Error spans into the standard library are missing because rust-src is not installed

[RalfJung]

Consider this code:

#![feature(const_swap)]
#![feature(const_mut_refs)]
use std::{
    mem::{self, MaybeUninit},
    ptr,
};

const X: () = {
    let mut ptr1 = &1;
    let mut ptr2 = &2;

    // Swap them, bytewise.
    unsafe {
        ptr::swap_nonoverlapping(
            &mut ptr1 as *mut _ as *mut MaybeUninit<u8>,
            &mut ptr2 as *mut _ as *mut MaybeUninit<u8>,
            mem::size_of::<&i32>(),
        );
    }
};

Locally, this renders an error like this:

error: any use of this value will cause an error
    --> /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1151:9
     |
1151 |           copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
     |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |           |
     |           unable to turn pointer into raw bytes
     |           inside `std::ptr::read::<MaybeUninit<u8>>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1151:9
     |           inside `mem::swap_simple::<MaybeUninit<u8>>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:768:17
     |           inside `ptr::swap_nonoverlapping_simple::<MaybeUninit<u8>>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:968:9
     |           inside `swap_nonoverlapping::<MaybeUninit<u8>>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:949:14
     |           inside `X` at test.rs:14:9
     |
    ::: test.rs:8:1
     |
8    | / const X: () = {
9    | |     let mut ptr1 = &1;
10   | |     let mut ptr2 = &2;
11   | |
...    |
19   | |     }
20   | | };
     | |__-
     |
     = note: `#[deny(const_err)]` on by default
     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>

However, on the playground, it looks like this:

error: any use of this value will cause an error
   |
  ::: src/lib.rs:8:1
   |
8  | / const X: () = {
9  | |     let mut ptr1 = &1;
10 | |     let mut ptr2 = &2;
11 | |
...  |
19 | |     }
20 | | };
   | |__-
   |
   = note: `#[deny(const_err)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, [see issue #71800 <https://github.com/rust-lang/rust/issues/71800>](https://github.com/rust-lang/rust/issues/71800)

In particular, the actual message of what went wrong ("unable to turn pointer into raw bytes") and the span of where that happened are missing!

[shepmaster]

This is because there's no source files for the compiler to point to:

% rustup component remove rust-src --toolchain nightly
info: removing component 'rust-src'

% cargo +nightly build
   Compiling rr v0.1.0 (/private/tmp/rr)
warning: constant is never used: `X`
  --> src/lib.rs:8:1
   |
8  | / const X: () = {
9  | |     let mut ptr1 = &1;
10 | |     let mut ptr2 = &2;
11 | |
...  |
19 | |     }
20 | | };
   | |__^
   |
   = note: `#[warn(dead_code)]` on by default

error: any use of this value will cause an error
     |
    ::: src/lib.rs:8:1
     |
8    | / const X: () = {
9    | |     let mut ptr1 = &1;
10   | |     let mut ptr2 = &2;
11   | |
...    |
19   | |     }
20   | | };
     | |__-
     |
     = note: `#[deny(const_err)]` on by default
     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>

warning: `rr` (lib) generated 1 warning
error: could not compile `rr` due to previous error; 1 warning emitted

If we install them, they reappear:

% rustup component add rust-src --toolchain nightly
info: downloading component 'rust-src'
info: installing component 'rust-src'

% cargo +nightly build
   Compiling rr v0.1.0 (/private/tmp/rr)
warning: constant is never used: `X`
  --> src/lib.rs:8:1
   |
8  | / const X: () = {
9  | |     let mut ptr1 = &1;
10 | |     let mut ptr2 = &2;
11 | |
...  |
19 | |     }
20 | | };
   | |__^
   |
   = note: `#[warn(dead_code)]` on by default

error: any use of this value will cause an error
    --> /Users/shep/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1104:9
     |
1104 |           copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
     |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |           |
     |           unable to turn pointer into raw bytes
     |           inside `std::ptr::read::<MaybeUninit<u8>>` at /Users/shep/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1104:9
     |           inside `mem::swap_simple::<MaybeUninit<u8>>` at /Users/shep/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/mem/mod.rs:768:17
     |           inside `ptr::swap_nonoverlapping_simple::<MaybeUninit<u8>>` at /Users/shep/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:921:9
     |           inside `swap_nonoverlapping::<MaybeUninit<u8>>` at /Users/shep/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:902:14
     |           inside `X` at src/lib.rs:14:9
     |
    ::: src/lib.rs:8:1
     |
8    | / const X: () = {
9    | |     let mut ptr1 = &1;
10   | |     let mut ptr2 = &2;
11   | |
...    |
19   | |     }
20   | | };
     | |__-
     |
     = note: `#[deny(const_err)]` on by default
     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>

warning: `rr` (lib) generated 1 warning
error: could not compile `rr` due to previous error; 1 warning emitted

[shepmaster]

the actual message of what went wrong ("unable to turn pointer into raw bytes")

I think that this should be reported separately as an upstream Rust bug.

[RalfJung]

Ah, good catch about the rust-src package! Reported as rust-lang/rust#97699.

Would it be an option for the playground to have rust-src installed?