Rollup merge of #71095 - pickfire:box-from-array, r=dtolnay

impl From<[T; N]> for Box<[T]>

Based on #68692

Author: RalfJung

src/liballoc/boxed.rs

@@ -865,6 +865,25 @@ impl From<Box<str>> for Box<[u8]> {
     }
 }
 
+#[stable(feature = "box_from_array", since = "1.45.0")]
+impl<T, const N: usize> From<[T; N]> for Box<[T]>
+where
+    [T; N]: LengthAtMost32,
+{
+    /// Converts a `[T; N]` into a `Box<[T]>`
+    ///
+    /// This conversion moves the array to newly heap-allocated memory.
+    ///
+    /// # Examples
+    /// ```rust
+    /// let boxed: Box<[u8]> = Box::from([4, 2]);
+    /// println!("{:?}", boxed);
+    /// ```
+    fn from(array: [T; N]) -> Box<[T]> {
+        box array
+    }
+}
+
 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
 impl<T, const N: usize> TryFrom<Box<[T]>> for Box<[T; N]>
 where

src/test/ui/const-generics/array-impls/alloc-traits-impls-length-32.rs

@@ -18,6 +18,10 @@ pub fn yes_array_into_vec<T>() -> Vec<T> {
     [].into()
 }
 
+pub fn yes_array_into_box<T>() -> Box<[T]> {
+    [].into()
+}
+
 use std::collections::VecDeque;
 
 pub fn yes_vecdeque_partial_eq_array<A, B>() -> impl PartialEq<[B; 32]>

src/test/ui/const-generics/array-impls/alloc-types-no-impls-length-33.rs

@@ -12,6 +12,8 @@ pub fn no_box() {
     let boxed_array = <Box<[i32; 33]>>::try_from(boxed_slice);
     //~^ ERROR the trait bound `std::boxed::Box<[i32; 33]>: std::convert::From<std::boxed::Box<[i32]>>` is not satisfied
     //~^^ ERROR the trait bound `std::boxed::Box<[i32; 33]>: std::convert::TryFrom<std::boxed::Box<[i32]>>` is not satisfied
+    let boxed_slice = <Box<[i32]>>::from([0; 33]);
+    //~^ 15:42: 15:49: arrays only have std trait implementations for lengths 0..=32 [E0277]
 }
 
 pub fn no_rc() {

src/test/ui/const-generics/array-impls/alloc-types-no-impls-length-33.stderr

@@ -18,10 +18,23 @@ LL |     let boxed_array = <Box<[i32; 33]>>::try_from(boxed_slice);
              <std::boxed::Box<(dyn std::error::Error + 'static)> as std::convert::From<&str>>
              <std::boxed::Box<(dyn std::error::Error + 'static)> as std::convert::From<std::borrow::Cow<'a, str>>>
              <std::boxed::Box<(dyn std::error::Error + 'static)> as std::convert::From<std::string::String>>
-           and 21 others
+           and 22 others
    = note: required because of the requirements on the impl of `std::convert::Into<std::boxed::Box<[i32; 33]>>` for `std::boxed::Box<[i32]>`
    = note: required because of the requirements on the impl of `std::convert::TryFrom<std::boxed::Box<[i32]>>` for `std::boxed::Box<[i32; 33]>`
 
+error[E0277]: arrays only have std trait implementations for lengths 0..=32
+  --> $DIR/alloc-types-no-impls-length-33.rs:15:42
+   |
+LL |     let boxed_slice = <Box<[i32]>>::from([0; 33]);
+   |                                          ^^^^^^^
+   |                                          |
+   |                                          expected an implementor of trait `std::convert::From<[{integer}; 33]>`
+   |                                          help: consider borrowing here: `&[0; 33]`
+   |
+   = note: the trait bound `[i32; 33]: std::convert::From<[{integer}; 33]>` is not satisfied
+   = note: required because of the requirements on the impl of `std::convert::From<[i32; 33]>` for `std::boxed::Box<[i32]>`
+   = note: required by `std::convert::From::from`
+
 error[E0277]: the trait bound `std::boxed::Box<[i32; 33]>: std::convert::TryFrom<std::boxed::Box<[i32]>>` is not satisfied
   --> $DIR/alloc-types-no-impls-length-33.rs:12:23
    |
@@ -32,7 +45,7 @@ LL |     let boxed_array = <Box<[i32; 33]>>::try_from(boxed_slice);
              <std::boxed::Box<[T; N]> as std::convert::TryFrom<std::boxed::Box<[T]>>>
 
 error[E0277]: the trait bound `std::rc::Rc<[i32; 33]>: std::convert::From<std::rc::Rc<[i32]>>` is not satisfied
-  --> $DIR/alloc-types-no-impls-length-33.rs:19:23
+  --> $DIR/alloc-types-no-impls-length-33.rs:21:23
    |
 LL |     let boxed_array = <Rc<[i32; 33]>>::try_from(boxed_slice);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::convert::From<std::rc::Rc<[i32]>>` is not implemented for `std::rc::Rc<[i32; 33]>`
@@ -47,7 +60,7 @@ LL |     let boxed_array = <Rc<[i32; 33]>>::try_from(boxed_slice);
    = note: required because of the requirements on the impl of `std::convert::TryFrom<std::rc::Rc<[i32]>>` for `std::rc::Rc<[i32; 33]>`
 
 error[E0277]: the trait bound `std::rc::Rc<[i32; 33]>: std::convert::TryFrom<std::rc::Rc<[i32]>>` is not satisfied
-  --> $DIR/alloc-types-no-impls-length-33.rs:19:23
+  --> $DIR/alloc-types-no-impls-length-33.rs:21:23
    |
 LL |     let boxed_array = <Rc<[i32; 33]>>::try_from(boxed_slice);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::convert::TryFrom<std::rc::Rc<[i32]>>` is not implemented for `std::rc::Rc<[i32; 33]>`
@@ -56,7 +69,7 @@ LL |     let boxed_array = <Rc<[i32; 33]>>::try_from(boxed_slice);
              <std::rc::Rc<[T; N]> as std::convert::TryFrom<std::rc::Rc<[T]>>>
 
 error[E0277]: the trait bound `std::sync::Arc<[i32; 33]>: std::convert::From<std::sync::Arc<[i32]>>` is not satisfied
-  --> $DIR/alloc-types-no-impls-length-33.rs:26:23
+  --> $DIR/alloc-types-no-impls-length-33.rs:28:23
    |
 LL |     let boxed_array = <Arc<[i32; 33]>>::try_from(boxed_slice);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::convert::From<std::sync::Arc<[i32]>>` is not implemented for `std::sync::Arc<[i32; 33]>`
@@ -71,14 +84,14 @@ LL |     let boxed_array = <Arc<[i32; 33]>>::try_from(boxed_slice);
    = note: required because of the requirements on the impl of `std::convert::TryFrom<std::sync::Arc<[i32]>>` for `std::sync::Arc<[i32; 33]>`
 
 error[E0277]: the trait bound `std::sync::Arc<[i32; 33]>: std::convert::TryFrom<std::sync::Arc<[i32]>>` is not satisfied
-  --> $DIR/alloc-types-no-impls-length-33.rs:26:23
+  --> $DIR/alloc-types-no-impls-length-33.rs:28:23
    |
 LL |     let boxed_array = <Arc<[i32; 33]>>::try_from(boxed_slice);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::convert::TryFrom<std::sync::Arc<[i32]>>` is not implemented for `std::sync::Arc<[i32; 33]>`
    |
    = help: the following implementations were found:
              <std::sync::Arc<[T; N]> as std::convert::TryFrom<std::sync::Arc<[T]>>>
 
-error: aborting due to 7 previous errors
+error: aborting due to 8 previous errors
 
 For more information about this error, try `rustc --explain E0277`.