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

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[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

let mut 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.