MIR dump prints the location of each statement as a comment

[oli-obk]

originally mentioned in #69916 (comment)

Basically all the lines look like

return;                          // bb2[0]: scope 0 at $DIR/const-promotion-extern-static.rs:13:1: 13:56

which is a lot of noise if used with mir-opt tests and diff mode, because mir optimizations that don't even touch the current statement may still cause diff fallout because they touch the comments.

Should we just stop emitting the location? Or maybe just specifically for mir-opt tests by creating a new -Z flag?

cc @nikomatsakis @pnkfelix @matthewjasper @eddyb

[Centril]

That info seems useful to me when e.g., dumping the MIR in the playground -- I think if we're going to make tweaks, let's do it only for mir-opt tests (but please find a way to thread this info without hacks in Debug impls).

[eddyb]

The comment is printed outside of any formatting impls so we should be fine.

@Centril do you mean the bb2[0], or the scope/span?

I specifically meant the mir::Location i.e. the bbX[Y], maybe this issue is unclear.

[Centril]

@eddyb both seem useful :)

[eddyb]

But have you actually used it? I can't remember having used it or imagine how, other than maybe correlating it with some debug! output?
If it was missing, worst case you have to count statements or something.

We could maybe normalize // bb\d+\[\d+\]: to just // by default for mir-opt tests in compiletest, that might be enough.

[eddyb]

Looks like printing mir::Locations started with #46319, so they're likely used for debugging MIR/NLL borrowck, for which we've already pinged @nikomatsakis and @matthewjasper, I guess?

[Centril]

I don't remember it vividly, but I recall that the info was useful when doing my initial investigation in #67565. Could be wrong though.

If it was missing, worst case you have to count statements or something.

Yeah I could do that, but it was a bit of a time saver not to have to do that work in my head. :)

[oli-obk]

We're not printing the basic blocks without -Zverbose anymore, so I think there's nothing actionable here