&[T] can be converted into [T; N] but &mut [T] can't

[rhysd]

I tried this code:

fn main() {
    use std::convert::TryFrom;

    let s: &[i32] = &[1, 2, 3, 4, 5];
    let _: [i32; 5] = TryFrom::try_from(s).unwrap();

    let s: &mut [i32] = &mut [1, 2, 3, 4, 5];
    let _: [i32; 5] = TryFrom::try_from(s).unwrap();
}

I expected to see this happen: The source is compiled without any error.

Instead, this happened: Compile error as follows.

error[E0277]: the trait bound `[i32; 5]: From<&mut [i32]>` is not satisfied
   --> foo.rs:8:23
    |
8   |     let _: [i32; 5] = TryFrom::try_from(s).unwrap();
    |                       ^^^^^^^^^^^^^^^^^ the trait `From<&mut [i32]>` is not implemented for `[i32; 5]`
    |
    = note: required because of the requirements on the impl of `Into<[i32; 5]>` for `&mut [i32]`
    = note: required because of the requirements on the impl of `TryFrom<&mut [i32]>` for `[i32; 5]`
note: required by `try_from`
   --> /Users/rhysd/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/src/rust/library/core/src/convert/mod.rs:477:5
    |
477 |     fn try_from(value: T) -> Result<Self, Self::Error>;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0277]: the trait bound `[i32; 5]: From<&mut [i32]>` is not satisfied
 --> foo.rs:8:23
  |
8 |     let _: [i32; 5] = TryFrom::try_from(s).unwrap();
  |                       ^^^^^^^^^^^^^^^^^^^^ the trait `From<&mut [i32]>` is not implemented for `[i32; 5]`
  |
  = note: required because of the requirements on the impl of `Into<[i32; 5]>` for `&mut [i32]`
  = note: required because of the requirements on the impl of `TryFrom<&mut [i32]>` for `[i32; 5]`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0277`.

Is there any reason TryFrom<&mut '_ [T]> for [T; N] is not implemented though TryFrom<&'_ [T]> for [T; N] is implemented?

Since the implementation is lack, I needed to convert the mutable slice into immutable slice like below.

let s: &mut [T] = ...;
let a: [T; N] = (&*s).try_into().unwrap();

Meta

rustc --version --verbose:

rustc 1.56.1 (59eed8a2a 2021-11-01)
binary: rustc
commit-hash: 59eed8a2aac0230a8b53e89d4e99d55912ba6b35
commit-date: 2021-11-01
host: x86_64-apple-darwin
release: 1.56.1
LLVM version: 13.0.0

[scottmcm]

To answer the historical reason: these implementations previously had to be generated with macros. See how ugly that made https://doc.rust-lang.org/1.14.0/std/primitive.array.html, for example. And all those impls actually slowed down compilation, so the easily-worked-around ones were omitted.

rust/library/core/src/array/equality.rs

Lines 106 to 108 in f7c4829

	// NOTE: some less important impls are omitted to reduce code bloat
	// __impl_slice_eq2! { [A; $N], &'b [B; $N] }
	// __impl_slice_eq2! { [A; $N], &'b mut [B; $N] }

Now that const generics exist, though, that problem has pretty much gone away, so filling out the matrix of conversions is more reasonable.