Add a `--emit=nameres` for IDEs

[davidbarsky]

Proposal

As discussed in this Zulip topic (and several others in t-compiler/rust-analyzer), name resolution in rust-analyzer is slow and extremely serial: it gates all IDE functionality until name resolution is completed for the entire dependency graph. While there are several promising avenues for us to improve rust-analyzer's performance without introducing external state, I'm curious how effective reusing rustc's name resolution results—exposed via an additive --emit=nameres mode—would be in speeding up rust-analyzer.

I don't expect this to be particularly helpful for Cargo-based users of rust-analyzer (as rust-analyzer largely requires the results of a cargo metadata execution...), but my employer's usage of Buck2 make this a somewhat viable experiment for two reasons:

• Buck2 is pervasively used with remote execution/caching, which means that a cargo check-equivalent build is effectively free for most users (I assume a cargo check is a strict superset of the work required to produce a --emit=nameres).
• the process to create a rust-project.json (rust-analyzer's cargo-metadata equivalent for non-Cargo build systems) from Buck is roughly equivalent to running a cargo check for build script/procedural macro reasons.

This mode does not need to be stabilized as I can either use a nightly or a RUSTC_BOOTSTRAP'd rustc on the stable channel.

Mentors or Reviewers

@oli-obk

Process

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

• [x] 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

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.

[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.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

 @rustbot concern reason-for-concern 
 <description of the concern> 

Concerns can be lifted with:

 @rustbot resolve reason-for-concern 

See documentation at https://forge.rust-lang.org

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

[apiraino]

Closing MCP after discussion on Zulip