normally store a
@@ -735,9 +757,7 @@ impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}
 #[rustc_paren_sugar]
 #[unstable(feature = "fnbox",
            reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
-pub trait FnBox<A> {
+pub trait FnBox<A>: FnOnce<A> {
-    type Output;
-
     fn call_box(self: Box<Self>, args: A) -> Self::Output;
 }
 
@@ -746,33 +766,11 @@ pub trait FnBox<A> {
 impl<A, F> FnBox<A> for F
     where F: FnOnce<A>
 {
-    type Output = F::Output;
-
     fn call_box(self: Box<F>, args: A) -> F::Output {
         self.call_once(args)
     }
 }
 
-#[unstable(feature = "fnbox",
-           reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
-impl<A, R> FnOnce<A> for Box<dyn FnBox<A, Output = R> + '_> {
-    type Output = R;
-
-    extern "rust-call" fn call_once(self, args: A) -> R {
-        self.call_box(args)
-    }
-}
-
-#[unstable(feature = "fnbox",
-           reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
-impl<A, R> FnOnce<A> for Box<dyn FnBox<A, Output = R> + Send + '_> {
-    type Output = R;
-
-    extern "rust-call" fn call_once(self, args: A) -> R {
-        self.call_box(args)
-    }
-}
-
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 

src/liballoc/lib.rs

@@ -107,6 +107,7 @@
 #![feature(unboxed_closures)]
 #![feature(unicode_internals)]
 #![feature(unsize)]
+#![feature(unsized_locals)]
 #![feature(allocator_internals)]
 #![feature(on_unimplemented)]
 #![feature(rustc_const_unstable)]

src/test/run-pass/unsized-locals/box-fnonce.rs

@@ -0,0 +1,8 @@
+fn call_it<T>(f: Box<dyn FnOnce() -> T>) -> T {
+    f()
+}
+
+fn main() {
+    let s = "hello".to_owned();
+    assert_eq!(&call_it(Box::new(|| s)) as &str, "hello");
+}

src/test/run-pass/unsized-locals/fnbox-compat.rs

@@ -0,0 +1,12 @@
+#![feature(fnbox)]
+
+use std::boxed::FnBox;
+
+fn call_it<T>(f: Box<dyn FnBox() -> T>) -> T {
+    f()
+}
+
+fn main() {
+    let s = "hello".to_owned();
+    assert_eq!(&call_it(Box::new(|| s)) as &str, "hello");
+}

src/test/ui/borrowck/two-phase-nonrecv-autoref.ast.nll.stderr

@@ -7,17 +7,15 @@ LL |         f(f(10));
    |         first mutable borrow occurs here
    |         first borrow later used by call
 
-error[E0382]: use of moved value: `*f`
+error[E0382]: use of moved value: `f`
   --> $DIR/two-phase-nonrecv-autoref.rs:69:11
    |
 LL |     fn twice_ten_so<F: FnOnce(i32) -> i32>(f: Box<F>) {
-   |                     - consider adding a `Copy` constraint to this type argument
+   |                                            - move occurs because `f` has type `std::boxed::Box<F>`, which does not implement the `Copy` trait
 LL |         f(f(10));
    |         - ^ value used here after move
    |         |
    |         value moved here
-   |
-   = note: move occurs because `*f` has type `F`, which does not implement the `Copy` trait
 
 error[E0499]: cannot borrow `*f` as mutable more than once at a time
   --> $DIR/two-phase-nonrecv-autoref.rs:76:11
@@ -28,30 +26,18 @@ LL |         f(f(10));
    |         first mutable borrow occurs here
    |         first borrow later used by call
 
-error[E0161]: cannot move a value of type dyn std::ops::FnOnce(i32) -> i32: the size of dyn std::ops::FnOnce(i32) -> i32 cannot be statically determined
+error[E0382]: use of moved value: `f`
-  --> $DIR/two-phase-nonrecv-autoref.rs:85:9
-   |
-LL |         f(f(10));
-   |         ^
-
-error[E0161]: cannot move a value of type dyn std::ops::FnOnce(i32) -> i32: the size of dyn std::ops::FnOnce(i32) -> i32 cannot be statically determined
-  --> $DIR/two-phase-nonrecv-autoref.rs:85:11
-   |
-LL |         f(f(10));
-   |           ^
-
-error[E0382]: use of moved value: `*f`
   --> $DIR/two-phase-nonrecv-autoref.rs:85:11
    |
+LL |     fn twice_ten_oo(f: Box<FnOnce(i32) -> i32>) {
+   |                     - move occurs because `f` has type `std::boxed::Box<dyn std::ops::FnOnce(i32) -> i32>`, which does not implement the `Copy` trait
 LL |         f(f(10));
    |         - ^ value used here after move
    |         |
    |         value moved here
-   |
-   = note: move occurs because `*f` has type `dyn std::ops::FnOnce(i32) -> i32`, which does not implement the `Copy` trait
 
 error[E0502]: cannot borrow `a` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:129:27
+  --> $DIR/two-phase-nonrecv-autoref.rs:125:27
    |
 LL |     double_access(&mut a, &a);
    |     ------------- ------  ^^ immutable borrow occurs here
@@ -60,7 +46,7 @@ LL |     double_access(&mut a, &a);
    |     mutable borrow later used by call
 
 error[E0502]: cannot borrow `i` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:157:7
+  --> $DIR/two-phase-nonrecv-autoref.rs:153:7
    |
 LL |     i[i[3]] = 4;
    |     --^----
@@ -70,7 +56,7 @@ LL |     i[i[3]] = 4;
    |     mutable borrow later used here
 
 error[E0502]: cannot borrow `i` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:163:7
+  --> $DIR/two-phase-nonrecv-autoref.rs:159:7
    |
 LL |     i[i[3]] = i[4];
    |     --^----
@@ -79,7 +65,7 @@ LL |     i[i[3]] = i[4];
    |     mutable borrow occurs here
    |     mutable borrow later used here
 
-error: aborting due to 9 previous errors
+error: aborting due to 7 previous errors
 
-Some errors occurred: E0161, E0382, E0499, E0502.
+Some errors occurred: E0382, E0499, E0502.
-For more information about an error, try `rustc --explain E0161`.
+For more information about an error, try `rustc --explain E0382`.

src/test/ui/borrowck/two-phase-nonrecv-autoref.ast.stderr

@@ -15,7 +15,7 @@ LL |         f(f(10));
    |         | second mutable borrow occurs here
    |         first mutable borrow occurs here
 
-error[E0382]: use of moved value: `*f`
+error[E0382]: use of moved value: `f`
   --> $DIR/two-phase-nonrecv-autoref.rs:69:11
    |
 LL |         f(f(10));
@@ -23,7 +23,7 @@ LL |         f(f(10));
    |         |
    |         value moved here
    |
-   = note: move occurs because `*f` has type `F`, which does not implement the `Copy` trait
+   = note: move occurs because `f` has type `std::boxed::Box<F>`, which does not implement the `Copy` trait
 
 error[E0499]: cannot borrow `*f` as mutable more than once at a time
   --> $DIR/two-phase-nonrecv-autoref.rs:76:11
@@ -34,7 +34,7 @@ LL |         f(f(10));
    |         | second mutable borrow occurs here
    |         first mutable borrow occurs here
 
-error[E0382]: use of moved value: `*f`
+error[E0382]: use of moved value: `f`
   --> $DIR/two-phase-nonrecv-autoref.rs:85:11
    |
 LL |         f(f(10));
@@ -42,10 +42,10 @@ LL |         f(f(10));
    |         |
    |         value moved here
    |
-   = note: move occurs because `*f` has type `(dyn std::ops::FnOnce(i32) -> i32 + 'static)`, which does not implement the `Copy` trait
+   = note: move occurs because `f` has type `std::boxed::Box<(dyn std::ops::FnOnce(i32) -> i32 + 'static)>`, which does not implement the `Copy` trait
 
 error[E0502]: cannot borrow `a` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:129:28
+  --> $DIR/two-phase-nonrecv-autoref.rs:125:28
    |
 LL |     double_access(&mut a, &a);
    |                        -   ^- mutable borrow ends here
@@ -54,7 +54,7 @@ LL |     double_access(&mut a, &a);
    |                        mutable borrow occurs here
 
 error[E0502]: cannot borrow `a` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:135:9
+  --> $DIR/two-phase-nonrecv-autoref.rs:131:9
    |
 LL |     a.m(a.i(10));
    |     -   ^      - mutable borrow ends here
@@ -63,7 +63,7 @@ LL |     a.m(a.i(10));
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `i` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:157:7
+  --> $DIR/two-phase-nonrecv-autoref.rs:153:7
    |
 LL |     i[i[3]] = 4;
    |     - ^   - mutable borrow ends here
@@ -72,7 +72,7 @@ LL |     i[i[3]] = 4;
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `i` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:163:7
+  --> $DIR/two-phase-nonrecv-autoref.rs:159:7
    |
 LL |     i[i[3]] = i[4];
    |     - ^   - mutable borrow ends here
@@ -81,7 +81,7 @@ LL |     i[i[3]] = i[4];
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `v` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:172:12
+  --> $DIR/two-phase-nonrecv-autoref.rs:168:12
    |
 LL |     v.push(v.len());
    |     -      ^      - mutable borrow ends here
@@ -90,7 +90,7 @@ LL |     v.push(v.len());
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `s` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:183:9
+  --> $DIR/two-phase-nonrecv-autoref.rs:179:9
    |
 LL |     s.m(s.i(10));
    |     -   ^      - mutable borrow ends here
@@ -99,7 +99,7 @@ LL |     s.m(s.i(10));
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `t` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:188:9
+  --> $DIR/two-phase-nonrecv-autoref.rs:184:9
    |
 LL |     t.m(t.i(10));
    |     -   ^      - mutable borrow ends here

src/test/ui/borrowck/two-phase-nonrecv-autoref.nll.stderr

@@ -7,17 +7,15 @@ LL |         f(f(10));
    |         first mutable borrow occurs here
    |         first borrow later used by call
 
-error[E0382]: use of moved value: `*f`
+error[E0382]: use of moved value: `f`
   --> $DIR/two-phase-nonrecv-autoref.rs:69:11
    |
 LL |     fn twice_ten_so<F: FnOnce(i32) -> i32>(f: Box<F>) {
-   |                     - consider adding a `Copy` constraint to this type argument
+   |                                            - move occurs because `f` has type `std::boxed::Box<F>`, which does not implement the `Copy` trait
 LL |         f(f(10));
    |         - ^ value used here after move
    |         |
    |         value moved here
-   |
-   = note: move occurs because `*f` has type `F`, which does not implement the `Copy` trait
 
 error[E0499]: cannot borrow `*f` as mutable more than once at a time
   --> $DIR/two-phase-nonrecv-autoref.rs:76:11
@@ -28,30 +26,18 @@ LL |         f(f(10));
    |         first mutable borrow occurs here
    |         first borrow later used by call
 
-error[E0161]: cannot move a value of type dyn std::ops::FnOnce(i32) -> i32: the size of dyn std::ops::FnOnce(i32) -> i32 cannot be statically determined
+error[E0382]: use of moved value: `f`
-  --> $DIR/two-phase-nonrecv-autoref.rs:85:9
-   |
-LL |         f(f(10));
-   |         ^
-
-error[E0161]: cannot move a value of type dyn std::ops::FnOnce(i32) -> i32: the size of dyn std::ops::FnOnce(i32) -> i32 cannot be statically determined
-  --> $DIR/two-phase-nonrecv-autoref.rs:85:11
-   |
-LL |         f(f(10));
-   |           ^
-
-error[E0382]: use of moved value: `*f`
   --> $DIR/two-phase-nonrecv-autoref.rs:85:11
    |
+LL |     fn twice_ten_oo(f: Box<FnOnce(i32) -> i32>) {
+   |                     - move occurs because `f` has type `std::boxed::Box<dyn std::ops::FnOnce(i32) -> i32>`, which does not implement the `Copy` trait
 LL |         f(f(10));
    |         - ^ value used here after move
    |         |
    |         value moved here
-   |
-   = note: move occurs because `*f` has type `dyn std::ops::FnOnce(i32) -> i32`, which does not implement the `Copy` trait
 
 error[E0502]: cannot borrow `a` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:129:27
+  --> $DIR/two-phase-nonrecv-autoref.rs:125:27
    |
 LL |     double_access(&mut a, &a);
    |     ------------- ------  ^^ immutable borrow occurs here
@@ -60,7 +46,7 @@ LL |     double_access(&mut a, &a);
    |     mutable borrow later used by call
 
 error[E0502]: cannot borrow `i` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:157:7
+  --> $DIR/two-phase-nonrecv-autoref.rs:153:7
    |
 LL |     i[i[3]] = 4;
    |     --^----
@@ -70,7 +56,7 @@ LL |     i[i[3]] = 4;
    |     mutable borrow later used here
 
 error[E0502]: cannot borrow `i` as immutable because it is also borrowed as mutable
-  --> $DIR/two-phase-nonrecv-autoref.rs:163:7
+  --> $DIR/two-phase-nonrecv-autoref.rs:159:7
    |
 LL |     i[i[3]] = i[4];
    |     --^----
@@ -79,7 +65,7 @@ LL |     i[i[3]] = i[4];
    |     mutable borrow occurs here
    |     mutable borrow later used here
 
-error: aborting due to 9 previous errors
+error: aborting due to 7 previous errors
 
-Some errors occurred: E0161, E0382, E0499, E0502.
+Some errors occurred: E0382, E0499, E0502.
-For more information about an error, try `rustc --explain E0161`.
+For more information about an error, try `rustc --explain E0382`.

src/test/ui/borrowck/two-phase-nonrecv-autoref.rs

@@ -67,9 +67,9 @@ fn overloaded_call_traits() {
     }
     fn twice_ten_so<F: FnOnce(i32) -> i32>(f: Box<F>) {
         f(f(10));
-        //[nll]~^   ERROR use of moved value: `*f`
+        //[nll]~^   ERROR use of moved value: `f`
-        //[g2p]~^^  ERROR use of moved value: `*f`
+        //[g2p]~^^  ERROR use of moved value: `f`
-        //[ast]~^^^ ERROR use of moved value: `*f`
+        //[ast]~^^^ ERROR use of moved value: `f`
     }
 
     fn twice_ten_om(f: &mut FnMut(i32) -> i32) {
@@ -83,13 +83,9 @@ fn overloaded_call_traits() {
     }
     fn twice_ten_oo(f: Box<FnOnce(i32) -> i32>) {
         f(f(10));
-        //[nll]~^          ERROR cannot move a value of type
+        //[nll]~^   ERROR use of moved value: `f`
-        //[nll]~^^         ERROR cannot move a value of type
+        //[g2p]~^^  ERROR use of moved value: `f`
-        //[nll]~^^^        ERROR use of moved value: `*f`
+        //[ast]~^^^ ERROR use of moved value: `f`
-        //[g2p]~^^^^       ERROR cannot move a value of type
-        //[g2p]~^^^^^      ERROR cannot move a value of type
-        //[g2p]~^^^^^^     ERROR use of moved value: `*f`
-        //[ast]~^^^^^^^    ERROR use of moved value: `*f`
     }
 
     twice_ten_sm(&mut |x| x + 1);

src/test/ui/span/borrowck-call-is-borrow-issue-12224.nll.stderr

@@ -18,7 +18,7 @@ LL | fn test2<F>(f: &F) where F: FnMut() {
 LL |     (*f)();
    |     ^^^^ `f` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
-error[E0596]: cannot borrow `*f.f` as mutable, as it is behind a `&` reference
+error[E0596]: cannot borrow `f.f` as mutable, as it is behind a `&` reference
   --> $DIR/borrowck-call-is-borrow-issue-12224.rs:34:5
    |
 LL | fn test4(f: &Test) {

src/test/ui/span/borrowck-call-is-borrow-issue-12224.rs

@@ -32,7 +32,7 @@ fn test3<F>(f: &mut F) where F: FnMut() {
 
 fn test4(f: &Test) {
     f.f.call_mut(())
-    //~^ ERROR: cannot borrow `Box` content `*f.f` of immutable binding as mutable
+    //~^ ERROR: cannot borrow field `f.f` of immutable binding as mutable
 }
 
 fn test5(f: &mut Test) {

src/test/ui/span/borrowck-call-is-borrow-issue-12224.stderr

@@ -19,13 +19,13 @@ LL | fn test2<F>(f: &F) where F: FnMut() {
 LL |     (*f)();
    |     ^^^^ cannot borrow as mutable
 
-error[E0596]: cannot borrow `Box` content `*f.f` of immutable binding as mutable
+error[E0596]: cannot borrow field `f.f` of immutable binding as mutable
   --> $DIR/borrowck-call-is-borrow-issue-12224.rs:34:5
    |
 LL | fn test4(f: &Test) {
    |             ----- use `&mut Test` here to make mutable
 LL |     f.f.call_mut(())
-   |     ^^^ cannot borrow as mutable
+   |     ^^^ cannot mutably borrow field of immutable binding
 
 error[E0504]: cannot move `f` into closure because it is borrowed
   --> $DIR/borrowck-call-is-borrow-issue-12224.rs:56:13