Remove Spans from HIR

[cjgillot]

What is this issue?

This is a major change proposal, which means a proposal to make a notable change to the compiler -- one that either alters the architecture of some component, affects a lot of people, or makes a small but noticeable public change (e.g., adding a compiler flag). You can read more about the MCP process on https://forge.rust-lang.org/.

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

MCP Checklist

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A note on stability. If your change is proposing a new stable feature, such as a -C flag, then a full team checkoff will be required before the feature can be landed. Often it is better to start with an unstable flag, like a -Z flag, and then move to stabilize as a secondary step.

TL;DR

The HIR tree currently embeds spans copied from the AST. This makes the HIR unstable between incremental sessions. This MCP proposes to remove spans from the HIR tree, and to store them in a separate data structure.

Links and Details

At the moment, the HIR contains spans in three forms:

1. directly associated to nodes (Item, Pat...) ;
2. inside Indents ;
3. to track ad-hoc semantic points (see ExprKind).

This MCP proposes to remove all those spans, and to store them in a side table.
This side table would be built during AST->HIR lowering.
The span information corresponding to a HIR node is then accessible through a query.

Preliminary work: rust-lang/rust#72015
The PR explores the easy case: removing spans for nodes which have an associated HidId.

The two other cases (ad-hoc spans & Idents) need further design work.
Some may be approached by adding a HirId.

Mentors or Reviewers

Who will review this work? If you are being mentored by someone, then list
their name here. If you are a compiler team member/contributor, and you
intend to mentor someone else, then you can put your own name here. You can
also leave it blank if you are looking for a reviewer. (Multiple names are ok
too.)

[pnkfelix]

you can find associated zulip discussion here: https://zulip-archive.rust-lang.org/233931tcompilermajorchanges/98972RemoveSpansfromHIRcompilerteam294.html

[pnkfelix]

@rustbot second

[nikomatsakis]

Closing, this is accepted.