Add leading 0x to offset in Debug fmt of Pointer

[divergentdave]

Currently the Debug format for Pointer prints its offset in hexadecimal, for example, alloc38657819+e2 or alloc35122748+64. This PR adds a leading 0x to the offset, in order to make it apparent that it is indeed a hexadecimal number. This came up during discussion of rust-lang/miri#1354. r? @RalfJung

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @RalfJung (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[RalfJung]

Thanks!

I think this is much better, when I see alloc137+13 I would not expect the offset to be hex.
But Cc @oli-obk and @eddyb who reworked a lot of MIR printing code recently; what do you think?

[oli-obk]

🤷 I don't have a strong opinion. It'll make the relocation size problem in the hex dump worse though, which I think is the main reason I tried to keep it as short as possible

[RalfJung]

I wonder why hex dumps do not show up in the diff here?

At least for pointers outside of hex dumps, I think this is a clear improvement. Even within hex dumps, I'd rather shorten alloc to a or so, that's less confusing IMO.

[RalfJung]

For example, src/test/mir-opt/const_allocation/32bit/rustc.main.ConstProp.after.mir at the bottom still looks like

alloc17 (size: 48, align: 4) {
0x00 │ 00 00 00 00 __ __ __ __ ╾alloc4+0─╼ 00 00 00 00 │ ....░░░░╾──╼....
0x10 │ 00 00 00 00 __ __ __ __ ╾alloc8+0─╼ 02 00 00 00 │ ....░░░░╾──╼....
0x20 │ 01 00 00 00 2a 00 00 00 ╾alloc13+0╼ 03 00 00 00 │ ....*...╾──╼....
}

How can that be?

[oli-obk]

Likely because the refactoring for the alloc42+69 change happened in parallel to the hex dump PR, so the hex dump PR repeated the printing logic

[RalfJung]

Ah, indeed it does:

rust/src/librustc_mir/util/pretty.rs

Line 725 in 66f7a5d

letmut target = format!("{}+{}", target_id, offset);

So should we take advantage of that and leave the dumps without 0x? Seems oddly inconsistent though. (And btw this needs a FIXME to also print the tag.)

What about landing this PR to change just Pointer printing, and then I can make a follow-up where we play with various options for the hex-dumps?

[oli-obk]

Yea, makes sense, let's merge this PR

[RalfJung]

@bors r+

[bors]

📌 Commit c16b6e0 has been approved by RalfJung

[RalfJung]

Turns out MIR allocation dumping was actually using decimal, while normal Pointer printing was hex, but both looked exactly the same... ouch.

[eddyb]

@RalfJung I'm surprised you didn't notice #71459 (comment) had no :x, heh. It seemed to be the point of linking that snippet

I never expected hex offsets, since I had only seen decimal ones thus far.

[RalfJung]

Pointer printing was using hex offsets since at least since d01ef7d.