Rollup merge of #97594 - WaffleLapkin:array_tuple_conv, r=ChrisDenton

Implement tuple<->array convertions via `From`

This PR adds the following impls that convert between homogeneous tuples and arrays of the corresponding lengths:
```rust
impl<T> From<[T; 1]> for (T,) { ... }
impl<T> From<[T; 2]> for (T, T) { ... }
/* ... */
impl<T> From<[T; 12]> for (T, T, T, T, T, T, T, T, T, T, T, T) { ... }

impl<T> From<(T,)> for [T; 1] { ... }
impl<T> From<(T, T)> for [T; 2] { ... }
/* ... */
impl<T> From<(T, T, T, T, T, T, T, T, T, T, T, T)> for [T; 12] { ... }
```

IMO these are quite uncontroversial but note that they are, just like any other trait impls, insta-stable.

Author: Manishearth

core/src/primitive_docs.rs

@@ -610,6 +610,9 @@ mod prim_pointer {}
 /// if the element type allows it. As a stopgap, trait implementations are
 /// statically generated up to size 32.
 ///
+/// Arrays of sizes from 1 to 12 (inclusive) implement [`From<Tuple>`], where `Tuple`
+/// is a homogenous [prim@tuple] of appropriate length.
+///
 /// Arrays coerce to [slices (`[T]`)][slice], so a slice method may be called on
 /// an array. Indeed, this provides most of the API for working with arrays.
 ///
@@ -672,6 +675,13 @@ mod prim_pointer {}
 /// move_away(roa);
 /// ```
 ///
+/// Arrays can be created from homogenous tuples of appropriate length:
+///
+/// ```
+/// let tuple: (u32, u32, u32) = (1, 2, 3);
+/// let array: [u32; 3] = tuple.into();
+/// ```
+///
 /// # Editions
 ///
 /// Prior to Rust 1.53, arrays did not implement [`IntoIterator`] by value, so the method call
@@ -774,6 +784,7 @@ mod prim_pointer {}
 /// [`Borrow`]: borrow::Borrow
 /// [`BorrowMut`]: borrow::BorrowMut
 /// [slice pattern]: ../reference/patterns.html#slice-patterns
+/// [`From<Tuple>`]: convert::From
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_array {}
 
@@ -1000,7 +1011,9 @@ mod prim_str {}
 /// * [`Debug`]
 /// * [`Default`]
 /// * [`Hash`]
+/// * [`From<[T; N]>`][from]
 ///
+/// [from]: convert::From
 /// [`Debug`]: fmt::Debug
 /// [`Hash`]: hash::Hash
 ///
@@ -1051,6 +1064,13 @@ mod prim_str {}
 /// assert_eq!(y, 5);
 /// ```
 ///
+/// Homogenous tuples can be created from arrays of appropriate length:
+///
+/// ```
+/// let array: [u32; 3] = [1, 2, 3];
+/// let tuple: (u32, u32, u32) = array.into();
+/// ```
+///
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_tuple {}
 

core/src/tuple.rs

@@ -100,6 +100,26 @@ macro_rules! tuple_impls {
                 }
             }
         }
+
+        #[stable(feature = "array_tuple_conv", since = "1.63.0")]
+        impl<T> From<[T; ${count(T)}]> for ($(${ignore(T)} T,)+) {
+            #[inline]
+            #[allow(non_snake_case)]
+            fn from(array: [T; ${count(T)}]) -> Self {
+                let [$($T,)+] = array;
+                ($($T,)+)
+            }
+        }
+
+        #[stable(feature = "array_tuple_conv", since = "1.63.0")]
+        impl<T> From<($(${ignore(T)} T,)+)> for [T; ${count(T)}] {
+            #[inline]
+            #[allow(non_snake_case)]
+            fn from(tuple: ($(${ignore(T)} T,)+)) -> Self {
+                let ($($T,)+) = tuple;
+                [$($T,)+]
+            }
+        }
     }
 }
 

std/src/primitive_docs.rs

@@ -610,6 +610,9 @@ mod prim_pointer {}
 /// if the element type allows it. As a stopgap, trait implementations are
 /// statically generated up to size 32.
 ///
+/// Arrays of sizes from 1 to 12 (inclusive) implement [`From<Tuple>`], where `Tuple`
+/// is a homogenous [prim@tuple] of appropriate length.
+///
 /// Arrays coerce to [slices (`[T]`)][slice], so a slice method may be called on
 /// an array. Indeed, this provides most of the API for working with arrays.
 ///
@@ -672,6 +675,13 @@ mod prim_pointer {}
 /// move_away(roa);
 /// ```
 ///
+/// Arrays can be created from homogenous tuples of appropriate length:
+///
+/// ```
+/// let tuple: (u32, u32, u32) = (1, 2, 3);
+/// let array: [u32; 3] = tuple.into();
+/// ```
+///
 /// # Editions
 ///
 /// Prior to Rust 1.53, arrays did not implement [`IntoIterator`] by value, so the method call
@@ -774,6 +784,7 @@ mod prim_pointer {}
 /// [`Borrow`]: borrow::Borrow
 /// [`BorrowMut`]: borrow::BorrowMut
 /// [slice pattern]: ../reference/patterns.html#slice-patterns
+/// [`From<Tuple>`]: convert::From
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_array {}
 
@@ -1000,7 +1011,9 @@ mod prim_str {}
 /// * [`Debug`]
 /// * [`Default`]
 /// * [`Hash`]
+/// * [`From<[T; N]>`][from]
 ///
+/// [from]: convert::From
 /// [`Debug`]: fmt::Debug
 /// [`Hash`]: hash::Hash
 ///
@@ -1051,6 +1064,13 @@ mod prim_str {}
 /// assert_eq!(y, 5);
 /// ```
 ///
+/// Homogenous tuples can be created from arrays of appropriate length:
+///
+/// ```
+/// let array: [u32; 3] = [1, 2, 3];
+/// let tuple: (u32, u32, u32) = array.into();
+/// ```
+///
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_tuple {}