const_err spans not rendered when rust-src not installed

[RalfJung]

This code causes a const_err somewhere in the standard library:

#![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>(),
        );
    }
};

When the rust-src package is not installed, the error is pretty unreadable:

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)

This is missing the key information of what goes wrong! With rust-src, the following is added, making it much clearer:

    --> /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

If the sources are missing, we should probably still show the error, even if we cannot attach it to any code.
Cc @rust-lang/wg-const-eval

[RalfJung]

Cc @rust-lang/wg-diagnostics : IMO it is a bug in the rustc diagnostic printing that it entirely omits information attached to spans where the source is missing. Is there anything we can do about that?

[estebank]

We could emit the information as a note instead of a label, but the wording would likely refer to "this blah", making it seem like it's referring to whatever is being pointed at in the main window. That might cause things to be misinterpreted. Another option would be to emit a note when rustc-src isn't present asking the user to install it when we encounter cases like this one.

[oli-obk]

Has this been resolved by #103970 ?

[albertlarsan68]

The error message is

error[E0080]: evaluation of constant value failed
   |
  ::: src\main.rs:14:9
   |
14 | /         ptr::swap_nonoverlapping(
15 | |             &mut ptr1 as *mut _ as *mut MaybeUninit<u8>,
16 | |             &mut ptr2 as *mut _ as *mut MaybeUninit<u8>,
17 | |             mem::size_of::<&i32>(),
18 | |         );
   | |_________- inside `X` at src\main.rs:14:9
   |
   = help: this code performed an operation that depends on the underlying bytes representing a pointer
   = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported

without rust-src installed, and

error[E0080]: evaluation of constant value failed
    --> $HOME\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\core\src\ptr\mod.rs:1139:9
     |
929  |       unsafe { swap_nonoverlapping_simple_untyped(x, y, count) }
     |                ----------------------------------------------- inside `swap_nonoverlapping::<MaybeUninit<u8>>` at $HOME\.rustup\toolchains\nightly-x86948  |           mem::swap_simple::<MaybeUninit<T>>(x, y);
     |           ---------------------------------------- inside `ptr::swap_nonoverlapping_simple_untyped::<MaybeUninit<u8>>` at $HOME\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\core\src\ptr\mod.rs:948:9
...
1139 |           copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
     |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |           |
     |           unable to copy parts of a pointer from memory at alloc6+0x1
     |           inside `std::ptr::read::<MaybeUninit<MaybeUninit<u8>>>` at $HOME\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\core\src\ptr\mod.rs:1139:9
     |
    ::: $HOME\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\core\src\mem\mod.rs:776:17
     |
776  |           let a = ptr::read(x);
     |                   ------------ inside `mem::swap_simple::<MaybeUninit<MaybeUninit<u8>>>` at $HOME\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\core\src\mem\mod.rs:776:17
     |
    ::: src\main.rs:14:9
     |
14   | /         ptr::swap_nonoverlapping(
15   | |             &mut ptr1 as *mut _ as *mut MaybeUninit<u8>,
16   | |             &mut ptr2 as *mut _ as *mut MaybeUninit<u8>,
17   | |             mem::size_of::<&i32>(),
18   | |         );
     | |_________- inside `X` at src\main.rs:14:9
     |
     = help: this code performed an operation that depends on the underlying bytes representing a pointer
     = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported

with the rust-src component installed with rustc 1.67.0-nightly (6284998a2 2022-11-12) on x86_64-pc-windows-msvc

[RalfJung]

So it does not seem entirely resolved -- the 'unable to copy parts of a pointer from memory at alloc6+0x1' is still missing completely when rust-src is not installed.

[oli-obk]

ugh... I'm slowly wondering if we should enable -Z simulate-remapped-rust-src-base=/rustc/xyz -Z translate-remapped-path-to-local-path=no for all ui tests just to actually see the way diagnostics are shown to users normally

[RalfJung]

Yeah if that is possible that would probably be good.

[oli-obk]

Well... either way, I found a way to fix this specific diagnostic, so once that is landed I'll experiment with unconditionally setting that flag on all ui tests.