Attribute handling reworks

[jdonszelmann]

Proposal

Attributes are only partially lowered, and parsed in no less than 60 different places all around the compiler. I filed an issue earlier on rust-lang/rust but on the recomendation of @BoxyUwU and @WaffleLapkin I'm filing this as an MCP. Usually a proposal goes here, but I think my original issue already rougly covers that. I will, below, discuss steps in which we can implement this change.

• Refactor attributes into separate ast::Attribute and hir::Attribute Hir attributes rust#131808
• Make hir::Attribute with one variant that represents "old style" attributes and contains the current structure, and where more variants can be added that are pre-parsed
• Refactor rustc_attr to be the place where attributes are parsed and checked, making this a central place for this.
• Slowly move over every built-in attribute to this location, except for custom tool attributes which will remain unparsed

Mentors or Reviewers

I'll certainly talk a lot to @WaffleLapkin and @m-ou-se about this as I work with them daily. Apart from that, maybe more people are interested? Possibly @petrochenkov should second this too.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

There was already a zulip stream for this: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Attribute.20handling.20reworks

Let me also ping @jieyouxu and @bjorn3 here as they commented on the orginal proposal.

[rustbot]

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.

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cc @rust-lang/compiler @rust-lang/compiler-contributors

[WaffleLapkin]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted