Extend -Zdump-mir output with two-phase borrow info

[pnkfelix]

This should cause no breakage since its just injecting a comment /*2pb*/ (and the -Zdump-mir output is unstable anyway).

[rustbot]

r? @lcnr

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[lcnr]

generally in favor even though the representation isn't great. I am not sure how long it would take me to figure out what /* 2pb */ means. Can't quickly think of a better alternative though '^^

r? @oli-obk

[oli-obk]

We could include more information in the // comments after the statement, by adding an arm for Rvalue::Ref in

rust/compiler/rustc_middle/src/mir/pretty.rs

Line 501 in 3af7df9

fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {

[pnkfelix]

@oli-obk can you spell out how much more info you think you'd want? Right now this really is just a boolean flag, so the presence/absence of a tag like /*2pb*/ is all I could think of to include.

(Maybe you're saying you'd want to know about when the reservation begins versus when the exclusive access itself actually begins, which I would agree would be superior, but I do not think that information is directly reflected in the MIR structure, and I think I'd bias towards making the dump_mir output merely reflect existing structure rather than have it attempt further deductions in the dump_mir code itself...)

[pnkfelix]

(I have been musing whether there would be value in having the MIR itself use general structure where every mutable borrow is modelled as a two-phase borrow, with distinct "reserve-mut" and "exercise-the-reservation" operations. But that is a huge change that would need far broader discussion.)

[oli-obk]

Right now this really is just a boolean flag, so the presence/absence of a tag like /*2pb*/ is all I could think of to include.

Oh, I don't mean more information, just clarifying what 2pb means in a comment, similar to how we have comments for constants giving their type and potentially verbose value

[oli-obk]

r=me with either the new comments added or not ^^

@rustbot author

[JohnCSimon]

@pnkfelix Ping from triage: Can you post your status on this PR? This has sat idle for a few months.

[JohnCSimon]

@pnkfelix

Ping from triage: I'm closing this due to inactivity, Please reopen when you are ready to continue with this.
Note: if you are going to continue please open the PR BEFORE you push to it, else you won't be able to reopen - this is a quirk of github.
Thanks for your contribution.

@rustbot label: +S-inactive